FIND OUT FOR YOURSELF (COAX LENGTH)
SEAN FOLEY
in order to to present a formidable case for using electrically correct
lengths of coaxial cable, i will be using several different reference
sources. i will footnote the source as needed. i will also give a real
world example as to how you can find the resonant length of any cable that
you would like to use. let's start out with the bookwork first. to find
out how long an electrical radiator in free space is, we can use the
formula 468 divided by frequency (1). for example, let's use the center of
the cb band. this works out to 27.205/468 = 17.202 . theoretically in
free space, that's how long a half wave cb antenna should be. there are
several varying factors involved in this though, and one of these is the
diameter of radiator. as stated previously, this is in free space and real
world results may vary slightly. this does give us a rough estimate to
start with though. coax is slightly different than antennas in the fact
that the signal is shielded and not in free space. this causes the
electricity to change the speed that it is traveling. this is referred to
as velocity of propagation, or more commonly, the velocity factor (1,2).
therefore, we have a different length for a physical half wave and an
electrical half wave. the velocity factor is affected by several
different things, but the most important is the type of insulating
dielectric used to separate the center conductor from the shield in the
cable. common coaxial cables use several different types of dielectric
insulator. the more common are poly as used in RG 213 and some RG 8, RG
58, etc. this type of insulator appears to look semi-clear and is usually
pretty stiff. this dielectric has relatively high power handling
characteristics but slightly higher loss as compared to some of the other
types. the velocity factor of this type of cable is typically rated at 66%
or .66 for calculating purposes. foam is another common insulator used in
the coax industry. common coax that uses foam as an insulator are RG 8X,
which is also called RG 8M (mini 8). it can also be found in some RG 8 and
RG 58. this type of insulator is typically white in color and pretty soft
and flexible. due to the lower density of foam, the power handling is much
lower than that of a poly insulated coax, but it is also lower in loss.
high heat is detrimental to foam coax, especially in areas where bends are
located. the velocity factor of foam is 79% or .79 for calculating
purposes. teflon is another insulator used in coax, but is usually
reserved for high powered use. it is white in color like foam, but is much
more rigid like poly. the teflon will withstand much higher heat levels
and is therefore better suited to high power situations. due to production
costs, this cable is quite expensive per foot. RG 393 is a common coax
using a teflon insulator. this cable also uses silver center conductors
with a double layer of silver shield. this is common coax for "the big
boys". the velocity factor for a teflon insulated coax is figured at 69%
or .69 for calculating purposes. another newer cable insulator is the air
dielectric. using spiral wraps of a poly type insulator to keep the hot
wire centered, the coax has a much lower contact area with the dielectric
insulator. this is used in such cables as beldens' 9913. the cable has
good power handling characteristics, very low loss and great shielding.
the only problem is that it doesn't like bending. due to the mostly
unsupported nature of the single solid center wire, sharp bends or kinks
tend to "uncenter" the hot wire. this causes an impedance bump along the
coax and can result in a major hot spot, or area of high current flow. in
high power installations, this can result in "pin holing" the insulator,
literally burning a hole through the insulator. this leaves a carbon
residue for the rf signal to follow. carbon is a conductor and this
results in a short in the cable, sometimes intermittently. this results in
bad things happening to your equipment. the "flexible" 9913 types use a
stranded center conductor. by using multiple strands, the center
conductors can tend to separate and spread out, changing the capacitance in
areas and also the impedance. it is called "flexible", but can cause even
more problems than the so called non flexible. this is great cable when
installed properly, but if using very high power with a poor installation,
it can result in major equipment damage. the velocity factor of this type
of cable is rated at 84% or .84 for calculating purposes. there are other
types of cable in use, but i think that these 4 will cover 99% of what most
cber's and a large quantity of hams are currently using. let's summarize
the velocity factors of the various cables: poly = .66 / foam = .79 /
teflon =.69 / air = .84 (1, 3) due to production tolerances and
manufacturing differences, there might be some variation from these specs,
so keep this in mind. we can now calculate the approximate electrical
length for a half wave of coax. this is the shortest length that gives us
the closest approximation of the impedance at the antenna feedpoint (2).
odd multiples of the electrical half wave of coax are most desired, as even
lengths tend to invert, or flip-flop the swr reading (4). this causes an
antenna system to look short when it is really long or vice-versa. you
would actually be adjusting your antenna in the wrong direction by using
even multiples of electrical half waves. here are the appr. lengths for
an electrical half wave of the following type insulators. here's the
formula for finding a half wavelength in free space: 27.205/468 = 17.20
now factoring in the velocity of propagation for the different cables, we
do this 17.20 x .66 = 11.4 ft. for poly coax. foam would be 17.20 x .79 =
13.6 ft. for foam coax. teflon would be 17.20 x .69 = 11.9 ft for teflon
coax. 9913 type would be 17.20 x .84 = 14.5 ft. for 9913 type. as i said,
these are close approximations only, due to production tolerances. to find
the exact length for your specific cable, we will need the following
instruments. a PL-259 connector, a 50 ohm 2 or 3 watt non-inductive
resistor, an swr meter (you can use the one in your radio if you like),
your coax and a safety pin. we will start out by inserting the one side of
the resistor into the center of the pl 259. push the resistor all the way
down until the lead comes through the tip of the pl 259. now solder the
tip of the pl 259 carefully and trim the excess resistor lead off. if we
take and bend the resistor lead that is exposed over to the outer side of
the pl and tape it there, we now have a small dummy load. it should show
a flat match on your swr meter on just about any freq that you go to. this
will approximate a 50 ohm antenna. we will not be using it in exactly the
same manner, but the end result is the same. now take and fill the area
inside the pl 259 around the resistor with silicone or some type of epoxy.
this is to keep the resistor centered so it doesn't make contact with
ground and give it support. to prep the coax, we need to strip the outer
jacket back about an inch. pull the shield into a single braid on one side
and trim the insulator and center wire almost flush with the outer jacket.
make sure you leave enough of the insulator so that the shield and center
can't touch. we will be wedging the free lead of the resistor into the
center conductor of the coax. make sure that the lead doesn't fold over
and touch the braid. you should probably trim the lead slightly. you may
have to loosen the center wires up a bit to insert the resistor lead. this
can be done using a pin or other small sharp object. the shield will be
connected to the outside part of the pl 259. this will put the resistor in
series with anything connected to the coax. we know that just connecting
the resistor to ground gives us a flat match almost everywhere. now by
hooking the coax in series with the resistor, we can measure how much the
electrical length of the cable changes the swr of the dummy load. as
discussed, connect the coax to the prepared pl 259. the far end of coax
should be open. by inserting the pin and making contact between the hot
and ground, we are closing the circuit and can alter the electrical length
of the coax by changing where we pin the coax at. yes this produces a
short in the coax, but the resistor is in place and acts just like a dummy
load. in effect, we will be viewing the swr of that specific length of
coax. i would start with 18' of coax and see what a farce this is. if you
don't want to start at 18', at least start about 1.5' longer than what the
equations come up with. in the real world, these are usually a little
shorter than needed (5). we can always pin down slowly to the length that
gives us the best reading. there will be a wide area that will give you a
very reasonable reading. the overall length will affect our bandwidth,
rocking the window of resonance. i usually tune for the 26 to 28 meg
range. this gives me an accurate swr view for the bootleg cb band, 100 up
and down. you can use this method at any freq, as long as the resistor
chosen when shorted to the pl 259 shows a flat match. use this as a
guideline for adjusting the length : higher swr on ch 40 = shorten
higher swr on ch 1 = lengthen. once you've found the resonant length, cut
the coax there. as long as you didn't pin down making the coax shorter,
you can put ends on this and use it for resonating mobile antennas. it
will now give you the most accurate swr reading within the range that you
resonated the coax for. to calculate longer lengths for base use, multiply
this length times odd numbers, like 3, 5, 7, etc. remember, using an even
multiple will invert the reading (4). if using an external swr meter, i
suggest directly coupling this to the signal generator/radio with a double
male connector. if this is not feasible, use the extreme shortest length
of cable possible to couple the radio and meter. this small amount of
jumper can sometimes play games with the readings. if you have an antenna
analyzer, watch the impedance follow the swr curve with the resonant coax
hooked up and then watch it with 18' or whatever length that you were using
previously. it will track much smoother with an even response. i would
suggest printing this out. you can then check it out for yourself and you
have it for reference. if you find it to be bullshit, then use it to line
your birdcage. one thing i ask though, please do not post comments until
you have actually tried this first hand and examined the results. if you
have questions, i will be glad to answer to the best of my ability.
Sean aka Bigfoot
(1) ARRL Handbook (2) Department of Army
Technical Manual # TM 11-666 entitled "Antennas and Radio Propagation (3)
Radio Shack catalogue (even these guys know, ha ha ha) (4) Lou Franklin,
an Extra Class Ham's book entitled, "The Screwdriver Experts Guide to CB
Radio". (5) years of reading, learning and working on radio gear .
George Warner
In article <01bc9b22$12b3d520$74fe...@bigfoot.realworld.com>, "SEAN FOLEY"
<big...@xsite.net> wrote:
> in order to to present a formidable case for using electrically correct
> lengths of coaxial cable, i will be using several different reference
> sources. i will footnote the source as needed.
[Information about calculating electrical length snipped]
I have no argument with how you find a half wavelength of coax;
my argument is why you find it.
> ...we can now calculate the approximate electrical
> length for a half wave of coax. this is the shortest length that gives us
> the closest approximation of the impedance at the antenna feedpoint (2).
OK so far.
> odd multiples of the electrical half wave of coax are most desired, as even
> lengths tend to invert, or flip-flop the swr reading (4). this causes an
> antenna system to look short when it is really long or vice-versa. you
> would actually be adjusting your antenna in the wrong direction by using
> even multiples of electrical half waves.
Now you've gone astray. The impedance repeats every half wavelength.
If one half wavelength of coax presents an input impedance equal to
its output impedance, why wouldn't a second half wavelength of coax
present an input impedance equal to its output impedance when its
output is tied to the input of the first half wavelength of coax?
It is odd multiples of a quarter wavelengths of transmission line
that "flip-flop" the impedance. However, they do not change the SWR.
"Look short when it is really long"!?!?!? Did this statement come out
of your reference?
By the way, what is the SWR for a short? Answer: Infinity.
What is the SWR for an open? Answer: Infinity.
Note that this "flip-flopped" impedance did not change the SWR.
[more electrical length calculations snipped]
No argument here. You have just described a method for finding a half
wavelength of coax. For those who had difficulty following the description,
it looks like this:
coax 1 center 50 ohms coax 2 center
Transmitter >--------------< <--/\/\/\---> >--------------|
| movable short
>--------------< <-----------> >--------------| (pin in coax)
coax 1 shield coax 2 shield
Because coax 2 will present an input impedance that looks like
the output impedance when it is one half wavelength long, shorting
it one half wavelength away will present a short at the input.
This will short the right side of the 50 ohm resistor to ground and
present a perfect match to coax 1.
> it will now give you the most accurate swr reading within the range that you
> resonated the coax for.
Now I disagree. You have made no case for it providing "the most accurate
swr reading."
> to calculate longer lengths for base use, multiply
> this length times odd numbers, like 3, 5, 7, etc.
Why? Again, this is wrong. Impedances repeat every half
wavelength -- not odd multiples of a half wavelength.
> remember, using an even
> multiple will invert the reading (4).
This is wrong. Either you have taken this out of context or
this is a really bad book.
> if using an external swr meter, i
> suggest directly coupling this to the signal generator/radio with a double
> male connector. if this is not feasible, use the extreme shortest length
> of cable possible to couple the radio and meter. this small amount of
> jumper can sometimes play games with the readings. if you have an antenna
> analyzer, watch the impedance follow the swr curve with the resonant coax
> hooked up and then watch it with 18' or whatever length that you were using
> previously. it will track much smoother with an even response. i would
> suggest printing this out. you can then check it out for yourself and you
> have it for reference. if you find it to be bullshit, then use it to line
> your birdcage. one thing i ask though, please do not post comments until
> you have actually tried this first hand and examined the results. if you
> have questions, i will be glad to answer to the best of my ability.
> Sean aka Bigfoot
>
> (1) ARRL Handbook (2) Department of Army
> Technical Manual # TM 11-666 entitled "Antennas and Radio Propagation (3)
> Radio Shack catalogue (even these guys know, ha ha ha) (4) Lou Franklin,
> an Extra Class Ham's book entitled, "The Screwdriver Experts Guide to CB
> Radio". (5) years of reading, learning and working on radio gear.
Sean, despite a mostly correct post, you haven't proven the
original claims. Here's why:
Here's what you have shown:
1) A system (coax 1 and series 50 ohm resistor) that requires its output
shorted for best match.
2) A system (coax 2) that presents a short when shorted one half
wavelength from the input.
This is a proven, accurate method for finding a half wavelength of coax
(or any multiple of 1/2 wavelength). There are variations for finding
odd multiples of a quarter wavelength.
Now consider the system of interest - typical of CB systems:
coax center
Transmitter >-------------< <---|
-
| | Unknown impedance
-
>-------------< <---|
coax shield
The original claims state that varying the coax length will
change the SWR reading. Furthermore, they state that the
most accurate SWR reading is at the load (antenna).
Neither of these claims is true. See my long post under
"Re: Help with high SWR" for my reasons why.
George Warner
warn...@loveboat.com
Professor
Sean,
George is right... you have proven nothing. A true coaxial cable should have
distributed circuit coefficients and in the case of your hook-up from
transmitter to antenna, should appear to be transparent to the system...
regardless of length.
--
The Professor
"WindowsNT Powered"
SEAN FOLEY
george, you have not performed the test. this is obvious. your drawing
and description are incorrect. go back and read the original post, step by
step. if you did and still can't see the light, i can't help you. we
start out by making a dummy load. we verify that the dummy load presents a
flat match to the transmitter. we then insert the dummy load in series at
the radio with the coax. there is no jumper between the radio and dummy
load (coax 1). the dummy load is in the pl 259 feeding the center
conductor of the coax in series. the shield goes to ground on the pl 259.
the other end of the coax is open. we close the circuit and vary the
length of the cable by inserting a pin through both conductors in the coax.
we know that the dummy load is flat anywhere we go. recheck swr with the
pin at the end of 18' of coax. the reading is completely different than
when we just had the dummy load in line by itself. all we have done is
added a length of coax, which you say can't or won't change our reading.
yet the reading is drastically different. as we come closer to the correct
length for an electrical half wave, we also see the swr coming down. to
simplify this, it means that all of the other lengths were introducing
false readings. we know that the dummy load is flat, yet we had swr
readings much higher. the length of the cable HAS effected our readings,
much in error also. the only variable that we changed was the length of
the cable. the radio stayed the same. the coax is 50 ohms throughout the
whole length as long as it is not defective. the dummy load is still 50
ohms, so the impedance of the circuit stayed the same. so what causes the
swr reading to change ? let alone take a non reactive 50 ohm circuit and
show some rather high readings ? if you performed this test, you would
have seen this firsthand. how can you take a reading of a verified perfect
match and turn it into a reading of a non perfect match by running odd
lengths of cable and still want to know why this is important ? this just
tells me that you verified the theory of how to find electrical half waves
of coax in a book. you did not perform this simple test, which i would
have been glad to refund any expenses incurred had you not seen the light.
nobody believes this works until they see it performed first hand, that's
why i said "FIND OUT FOR YOURSELF". not everything that you need to know
is written in a book. yes, i am hot. i rest my case. the proof is in the
pudding and i gave you the recipe. it's up to you to follow it. if anybody
has tried this, especially those with swr analyzer's, please report in with
your results. i will gladly eat crow if i'm wrong, but i'm not. best
wishes, Sean aka Bigfoot
Bill Eitner
I agree with George--if you put your 50 ohm resistor ACROSS
the coax ANY length of coax will return a 1:1 SWR. Antennas
are termination loads; they exist at the END of a run of coax,
not in series somewhere along a shorted run of coax.
Also, in Franklin's book he states that odd quarter-wavelengths
of coax will act as impedance inverters. He uses the odd half
wavelengths as an impedance repeater example, but he doesn't say
what even half-wavelengths do to an impedance measurement.
The bottom line as far as I'm concerned is field strength. If
you can prove that using resonant lengths of coax will result
in higher field strength than can be achieved by simply using
the shortest length possible (assuming a resonant, 50 ohm termin-
ating load) then you'll have something worth talking about.
--
-=[Bill Eitner]=-
George Warner
In article <01bc9bd6$62cc6400$6bfe...@bigfoot.realworld.com>, "SEAN FOLEY"
<big...@xsite.net> wrote:
> george, you have not performed the test. this is obvious. your drawing
> and description are incorrect. go back and read the original post, step by
> step. if you did and still can't see the light, i can't help you. we
> start out by making a dummy load. we verify that the dummy load presents a
> flat match to the transmitter. we then insert the dummy load in series at
> the radio with the coax. there is no jumper between the radio and dummy
> load (coax 1). the dummy load is in the pl 259 feeding the center
> conductor of the coax in series. the shield goes to ground on the pl 259.
> the other end of the coax is open. we close the circuit and vary the
> length of the cable by inserting a pin through both conductors in the coax.
> we know that the dummy load is flat anywhere we go. recheck swr with the
> pin at the end of 18' of coax.
OK, I apologize. Here is the corrected circuit diagram.
I merely had a piece of coax on the input of your circuit.
It was still terminated with your circuit. Everything I stated
still applies. The circuit is effectively equivalent. Because
coax does not change the SWR, my circuit was equivalent.
50 ohms coax center
Transmitter <--/\/\/\---> >--------------|
| movable short
<-----------> >--------------| (pin in coax)
PL259 shield coax shield
Everything that I said still applies; nothing has really changed.
Coax 1 had no effect on the circuit.
> the reading is completely different than
> when we just had the dummy load in line by itself.
No shit! What do you expect when you add an impedance in
series with 50 ohms? The only way you will get a good
match is if that series impedance is 0 ohms.
> all we have done is added a length of coax,
Yes, and, in this case, the coax must be a multiple of
a half wavelength in order to present the necessary 0 ohms
to get a good match.
This is not the same as varying the length of coax between
your transmitter and your antenna. This is equivalent to adding
a coax stub to the other side of your antenna.
> which you say can't or won't change our reading.
I say changing the length of coax between your transmitter and
your antenna will have no effect. This is a typical CB configuration.
You have created an atypical configuration which does not prove your
claim and misinterpreted the results.
> yet the reading is drastically different. as we come closer to the correct
> length for an electrical half wave, we also see the swr coming down. to
> simplify this, it means that all of the other lengths were introducing
> false readings.
No, it means no such thing. The readings were correct all along because
the transmitter was not terminated with 50 ohms.
> we know that the dummy load is flat, yet we had swr
> readings much higher.
Yes, because you weren't measuring the dummy load alone; you were
measuring 50 ohms plus some impedance seen at the input of the
coax. Unless this impedance were 0 ohms, the SWR readings would
be much higher.
> the length of the cable HAS effected our readings,
In this circuit, yes.
> much in error also.
No error. The high SWR reading was a correct reading. The
transmitter was not terminated with 50 ohms. The SWR will
not be 1:1.
> the only variable that we changed was the length of
> the cable. the radio stayed the same. the coax is 50 ohms throughout the
> whole length as long as it is not defective. the dummy load is still 50
> ohms, so the impedance of the circuit stayed the same.
The impedance of the circuit was NOT the same!
> so what causes the
> swr reading to change ? let alone take a non reactive 50 ohm circuit and
> show some rather high readings ? if you performed this test, you would
> have seen this firsthand.
The SWR reading changes because you are adding a capacitance or
inductance (depending on the coax length) in series with the 50 ohm
resistor.
Your test will work as described but you are misinterpreting the
results.
> how can you take a reading of a verified perfect
> match and turn it into a reading of a non perfect match by running odd
> lengths of cable and still want to know why this is important ?
If I add an impedance in series with 50 ohms, it will be a non-perfect
match unless that impedance is 0 ohms.
> this just
> tells me that you verified the theory of how to find electrical half waves
> of coax in a book. you did not perform this simple test, which i would
> have been glad to refund any expenses incurred had you not seen the light.
> nobody believes this works until they see it performed first hand, that's
> why i said "FIND OUT FOR YOURSELF". not everything that you need to know
> is written in a book.
Yes, your test will work. I have not performed it because I understand
how it works and that it will work as you describe.
Also, because I understand it, I know the relevance of the results.
I do not apply the misinterpreted results to another situation.
> yes, i am hot. i rest my case. the proof is in the
> pudding and i gave you the recipe. it's up to you to follow it. if anybody
> has tried this, especially those with swr analyzer's, please report in with
> your results. i will gladly eat crow if i'm wrong, but i'm not. best
> wishes, Sean aka Bigfoot
You are certainly no Perry Mason. You've presented a weak case with
holes big enough to drive a tractor-trailer through and, although I
don't think it is necessary, I will now give a closing argument.
The debate centers on whether or not the length of the coax
affects the SWR reading in a standard CB configuration
(i.e. radio -- coax -- load [antenna]). You argue that somehow a
half wavelength of coax gives the most accurate reading. To prove
your point, you present an atypical configuration
(i.e. radio -- load -- shorted coax).
In your configuration, the shorted coax must present 0 ohms at the
load or the radio will not see 50 ohms.
You then misinterpret the high SWR readings as bad readings because
they are not 1:1. The fact is, these high readings are indeed correct
because the radio is not terminated with a pure 50 ohm load; it is
50 ohms plus some impedance introduced by the coax.
I need not perform your test. It means nothing. Your interpretation
of the results is flawed. Get out the crow.
George Warner
warn...@loveboat.com
SEAN FOLEY
i have proven nothing ? once again, you guys have to say that "coax SHOULD
have" instead of "coax DOES have". you are speaking out of your books and
not first hand knowledge. there is no antenna in this test. it is just a
dummy load serially feeding coax. the setup goes like this: radio
connected to pl with resistor in it. this is fed into the coax. the coax
is shorted at the end and various points until we can duplicate the same
match as what the dummy load read by itself. if you would perform this
simple test, you WILL see the proof. i can't change what you think, that's
why i gave a hands on experience that YOU are in control of and could SEE
FOR YOURSELF. forget that the world is flat and do some discovering. you
will FIND OUT FOR YOURSELF if you do this simple test. YOU will know for
sure one way or the other. once you have done this simple test with the
dummy load and coax firsthand, then post some pearls of knowledge. step up
people and take the test, or are all of you so brainwashed/stubborn/lazy
that you can't think for yourselves ? are ALL of you sheep ? i don't mind
being wrong, i just want someone else to make the same effort so that we
can compare notes. i am not here to argue. i put up this posting so that
you folks may learn the things that others have taught and shown me.
Sean aka Bigfoot
.
Professor
Well come on Sean, aren't you going to attack George like you always attack
me? You've been rather nice to him. I always got statements like the
Professor is an asshole, etc. etc. but your treating George with kid gloves.
Didn't you read his reply, he's telling you that you are full of it. You
seem to spend most of your time on this board defending your knowledge...
but is there a trend???
--
The Professor
"WindowsNT Powered"
George Warner wrote in article ...
SEAN FOLEY
if lengths of coax don't affect the reading, how does it do this ? in
effect we have the same thing as a non reactive antenna at 50 ohms with the
coax connected to it. instead of radiating the power, the resistor is
absorbing it. as for sticking something in series and that it must be 0
ohms, that is sheer nonsense. an antenna or dummy load is in series with
the coax at all times in normal use. if it isn't, then we have an open
circuit. the antenna radiates the energy, the dummy load absorbs it. by
shorting the coax with the resistor in series it simply does the same
thing. it closes the circuit. if we have an antenna analyzer, we can
measure the exact resonant frequency of this specific length of cable.
this would be the length that measures flat, just as the dummy load alone
measures. when we can find the length that coincides with the frequency
range that we want to use, it too will show a flat match. as i stated
before, the dummy load showed flat. all we have done is broken the path to
ground on the resistive dummy load and connected the coax. you are now
measuring the electrical length of the cable. just as the cable has an
electrical length, it too will have tuning. that's why the swr reading
changes. george has already acknowledged that an electrical 1/2 wave of
coax will duplicate the impedance reading at the antenna. he has also
acknowledged that this method will find the electrical half wavelength. if
the swr reading is high it is because this length of cable is not
replicating the actual 50 ohm load. the cable itself is tuned somewhere
else and replicating the 50 ohm flat match of the dummy load at that
frequency. this is why an swr analyzer is nice. you can measure your
freq, swr bandwidth, impedance and capacitance all with the spin of a dial
at the same time. the swr has not changed as both george and i will agree.
the dummy load measured flat and it is still 50 ohms. the length of cable
is affecting the reading. if you think about it, all we have done is
reversed the coax and dummy load. it's still the same coax we normally use
and still the same dummy load. just like in a normal radio / dummy load
setup, the signal leaves the radio, travels through the resistor and
returns to ground. there is no difference in the circuit. once again we
come back to the same place. please hook this circuit up exactly as
described and see for yourself. theory and hands on learning are sometimes
different. i'm sure that most of you have seen rf do things that are
technically impossible. take care and good luck..........Sean aka Bigfoot
P.S. the fat lady isn't singing yet.
keep the crow on the back burner for whomever gets to eat it. no matter
how long it cooks, it still tastes the same.
.
Dale Wambaugh
Bill Eitner wrote:
> The bottom line as far as I'm concerned is field strength. If
> you can prove that using resonant lengths of coax will result
> in higher field strength than can be achieved by simply using
> the shortest length possible (assuming a resonant, 50 ohm termin-
> ating load) then you'll have something worth talking about.
> --
>
> -=[Bill Eitner]=-
I agree. If it doesn't show up in the air, it's all hot air!
To the same point, I took the kids to Grandpa's house tonight and I was
showing him this TNC magnet mount setup I bought to mount the rubber
duckie for my hand-held on top of the car. He's been a CB'er since the
mid-Sixties and a journeyman electrician to boot. He started going on
about how the coax on a mobile had to be 20' long or longer or damage to
the finals would occur over a period of time. I just let it slide!
Jon G.
SEAN FOLEY wrote:
>
> i have proven nothing ? once again, you guys have to say that "coax SHOULD
> have" instead of "coax DOES have". you are speaking out of your books and
> not first hand knowledge. there is no antenna in this test. it is just a
> dummy load serially feeding coax. the setup goes like this: radio
> connected to pl with resistor in it. this is fed into the coax. the coax
> is shorted at the end and various points until we can duplicate the same
> match as what the dummy load read by itself. if you would perform this
> simple test, you WILL see the proof. i can't change what you think, that's
> why i gave a hands on experience that YOU are in control of and could SEE
> FOR YOURSELF. forget that the world is flat and do some discovering. you
> will FIND OUT FOR YOURSELF if you do this simple test. YOU will know for
> sure one way or the other. once you have done this simple test with the
> dummy load and coax firsthand, then post some pearls of knowledge. step up
> people and take the test, or are all of you so brainwashed/stubborn/lazy
> that you can't think for yourselves ? are ALL of you sheep ? i don't mind
> being wrong, i just want someone else to make the same effort so that we
> can compare notes. i am not here to argue. i put up this posting so that
> you folks may learn the things that others have taught and shown me.
> Sean aka Bigfoot
Sean, I don't want to get in the middle of the flame wars here but I wanted
to share my experiance with different coax lengths.
I have been cb'ing for almost 30 years and in those 30 years I have lived in
various places and used different lengths of coax for my installations. I
have never had an swr problem due to length of coax.
I always check the coax before installing with a dummy load. Length didn't
make any difference in the swr reading.
Just my 2 cents if it's worth that. - Video Man
SEAN FOLEY
here are my responses to this, one by one. they are in capitals.
Professor <bgri...@ameritech.net> wrote in article
<5rlrsb$gsi$1...@nntp0.chicago.il.ameritech.net>...
> Well come on Sean, aren't you going to attack George like you always
attack
> me? You've been rather nice to him.
NO, I WILL NOT ATTACK
HIM. HE HAS PROVEN THAT HE KNOWS HIS THEORY. I MUST RESPECT HIM FOR THAT.
WE ARE DEBATING A VALID POINT AND NAME CALLING GOES NOWHERE TOWARD
RESOLVING THIS. AS FOR ATTACKING YOU, I HAVE JUST STATED THE FACTS. YOU
HAVE POSTED IGNORANT COMMENTS TIME AND TIME AGAIN WITHOUT EVER RESPONDING
TO THE PERSONS ORIGINAL QUESTION. WHEN YOU HAVE MADE CLAIMS, YOU HAVEN'T
PROVIDED ANY REASONABLE PROOF OR THEORY TO BACK THEM UP. YOU HAVE STILL
NEVER RESPONDED OR EVEN ACKNOWLEDGED THE FACT THAT I HAVE OFFERED TO KEY
DOWN WITH YOU. THIS HAS BEEN A LONG SILENT JOKE WITH MANY ON THE GROUP.
IF YOU FEEL THAT THIS IS WHAT A PERSONAL ATTACK CONSISTS OF, I AM JUST
GIVING YOU A REFLECTION OF WHAT WE SEE.
.
I always got statements like the
> Professor is an asshole, etc. etc. but your treating George with kid
gloves.
.I HAVE
NEVER CALLED YOU AN ASSHOLE OR ANY OTHER DEROGATORY NAME. MAYBE INFERRED
IT, BUT NEVER STATED IT. THIS MUST BE YOUR GUILTY CONSCIENCE. AS FOR HOW
I'M TREATING GEORGE, THAT IS SIMPLE. HE SEEMS TO BE A WELL RESPECTED
OPINION ON THIS GROUP. HE IS CARRYING THE TORCH FOR THE UNBELIEVERS. IF I
WERE TO OFFEND HIM, MY CHANCES OF GETTING HIM TO COMPLETE THE DEBATE AND DO
THE EXPERIMENT WOULD BE SLIM. IN ORDER TO ACHIEVE ANY RESULTS AND POSSIBLY
PULL IN ANY CONVERTS, I MUST PRESENT THE FACTS AND HAVE PROOF. IT MAY BE A
LONG AND SLOW RIDE, BUT WE'LL GET THERE. HOPEFULLY SOME OF THE OTHERS WILL
TRY THIS FOR THEMSELVES AND SPEAK UP ABOUT THEIR RESULTS.
.
> Didn't you read his reply, he's telling you that you are full of it.
OF COURSE I DID. HE IS
DEFENDING WHAT HE THINKS IS TRUE. DIFFERENCES OF OPINION OFTEN RESULT IN
BOISTEROUS ARGUMENTS, AND SOMETIMES THINGS SAID OUT OF ANGER. EMOTION AND
EGO CAN BE A POWERFUL COMBINATION. WE ARE BOTH FERVENT BELIEVERS IN OUR
"FAITHS" AND ARE RISKING OUR REPUTATIONS ON THIS.
.
You seem to spend most of your time on this board defending your
knowledge...
> but is there a trend???
>
> I POST CONTROVERSIAL ITEMS. I AM NOT AFRAID TO DEFEND MY POINT OF VIEW.
I ALSO POST RESPONSES TO QUESTIONS, GIVE INFORMATION AS TO WHERE TO LOCATE
ITEMS, HOW TO CORRECT PROBLEMS, ETC. I AM NOT AFRAID OF
LIFE, NOR THE RESULTS OF LIVING. BREAKTHROUGHS WERE RARELY MADE BY PEOPLE
ACCEPTING THE NORM. I'LL TAKE THIS AS A COMPLIMENT. THANK YOU.
SEAN AKA BIGFOOT
Bill Nelson
SEAN FOLEY (big...@xsite.net) wrote:
: i have proven nothing ? once again, you guys have to say that "coax SHOULD
: have" instead of "coax DOES have". you are speaking out of your books and
: not first hand knowledge. there is no antenna in this test. it is just a
: dummy load serially feeding coax. the setup goes like this: radio
Correct, you have proven nothing. There is no need to do the test, it will
perform exactly as you have described.
You do NOT have a dummy load feeding the coax. You have a dummy load in
series with a matching stub.
As others have pointed out, the experiment does not prove what you think
it does.
I posted a simple test to see if adding a line section will change the
SWR. It does NOT require an in-line resistor, or any such garbage. It is
a direct test of what you are claiming will happen.
Try it, and then report back to us.
: people and take the test, or are all of you so brainwashed/stubborn/lazy
: that you can't think for yourselves ? are ALL of you sheep ? i don't mind
: being wrong, i just want someone else to make the same effort so that we
: can compare notes. i am not here to argue. i put up this posting so that
: you folks may learn the things that others have taught and shown me.
I suggest you follow your own advice, and try the test I listed - unless
YOU are "brainwashed/stubborn/lazy".
--
Bill Nelson (bi...@peak.org)
SEAN FOLEY
that's the whole point Bill. without having an accurate swr reading, you
won't have a 50 ohm load. you THINK that the swr is good, and it may show
good, but your reading may be skewed. so it seems to reason, if we can
correct the impedance mismatch by getting a more accurate reading and
adjusting accordingly, we will have increased our power transfer and
radiated signal also. of course by bringing up the radiated signal, the
field strength would increase. i don't understand why you haven't
joined in on this one. you have all of the gear to do your own analysis
right under your nose. what's the hold-up ? i already know where you
stand on this, so please put my theory / experiment to the test. Sean aka
Bigfoot
George Warner
In article <01bc9c97$caec2680$6dfe...@bigfoot.realworld.com>, "SEAN FOLEY"
<big...@xsite.net> wrote:
> if lengths of coax don't affect the reading, how does it do this ? in
> effect we have the same thing as a non reactive antenna at 50 ohms with the
> coax connected to it. instead of radiating the power, the resistor is
> absorbing it. as for sticking something in series and that it must be 0
> ohms, that is sheer nonsense. an antenna or dummy load is in series with
> the coax at all times in normal use. if it isn't, then we have an open
> circuit. the antenna radiates the energy, the dummy load absorbs it. by
> shorting the coax with the resistor in series it simply does the same
> thing. it closes the circuit.
This is wrong. A shorted length of cable is not zero ohms at radio
frequencies. You don't seem to understand that the transmission line
in your circuit behaves like an inductor or capacitor. The formula for
the input impedance of a transmission line is:
Zin = Rc Zl + jRc tan(l)
---------------
Rc + jZl tan(l)
where:
Rc = characteristic impedance of the transmission line
Zl = impedance of load
l = length of transmission line in degrees or radians
(by the way, you can use this equation to show how quarter-wave
transmission lines invert impedances and half-wave transmission
lines leave impedances untouched)
In your circuit:
Rc = 50 ohms
Zl = 0 ohms (short)
l = variable but works at one half wavelength (180 degrees)
Substituting these values into the original equation, it
simplifies to:
Zin = j(50) tan(l)
Because this impedance is in series with a 50 ohm resistor,
the transmitter sees the following impedance:
50 + j(50) tan(l)
The only time your transmitter will get a good 50 ohm match
is when this equation equals 50. Substituting, we find that
the right hand term MUST be 0.
When does this right hand term go to zero? Answer: at multiples
of 180 degrees (multiples of one half wavelength)
At 0 to 90 degrees (0 to 1/4 wavelength) the term is positive (inductive).
At 90 to 180 degrees (1/4 to 1/2 wavelength) the term is negative
(capacitive).
> if we have an antenna analyzer, we can
> measure the exact resonant frequency of this specific length of cable.
> this would be the length that measures flat, just as the dummy load alone
> measures. when we can find the length that coincides with the frequency
> range that we want to use, it too will show a flat match. as i stated
> before, the dummy load showed flat. all we have done is broken the path to
> ground on the resistive dummy load and connected the coax.
Connecting the coax is like connecting an inductor or capacitor.
The terminating impedance will no longer be 50 ohms.
> you are now
> measuring the electrical length of the cable. just as the cable has an
> electrical length, it too will have tuning. that's why the swr reading
> changes. george has already acknowledged that an electrical 1/2 wave of
> coax will duplicate the impedance reading at the antenna. he has also
> acknowledged that this method will find the electrical half wavelength. if
> the swr reading is high it is because this length of cable is not
> replicating the actual 50 ohm load. the cable itself is tuned somewhere
> else and replicating the 50 ohm flat match of the dummy load at that
> frequency. this is why an swr analyzer is nice. you can measure your
> freq, swr bandwidth, impedance and capacitance all with the spin of a dial
> at the same time. the swr has not changed as both george and i will agree.
> the dummy load measured flat and it is still 50 ohms. the length of cable
> is affecting the reading. if you think about it, all we have done is
> reversed the coax and dummy load.
This is a significant difference.
> it's still the same coax we normally use
> and still the same dummy load. just like in a normal radio / dummy load
> setup, the signal leaves the radio, travels through the resistor and
> returns to ground.
Returns to ground through an inductor or capacitor.
> there is no difference in the circuit. once again we
> come back to the same place. please hook this circuit up exactly as
> described and see for yourself. theory and hands on learning are sometimes
> different.
Where do you think theory comes from? It comes from much hands-on
experience.
> i'm sure that most of you have seen rf do things that are
> technically impossible. take care and good luck..........Sean aka Bigfoot
>
> P.S. the fat lady isn't singing yet.
> keep the crow on the back burner for whomever gets to eat it. no matter
> how long it cooks, it still tastes the same.
George Warner
warn...@loveboat.com
SEAN FOLEY
here are a few things that we all agree on so far.
(1) an electrical 1/2 wave reproduces a mirror image impedance of what is
on the other end. (2) this exercise will find the
electrical half wave of the specific coax in question.
(3) 50 ohms represents an ideal antenna load, at least for
standard cb's.
.
anything from this point on gets fuzzy.
we can go one step further, and probably say in agreement, that this
exercise also verifies the ARRL'S mathematical formulas for calculating a
half wavelength of coax with velocity factors calculated in. this is
within a tolerance window for production differences, of course. i gave
the formulas for this in the original post. yes, as george and bill nelson
stated, this is known as a matching stub. it is called this, because as he
demonstrated mathematically, it is neutral. it is neither inductive or
capacitive and therefore does not alter the reading but gives an
approximate "match" from one end to the other. this is just as we have all
agreed, that an electrical half wavelength reflects what is at the other
end without influence, making it in effect electrically invisible.
granted, many (most) antennas are not exactly 50 ohms. this still gives us
the benefit of seeing exactly what impedance we do have at the antenna
without any or very little coaxial influence. using this "matching stub"
as a feedline will allow us to track the impedance as we change frequency
more accurately, as long as we are within the cables resonant window. this
allows us to tweak the system for that added finesse that we wouldn't know
it was capable of unless we looked directly at the feedpoint. this can be
verified by comparing the impedance readings of the antenna using the
resonated cable and a typical 18' section. with the tuned length, as the
impedance changes, you will see the swr curve follow it gently, just as it
should. using the 18' piece, you will see the impedance curve taking
drastic swings while the swr stays relatively still. how can an antenna
drastically change impedance and maintain a non reactive swr ? the answer
is that it can't. but you wouldn't know that unless you were using an
electrically invisible length of coax to compare it to or unless you
measured the swr directly at the feedpoint. for most installations,
measuring at the feedpoint can be pretty difficult, but this alleviates
that problem. just as george demonstrated, other lengths of cable will
alter the reading due to capacitive or inductive reactance. this is where
we see the benefit of resonating the cable. as we have all seen printed
here in this group, it is a bad thing when we are radiating / talking off
of our coax. the coax is meant to be simply a funnel to go from radio to
load. this helps to minimize this problem by showing us the exact swr of
the antenna, so that we can make the necessary adjustments. this length of
cable does not hide resistive grounds, poor swr due to nearby reflected
images, etc. like random lengths of cable do. they do this by combining
the antennas reactive impedance with the cables own capacitive or inductive
reactance, which then easily play games with an swr meter. this way we know
just what we are dealing with at the antenna, because the coax itself is
electrically "invisible" or "matched". using the resonant coax, we would
find that the impedance of a typical wilson 2000 is closer to 37 ohms at
resonance instead of 50. this tells us that we are not achieving optimum
power transfer from the radio to the antenna, but that the antenna will
still work well. this is actually very close to the same impedance as a
102" whip. it also lets us see what the true bandwidth of the antenna is,
without influence from the cables reactance. the benefits of the cable are
that it has little to no influence on our standing wave or impedance
measurements, which is why i put up this post in the first place. this
simply means that we are getting more accurate swr readings. all of the
hoopla about the test circuit was just to make it easy for anyone to find
the electrical half wave length. all of you have agreed that this is the
least reactive length of cable. this circuit is used by MFJ in their swr
analyzer manuals. it is also recommended by Solarcon and several other
manufacturers. it helps take the guess work out of the equation and gives
us solid data that is easier to interpret if there is a problem. i have
gotten you to agree that the electrical half wavelength of cable does
exactly what i wanted it to do. be invisible. Sean aka Bigfoot
Kelly Flowers
SEAN FOLEY wrote:
>
> snip snip
Sean:
The fact remains that line length DOES NOT affect SWR. If you use cheap
shit "power" meters which are essentially voltage measuring devices, you
may see differences. This is not because the actual SWR is different,
its because you're NOT using a true rms reading power meter like a Bird
43 for example.
Since you like to quote from the ARRL handbook, let me offer the
following. It's from the 1981 version, Chapter 19 on Transmission
lines, page 19-4. (Unfortunately, my new one is loaned out!)
"In case the load(antenna) is not purely resistive, and/or is not equal
to the characteristic impedance, there will be standing waves. No
adjustments that can be made at the input end of the line can change the
SWR, nor is it affected by changing the line length."
NOTE THE LAST PART OF THE SENTENCE!!!!!
You've mixed up the concept of impedance transformation and SWR. Line
length will cause the impedance seen by the radio to be
different(assuming it's not a perfect 50 ohm "real" load) but IT DOES
NOT CHANGE THE SWR. LET ME REPEAT, IT DOES NOT CHANGE THE SWR!!!!!!
All other things being equal, the only thing that will change when you
change the line length is the loss associated with the line itself.
These differences for the lenghts you keep mentioning are essentially 0.
I won't belabor the math to prove this to you but if you insist, I'll
dig out my Transmission Lines textbook from college and email you.
This fact about line length not affecting SWR(excluding line loss
effects for long lengths) is common knowledge among anyone who has
studied Transmission Line theory.
You better think twice before you question the laws of Physics using
data obtained from mediocre instrumentation.
Kelly Flowers, P.E.
ARS NN8E
Bill Eitner
I believe in tuning antennas before installation and/or inserting
the SWR meter at the feedpoint. Both of these methods negate the
need to resonante the coax.
Remember, I've been at this since 1979--I've been there, done that.
When an antenna is resonant and a 50 ohm load field strength only
increases with significant reductions is total feedline length.
It does NOT increase more when you duplicate what would be an odd
integer multiple of a velocity factor corrected half wavelength.
When this isn't the case you have an antenna that needs work.
The time spent fooling around resonating coax would be much better
spent solving the antenna problem--that's the bottom line.
Bill Harris
Jon G. wrote:
>
> Sean, I don't want to get in the middle of the flame wars here but I wanted
> to share my experiance with different coax lengths.
>
> I have been cb'ing for almost 30 years and in those 30 years I have lived in
> various places and used different lengths of coax for my installations. I
> have never had an swr problem due to length of coax.
> I always check the coax before installing with a dummy load. Length didn't
> make any difference in the swr reading.
>
> Just my 2 cents if it's worth that. - Video Man
Exactly. I don't understand this business of placing the 50 ohm load in
series with the coax at the transmitter. Just terminate the coax with 50
ohms at the far end and try any lengths of coax you want and you will
see it will not change the swr. Length of coax does not affect the swr.
George Warner
In article <01bc9d05$a5d79e60$74fe...@bigfoot.realworld.com>, "SEAN FOLEY"
<big...@xsite.net> wrote:
> here are a few things that we all agree on so far.
> (1) an electrical 1/2 wave reproduces a mirror image impedance of
> what is on the other end.
> (2) this exercise will find the
> electrical half wave of the specific coax in question.
> (3) 50 ohms represents an ideal antenna load, at least for
> standard cb's.
Yes.
[snip]
> ...[a half wavelength coax is] in effect electrically invisible.
> granted, many (most) antennas are not exactly 50 ohms. this still
> gives us the benefit of seeing exactly what impedance we do have
> at the antenna without any or very little coaxial influence.
It does let you see the exact impedance of the antenna but is
that any benefit? The SWR was the same at every point in between.
Why bother getting the exact impedance when it really doesn't
matter? You are better off striving to minimize the coax length
to reduce those losses.
> using this "matching stub"
> as a feedline will allow us to track the impedance as we change frequency
> more accurately, as long as we are within the cables resonant window. this
> allows us to tweak the system for that added finesse that we wouldn't know
> it was capable of unless we looked directly at the feedpoint.
There is no added finesse. The SWR at the coax imput tracks the
antenna SWR directly regardless of coax length.
> this can be
> verified by comparing the impedance readings of the antenna using the
> resonated cable and a typical 18' section. with the tuned length, as the
> impedance changes, you will see the swr curve follow it gently, just as it
> should. using the 18' piece, you will see the impedance curve taking
> drastic swings while the swr stays relatively still.
If you were to plot the coax input impedance and the antenna impedance
on a Smith chart, you would find that they track identically. The only
difference is a rotational offset. By restricting the coax length to
one half wavelength, you merely make that offset to be one full rotation;
thus, the two impedances overlay each other.
> how can an antenna
> drastically change impedance and maintain a non reactive swr ? the answer
> is that it can't. but you wouldn't know that unless you were using an
> electrically invisible length of coax to compare it to or unless you
> measured the swr directly at the feedpoint.
If you measure SWR (as most of us do), you don't care how the impedance
changed because it hasn't changed the SWR. If you somehow measure the
impedance instead of the SWR, you can simply plot the impedances on
a Smith chart to visualize what is happening. No drastic change is
occurring.
> for most installations,
> measuring at the feedpoint can be pretty difficult, but this alleviates
> that problem. just as george demonstrated, other lengths of cable will
> alter the reading due to capacitive or inductive reactance.
You are comparing "most installations" to your "circuit." They are
not the same.
The reading was changing in your circuit because you were
actually changing the SWR.
Most installations will not see a change in reading due to changes
in coax length because the SWR does not change.
> this is where we see the benefit of resonating the cable.
There is no benefit.
[jive about coax hiding measurements snipped]
> ...which then easily play games with an swr meter.
No, the coax plays no games with the SWR meter. The SWR meter reading
is correct.
[jive about the necessity for an "invisible" length of coax snipped]
All lengths of coax are "invisible" to SWR. No special length is
required to get an accurate SWR reading.
> be invisible. Sean aka Bigfoot
You don't have to.
George
warn...@loveboat.com
SEAN FOLEY
Kelly Flowers <kelly....@ab.com> wrote in article
<33DF7E...@ab.com>...
> SEAN FOLEY wrote:
> >
> > snip snip
>
> Sean:
>
> The fact remains that line length DOES NOT affect SWR. If you use cheap
> shit "power" meters which are essentially voltage measuring devices, you
> may see differences. This is not because the actual SWR is different,
> its because you're NOT using a true rms reading power meter like a Bird
> 43 for example.
> LET'S COMPARE EQUIPMENT, BOZO. I HAVE A WHOLE SHOP FULL, INCLUDING
SEVERAL BIRDS.
> Since you like to quote from the ARRL handbook, let me offer the
> following. It's from the 1981 version, Chapter 19 on Transmission
> lines, page 19-4. (Unfortunately, my new one is loaned out!)
>
> "In case the load(antenna) is not purely resistive, and/or is not equal
> to the characteristic impedance, there will be standing waves. No
> adjustments that can be made at the input end of the line can change the
> SWR, nor is it affected by changing the line length."
>
> NOTE THE LAST PART OF THE SENTENCE!!!!!
>
> You've mixed up the concept of impedance transformation and SWR. Line
> length will cause the impedance seen by the radio to be
> different(assuming it's not a perfect 50 ohm "real" load) but IT DOES
> NOT CHANGE THE SWR. LET ME REPEAT, IT DOES NOT CHANGE THE SWR!!!!!!
.
> HEY HAMHEAD, ELECTRICAL 1/2 WAVES REPLICATE THE TRUE IMPEDANCE AT THE
FEEDPOINT. EVERYONE INVOLVED IN THIS THREAD HAS ALREADY AGREED TO THIS. I
AM NOT TRYING TO CHANGE THE SWR, JUST SEE WHAT THE TRUE LOAD IS AT THE
ANTENNA. THE COAX IS JUST A FUNNEL. WE ARE RADIATING OFF OF THE ANTENNA,
NOT THE COAX. THAT'S WHY I WANT TO SEE THE ACTUAL ANTENNA IMPEDANCE.
> All other things being equal, the only thing that will change when you
> change the line length is the loss associated with the line itself.
> These differences for the lenghts you keep mentioning are essentially 0.
>
> I won't belabor the math to prove this to you but if you insist, I'll
> dig out my Transmission Lines textbook from college and email you.
> TAKE YOUR NOSES OUT OF THE BOOKS AND DO SOME REAL LEARNING IN THE FIELD.
TAKE ONE REFERENCE MOBIL ANTENNA AND SEVERAL DIFFERENT LENGTHS OF COAX AND
MEASURE THE SWR AT OPPOSITE ENDS OF THE BAND WITH EACH PIECE. SEE WHAT
HAPPENS. SOME WILL GIVE DIFFERENT READINGS > This fact about line length
not affecting SWR(excluding line loss
> effects for long lengths) is common knowledge among anyone who has
> studied Transmission Line theory. KEY WORD HERE, THEORY. KINDA LIKE
THE EARTH IS FLAT THEORY>
SEAN FOLEY
i grow tired of this idle chatter. forget the resonant lengths of cable
and we'll go back to joe schmo's cb setup. tune a heliwhip on a vehicle
using 18' of mini 8 and check swr on ch. 1 & 40.. then recheck the swr
readings with 12' of mini 8 on 1 and 40. i will guarantee that there will
be a difference in readings. this is as simple as we can get. i will be
at the peotone (south chicago) hamfest on sunday. if anyone that has been
monitoring this thread will be there, please let me know. i would like to
have an outside source to verify some testing. we can do it at the hamfest
at your convenience. please continue down as i have also posted responses
to georges responses. Sean aka Bigfoot .
George Warner <warn...@loveboat.com> wrote in article
<warnergt-ya0240800...@news.ptd.net>...
> In article <01bc9d05$a5d79e60$74fe...@bigfoot.realworld.com>, "SEAN
FOLEY"
> <big...@xsite.net> wrote:
>
> > here are a few things that we all agree on so far.
> > (1) an electrical 1/2 wave reproduces a mirror image impedance of
> > what is on the other end.
> > (2) this exercise will find the
> > electrical half wave of the specific coax in question.
> > (3) 50 ohms represents an ideal antenna load, at least for
> > standard cb's.
>
> Yes.
>
> [snip]
> > ...[a half wavelength coax is] in effect electrically invisible.
> > granted, many (most) antennas are not exactly 50 ohms. this still
> > gives us the benefit of seeing exactly what impedance we do have
> > at the antenna without any or very little coaxial influence.
>
> It does let you see the exact impedance of the antenna but is
> that any benefit? The SWR was the same at every point in between.
> Why bother getting the exact impedance when it really doesn't
> matter? You are better off striving to minimize the coax length
> to reduce those losses.
> YOU KEEP AGREEING THAT THE 1/2 WAVE IS ELECTRICALLY INVISIBLE AND
INTRODUCES NO FALSE READINGS, YET YOU DENY WHAT YOU YOURSELF POSTED
EARLIER. YOU STATED THAT DUE TO DIFFERENT LENGTHS HAVING EITHER CAPACITIVE
OR INDUCTIVE REACTANCE, NONE OF THE OTHER LENGTHS WOULD WORK IN THAT TEST
CIRCUIT. BOTH CAPACATIVE AND INDUCTIVE REACTANCE CAN SUBSTANTIALLY
CHANGE THE STABILITY OR TUNING OF A CIRCUIT. IF ALL LENGTHS READ THE SAME
SWR AND NO LENGTHS CHANGE THE SWR READING, WHY WOULD ONLY THAT ONE WORK ?
IT'S BECAUSE THE RESISTOR WAS 50 OHMS AND THE RESONANT LENGTH INTRODUCED NO
RETUNING (THROUGH CAPACITIVE OR INDUCTIVE REACTANCE) . THEREFORE THE SWR
METER SEES 50 OHMS. AS FOR WHY GO THROUGH ALL THE HASSLE, IT'S BECAUSE I
CAN SEE THE DIFFERENCE. I WOULDN'T MAKE EXTRA WORK FOR MYSELF. IT MAKES
ME NO MORE MONEY OR GIVES ME NO MORE PERSONAL BENEFIT. IT GIVES RESULTS
FOR MY CUSTOMERS.
> > using this "matching stub"
> > as a feedline will allow us to track the impedance as we change
frequency
> > more accurately, as long as we are within the cables resonant window.
this
> > allows us to tweak the system for that added finesse that we wouldn't
know
> > it was capable of unless we looked directly at the feedpoint.
>
> There is no added finesse. The SWR at the coax imput tracks the
> antenna SWR directly regardless of coax length.
> YOU STATE THAT, YET YOU HAVE DONE NO TESTING FIRSTHAND. DO YOU EVEN OWN
AN SWR ANALYZER THAT IS CAPABLE OF GIVING YOU THIS DIRECT INFO ? > > this
can be
> > verified by comparing the impedance readings of the antenna using the
> > resonated cable and a typical 18' section. with the tuned length, as
the
> > impedance changes, you will see the swr curve follow it gently, just as
it
> > should. using the 18' piece, you will see the impedance curve taking
> > drastic swings while the swr stays relatively still.
>
> If you were to plot the coax input impedance and the antenna impedance
> on a Smith chart, you would find that they track identically. The only
> difference is a rotational offset. By restricting the coax length to
> one half wavelength, you merely make that offset to be one full rotation;
> thus, the two impedances overlay each other.OR IN OTHER WORDS, GIVE AN
IDENTICAL READING INTRODUCING NO FALSE READINGS.
>
> > how can an antenna
> > drastically change impedance and maintain a non reactive swr ? the
answer
> > is that it can't. but you wouldn't know that unless you were using an
> > electrically invisible length of coax to compare it to or unless you
> > measured the swr directly at the feedpoint.
>
> If you measure SWR (as most of us do), you don't care how the impedance
> changed because it hasn't changed the SWR. If you somehow measure the
> impedance instead of the SWR, you can simply plot the impedances on
> a Smith chart to visualize what is happening. No drastic change is
> occurring.
> YOU SAY THAT IMPEDANCE DOESN'T MATTER OR CHANGE THE SWR, YET WE KNOW THAT
2:1 SWR IS EITHER 25 OHMS OR 100 OHMS, AND THAT 3:1 IS 150 OHMS OR 12.5
OHMS. OBVIOUSLY, IMPEDANCE AND SWR GO HAND IN HAND. HOW CAN ONE CHANGE
AND THE OTHER STAY THE SAME. THERE ARE DRASTIC CHANGES IN IMPEDANCE, AND
IF YOU HAD THE RIGHT EQUIPMENT AND TOOLS, AND TOOK THE TIME TO VERIFY IT
YOURSELF, YOU WOULD REALIZE THAT THIS WORKS.
> > for most installations,
> > measuring at the feedpoint can be pretty difficult, but this alleviates
> > that problem. just as george demonstrated, other lengths of cable will
> > alter the reading due to capacitive or inductive reactance.
>
> You are comparing "most installations" to your "circuit." They are
> not the same.
> The reading was changing in your circuit because you were
> actually changing the SWR. I THOUGHT THAT COAX CAN'T CHANGE THE SWR OR
SWR READING, AS IT REMAINS CONSTANT ALONG THE LENGTH OF THE LINE. YOU
CAN'T SAY ONE THING ONE TIME AND ANOTHER THING WHEN IT IS CONVENIENT.
> Most installations will not see a change in reading due to changes
> in coax length because the SWR does not change.
>
> > this is where we see the benefit of resonating the cable.
>
> There is no benefit.
>
> [jive about coax hiding measurements snipped]
> > ...which then easily play games with an swr meter.
>
> No, the coax plays no games with the SWR meter. The SWR meter reading
> is correct.
>
> [jive about the necessity for an "invisible" length of coax snipped]
>
> All lengths of coax are "invisible" to SWR. No special length is
> required to get an accurate SWR reading.
> YET THE ONLY ONE THAT WORKED SHOWING THE TRUE 50 OHM IMPEDANCE OF THE
INLINE DUMMY LOAD WAS THE "INVISIBLE" 1/2 WAVE SECTION. OH, YES, I KEEP
FORGETTING. ALL LENGTHS ARE INVISIBLE. ALL OF THE OTHER VARIOUS READINGS
OF 3:1 MUST BE RIGHT ALSO, BECAUSE 50 OHMS DOESN'T MEAN A FLAT MATCH.
AFTER ALL, IMPEDANCE DOESN'T MATTER ONE BIT.
SEAN FOLEY
JON, I AM NOT CLAIMING THAT THE LENGTH WILL CREATE A PROBLEM. AS LONG AS
THE COAX IS IN GOOD CONDITION, IT WILL WORK FINE. WHAT I AM STATING IS
THAT DIFFERENT LENGTHS HAVE DIFFERENT LEVELS OF ACCURATELY READING THE SWR.
SEAN PS YOU HAVE ME BEAT BY 10 YEARS OF CBING. I WAS KAYE 3535 IN
1976. THANKS FOR PIPING IN.
Squeeze the Weeze
Obviously, You haven't done the Vector addition or you'd see It doesn't
mean SQUAT
to the IMPEDANCE, which is what you actually are looking at.
Remember, a dummy load will give you perfect SWR no matter where you put it
on the cable....
--
-skweezieweezie
` X o'
-----ooo-0-ooo-----
~
Jsender104
The idea of using multiple 1/2 wavelengths is useful when feeding
antennas of unusal impedances such as random length wires. this allows
the use of an antenna tunner to match impedance of the transmitter to the
antenna without the feedline having an effect. The impedance of the
antenna is repeated at the output of the tunner. In a system totally
designed for a standrad impedance(50 ohm) the use of multiple 1/2 wave
length feedlines has little effect. If it does there is something wrong
with your antenna / feedline.
One way of checking an antenna system wher impedance of all the parts are
standradized (50 ohms) is by inserting a !/4 wave section of feedline into
the system. If Swr changes appreciably then you have a problem.
Kelly Flowers
Sean:
I will resist the urge to flame in kind. Your original statement was
that SWR changed with line length. It DOES NOT. Then you went thru
your diatribe about determining the 1/2 wavelength of a cable. IT
DOESN'T MATTER!!! As George has pointed out, why do you need to know
the exact impedance of the antenna by using a 1/2 wavelength cable????
Seems like your backpeddling now. I noticed you did not respond to the
statement out of the handbook. Why? Do you choose to ignore the facts
when it is convenient?
Do you always resort to derogatory statements in your replies? Do you
always turn your replies into a pissing contest about your having bigger
this or more of that? That doesn't do much for your credibility even
though you clearly think it does. Also, owning the equipment doesn't
mean you know how it works, its limitations, and how to use it. Your
replies demonstrate this in spades.
I enjoyed George's reference to a Smith chart, although I'm quite sure
Sean you wouldn't know one if it were staring you in the face. If you
did, you'd realize how silly you really sound with your claims about SWR
changing with line length.
Let's face it Sean, you're wrong, period, end of story. Accept it and
move on. Take that pent up rage and anger toward hams and engineers and
focus it toward learning more about the subject matter. You'll be
better off and so will we!
Kelly Flowers P.E.
ARS NN8E
CB KFX5805 (since 1968)
Bill Eitner
Squeeze the Weeze wrote:
>
> Obviously, You haven't done the Vector addition or you'd see It doesn't
> mean SQUAT
> to the IMPEDANCE, which is what you actually are looking at.
>
> Remember, a dummy load will give you perfect SWR no matter where you put it
> on the cable....
You'd better correct that to "a dummy load will give you a
perfect SWR no matter what length of cable you put it across
the end of". Putting it in series with one of the conductors
and shorting the end with a pin is the test circuit Sean
introduced as an equivalent for the standard CB circuit. In
his circuit moving the location of the short along the cable
(in effect shortening or lengthening it) causes different SWR
readings at the radio end.
Squeeze the Weeze
<snip>
Sorry 'bout that one, Bill. Too busy weedin' thru the crap.
Now, if you'll excuse me, I think I hear another one of those
"old wives" calling on the local good old boy channel......
SEAN FOLEY
as i said, check the swr of one mobile antenna with several various lengths
of cable. use the standard 9', 12', 18' and a resonant length 1/2 wave.
use a random length just for kicks. if you don't have access to this, i
will send them to you. you can verify that they are good first and then
test this out. if you have an analyzer, please use this for the test. if
not, you will still see an swr change, although we know that the antenna
hasn't changed impedance or tuning. do this at both ends of the band, ch 1
and 40. if you get the same readings, you are the only man in the U.S.
that can. this is why i use the resonant 1/2 wave. JUST DO IT AND SEE
FIRST HAND. YOU NEED THE EXERCISE. YOU HAVE BEEN READING WAY TOO MANY
BOOKS AND PLAYING WITH THE COMPUTER TOO MUCH. forget the test theory. i
just want you to actually try SOMETHING instead of giving me rhetoric. you
can tell me what you think all day long, but go test it in the real world.
PRETTY, PRETTY PLEASE WITH SUGAR ON TOP, PLEASE TEST THIS . Sean
SEAN FOLEY
just because you have been doing this for a while, doesn't mean that your
doing it right (no offense, please). i too, have been doing this for a
while. i received my first radio in either 75 or 76. my call was KAYE
3535. so if how long we have been doing this counts, i must be correct,
right ? how can you pretune an antenna without it being mounted ? the
ground plane varies so drastically from vehicle to vehicle, that it may
never really be tuned at all. it would be ballpark at best, like a francis.
as for measuring at the feedpoint, that works the same as the 1/2 wave
stub. as you said, been there, done that. Sean
George Warner
In article <01bc9d6f$b4d4b1e0$72fe...@bigfoot.realworld.com>, "SEAN FOLEY"
<big...@xsite.net> wrote:
> i grow tired of this idle chatter.
Me, too. Give it up.
[snip]
> George Warner <warn...@loveboat.com> wrote in article
> <warnergt-ya0240800...@news.ptd.net>...
> > In article <01bc9d05$a5d79e60$74fe...@bigfoot.realworld.com>, "SEAN
> FOLEY"
> > <big...@xsite.net> wrote:
[snip]
> YOU KEEP AGREEING THAT THE 1/2 WAVE IS ELECTRICALLY INVISIBLE AND
> INTRODUCES NO FALSE READINGS, YET YOU DENY WHAT YOU YOURSELF POSTED
> EARLIER.
I do no such thing.
> YOU STATED THAT DUE TO DIFFERENT LENGTHS HAVING EITHER CAPACITIVE
> OR INDUCTIVE REACTANCE,
There is one important qualifier here...different lengths of coax WITH
ONE END SHORTED appear to be capacitive or inductive or short (1/2 wave)
or open (1/4 wave) depending on the length. That is what your circuit
uses; that is not what a typical CB installation uses.
> NONE OF THE OTHER LENGTHS WOULD WORK IN THAT TEST CIRCUIT.
Yes, because your test circuit requires that it be a short which only
occurs when your shorted coax is 1/2 wavelength long.
> BOTH CAPACATIVE AND INDUCTIVE REACTANCE CAN SUBSTANTIALLY
> CHANGE THE STABILITY OR TUNING OF A CIRCUIT. IF ALL LENGTHS READ THE SAME
> SWR AND NO LENGTHS CHANGE THE SWR READING, WHY WOULD ONLY THAT ONE WORK ?
I just told you...because your atypical circuit requires that the 50 ohm
resistor be given an RF short to ground in order for the transmitter to
see a good 50 ohm load.
> IT'S BECAUSE THE RESISTOR WAS 50 OHMS AND THE RESONANT LENGTH INTRODUCED NO
> RETUNING (THROUGH CAPACITIVE OR INDUCTIVE REACTANCE) . THEREFORE THE SWR
> METER SEES 50 OHMS. AS FOR WHY GO THROUGH ALL THE HASSLE, IT'S BECAUSE I
> CAN SEE THE DIFFERENCE. I WOULDN'T MAKE EXTRA WORK FOR MYSELF. IT MAKES
> ME NO MORE MONEY OR GIVES ME NO MORE PERSONAL BENEFIT. IT GIVES RESULTS
> FOR MY CUSTOMERS.
You are mistaken.
[snip]
> > If you were to plot the coax input impedance and the antenna impedance
> > on a Smith chart, you would find that they track identically. The only
> > difference is a rotational offset. By restricting the coax length to
> > one half wavelength, you merely make that offset to be one full rotation;
>
> > thus, the two impedances overlay each other.
> OR IN OTHER WORDS, GIVE AN
> IDENTICAL READING INTRODUCING NO FALSE READINGS.
Earth to Sean! No false SWR readings are introduced at any coax length!
Not in your atypical test circuit; not in any standard CB configuration.
> YOU SAY THAT IMPEDANCE DOESN'T MATTER OR CHANGE THE SWR, YET WE KNOW THAT
> 2:1 SWR IS EITHER 25 OHMS OR 100 OHMS,
There are an infinite number of impedances that give an SWR of 2:1.
You can find them on a Smith chart by plotting a circle of radius 0.333
in the center. (0.333 is the reflection coefficient for a 2:1 SWR).
By the way, these are all the impedances you go through as you vary
the coax length (and all are a 2:1 SWR). Here are some sample impedances:
25 ohms
40 -j30 ohms (note: some capacitive reactance but still 2:1 SWR)
100 ohms
40 +j30 ohms (note the symmetry of the circle centered on the Smith chart)
> AND THAT 3:1 IS 150 OHMS OR 12.5
> OHMS. OBVIOUSLY, IMPEDANCE AND SWR GO HAND IN HAND. HOW CAN ONE CHANGE
> AND THE OTHER STAY THE SAME.
Easy, I just showed you. That is the magic of how a transmission
line transforms impedances.
Actually, there is no magic to it and it is much simpler to understand
if you stop analyzing it in terms of changing impedance. The reason
for the apparent impedance is the interaction with the reflected
signal on the coax. As you change the coax length, you change the
phase of the reflected signal because you are changing the round-trip
time required for the reflected signal to appear at the input.
Let's look at it in simpler terms of forward and reflected power:
1) The forward power is fixed.
2) It travels down the coax to the load.
3) A mismatch at the load causes some percentage of the signal to
be reflected back up the coax.
The SWR is found from the ratio of the reflected power to the
forward power.
Now consider what happens when you change the coax length -- in
these terms basically nothing happens! The load has not changed.
The percentage of reflected signal did not change.
The reflected signal does take a different amount of time to
return to the input - but so what!? Who cares!?
This is exactly what we are arguing. You say the impedance
(the time for the reflected signal to appear at the input)
is changing. I say, "so what?" It does not change the SWR
or the SWR reading. It does not matter. There is no reason
to worry about. Forget about the changing impedance; just
think about the reflected power. That is what the standard
SWR meter measures.
[snip]
> > You are comparing "most installations" to your "circuit." They are
> > not the same.
> > The reading was changing in your circuit because you were
> > actually changing the SWR.
> I THOUGHT THAT COAX CAN'T CHANGE THE SWR OR
> SWR READING, AS IT REMAINS CONSTANT ALONG THE LENGTH OF THE LINE. YOU
> CAN'T SAY ONE THING ONE TIME AND ANOTHER THING WHEN IT IS CONVENIENT.
It is correct that coax cannot change the SWR or SWR reading. SWR
remains constant along a coax.
In your circuit, we are not measuring the SWR of your shorted coax;
we are measuring the SWR of a 50 ohm resistor in series with your
shorted coax.
The SWR along your coax is infinity; that is the SWR for coax that
is terminated with a short. Regardless of the length of coax in
your circuit, the SWR on your coax is always infinity.
Now let's look at your total circuit with the resistor. The SWR
seen at the input of your circuit is determined by the impedance
seen at the input of your coax which is dependent on its length.
[snip]
> YET THE ONLY ONE THAT WORKED SHOWING THE TRUE 50 OHM IMPEDANCE OF THE
> INLINE DUMMY LOAD WAS THE "INVISIBLE" 1/2 WAVE SECTION.
That is correct. The only way you will see 50 ohms (the resistor
value) is if your coax presents 0 ohms (which only occurs at
1/2 wavelength).
> OH, YES, I KEEP
> FORGETTING. ALL LENGTHS ARE INVISIBLE.
That is correct. All coax lengths have no effect on SWR.
Your circuit's coax has an SWR of infinity regardless of its
length. However, your overall circuit requires a length of
1/2 wavelength to present a 50 ohm load at the input.
> ALL OF THE OTHER VARIOUS READINGS
> OF 3:1 MUST BE RIGHT ALSO, BECAUSE 50 OHMS DOESN'T MEAN A FLAT MATCH.
Yes, if you read an SWR of 3:1; it is correct. That means that your
50 ohm resistor is in series with some other impedance created by
the shorted coax stub. The overall circuit impedance would be:
50 +j57.7 ohms or
50 -j57.7 ohms
depending on the length of the coax.
(These are the only two impedances that give a 3:1 SWR when the
resistive term is fixed at 50 ohms).
> AFTER ALL, IMPEDANCE DOESN'T MATTER ONE BIT.
Wrong. Impedance matters BUT A TRANSMISSION LINE WILL ONLY
TRANSFORM TO IMPEDANCES OF CONSTANT SWR. A transmission line will
transform the impedance but it will NOT transform it to an
impedance that gives a different SWR
Adding or subtracting coax does not change the percentage of
power that is reflected at the load. How could it?
Go back to my forward/reflected power analysis. It all adds up.
George Warner
warn...@loveboat.com
Kelly Flowers
George Warner wrote:
> lots of good stuff snipped
Excellent explanation George!!! Thanks for taking the time.
I particularly liked the reflected power phase change piece.
I hope Sean will take a moment and reflect on why his claims can't be
true and have no basis in fact. I have found over the years that many
people have a difficult time understanding how impedance can change but
SWR does not.
Too bad Smith charts aren't taught to a broader audience!
Kelly
Bill Harris
Kelly Flowers wrote:
>
> Sean:
>
> I will resist the urge to flame in kind. Your original statement was
> that SWR changed with line length. It DOES NOT. Then you went thru
> your diatribe about determining the 1/2 wavelength of a cable. IT
> DOESN'T MATTER!!! As George has pointed out, why do you need to know
> the exact impedance of the antenna by using a 1/2 wavelength cable????
> Seems like your backpeddling now. I noticed you did not respond to the
> statement out of the handbook. Why? Do you choose to ignore the facts
> when it is convenient?
Most cb'ers do.
Frank PoWell
SEAN FOLEY wrote:
>
> i have proven nothing ? once again, you guys have to say that "coax SHOULD
> have" instead of "coax DOES have". you are speaking out of your books and
> not first hand knowledge. there is no antenna in this test. it is just a
> dummy load serially feeding coax. the setup goes like this: radio
> is shorted at the end and various points until we can duplicate the same
> match as what the dummy load read by itself. if you would perform this
> simple test, you WILL see the proof. i can't change what you think, that's
> why i gave a hands on experience that YOU are in control of and could SEE
> FOR YOURSELF. forget that the world is flat and do some discovering. you
> will FIND OUT FOR YOURSELF if you do this simple test. YOU will know for
> sure one way or the other. once you have done this simple test with the
> dummy load and coax firsthand, then post some pearls of knowledge. step up
> that you can't think for yourselves ? are ALL of you sheep ? i don't mind
> being wrong, i just want someone else to make the same effort so that we
> can compare notes. i am not here to argue. i put up this posting so that
> you folks may learn the things that others have taught and shown me.
> .
>
>
>
>
> > Sean,
> George is right... you have proven nothing. A true coaxial cable should
> have
> > distributed circuit coefficients and in the case of your hook-up from
> > transmitter to antenna, should appear to be transparent to the system...
> > regardless of length.
> >
> >
> > --
> > The Professor
> > "WindowsNT Powered"
> >
> >
> >
> >
cable between an antenna and an AEA SWR 1 t 1 HF Analyzer, and
guess what???? As the length of the coaxial cable changed, so
did the SWR!!!!!!!
--
/\_/\ I never met a cat |\ /| | Frank PoWell - N7KSK -
( o.o ) I didn't like! O.O | lkil...@xmission.com
\ - / ___,,,^..^,,,___ =(___)= | FPO...@tcs.lmco.com
/ \ ftp://ftp.xmission.com/pub/users/l/lkilgore/sources.html
Bill Eitner
SEAN FOLEY wrote:
>
> just because you have been doing this for a while, doesn't mean that your
> doing it right (no offense, please).
It doesn't mean I'm doing it wrong either. In fact, it
probably means that I have been doing it right because
I've tried resonating coax and I've found that it in and
of itself doesn't increase field strength when all other
factors remain the same.
In CB field strength is God Almighty--it's stupid to waste
your time (and the customers money in the case of a paid
tech) doing anything that won't significantly increase it.
In my opinion, resonating coax is nothing but a voodoo
technique that causes no improvement in performance.
> i too, have been doing this for a
> while. i received my first radio in either 75 or 76. my call was KAYE
> 3535.
Although irrelevant, mine was KBTL-6695.
> so if how long we have been doing this counts, i must be correct,
> right ?
That's not true, but it sounds like your kind of logic. ;-)
> how can you pretune an antenna without it being mounted ?
They're mounted. I use a trailer that I built with
mounts for a tri-pod. I mount base antennas in it
using a mast and pre-tune them prior to installation.
In some cases I've tuned beams by pointing them straight
up with the reflector against the ground. Either way
I've found that as long the antenna's final mounting
location does not contain any obstructions to the
antenna's immediate field (that means no tree's, no
TV antennas, no anything for at least a quarter wave-
length) the SWR is very close to the pretuned value
after it's mounted in it's final location.
There are many cases where it's too dangerous to access
the feed point and the adjustable parts of a base antenna
due to it's height and the use of a mast rather than a
tower. In those cases you have to pre-tune the antenna.
I guess you could argue that you use a resonant length
of coax and keep taking the antenna down to make adjustments.
I've simply found that to be unnecessary (not to mention a
waste of time and risky) in all but the most unusual
installations.
> the
> ground plane varies so drastically from vehicle to vehicle, that it may
> never really be tuned at all.
In mobiles I access the feedpoint. With mag-mounts I simply
mount it and tune for lowest SWR at the radio end. Years ago
I used to add a jumper to the coax to make it resonant, but
I found that the difference in readings was never significant
and the difference in field strength was even less significant
even over large chunks of spectrum. In my experience SWR and
field strength track together regardless of whether the coax
is a resonant length or not. That's why I don't worry about
it. The results simply don't justify the change from a "use
the shortest possible length" philosophy to a "use only reso-
nant lengths" philosophy. If you doubt an SWR measurement
simply tune for highest field strength and leave it at that.
> it would be ballpark at best, like a francis.
> as for measuring at the feedpoint, that works the same as the 1/2 wave
> stub.
Exactly, that's why I test at the feedpoint. The difference
is you can use shorter coax runs and increase field strength
by not worrying about resonating the coax. And whether or
not one believes that coax influences SWR readings, you've
done what it takes to make the argument meaningless in either
case. That's what I like the most--if I test at the feedpoint
I don't need to concern myself with either side of this debate.
This debate has been going non-stop for as long as I've been
in radio. I decided years ago that the smartest thing to
do would be to incorporate testing and tuning techniques that
would keep me from having to choose a side. Pre-tuning,
measuring SWR at the feedpoint whenever possible, and putting
more effort into increasing field strength than lowering SWR
are the techniques that will keep you out of this never-ending
argument. When a customer comes to me asking about all of this
I teach him about these techniques rather than trying to push
one philosophy over another. In the long run I think that's
the best way to go.
> as you said, been there, done that.
Although you haven't transcended this worthless debate which
can easily be worked around regardless of ones stand on this
rather petty issue. You're here now--do it!
The CB Guy
In article <33E1BE...@flash.net>, Bill Harris <bill...@flash.net>
writes:
>Most cb'ers do.
As do many hams.......you're point is?
SEAN FOLEY
i have always had a problem with certain headers not appearing. i many
times have the responses but no original text. sometimes the original post
takes days to show up, long after i have already seen the responses posted.
i have contacted my server about this and they can't help me. i have also
gone into deja news looking for headers that i have missed, as the
responses were on topics that i would have liked to respond to. i think
that everybody that reads this newsgroup knows that i am not afraid to
tackle an issue. if someone can forward this specific post to me, i would
be glad to respond. Sean aka Bigfoot
Bill Nelson
Bill Eitner (kd6...@earthlink.net) wrote:
: even over large chunks of spectrum. In my experience SWR and
: field strength track together regardless of whether the coax
: is a resonant length or not. That's why I don't worry about
This is true if the antenna is 50 ohms resistive, at resonance. Many are
not.
: it. The results simply don't justify the change from a "use
: the shortest possible length" philosophy to a "use only reso-
: nant lengths" philosophy. If you doubt an SWR measurement
: simply tune for highest field strength and leave it at that.
I never worry about SWR - I always tune for maximum field strength.
As long as my radio can handle any resulting SWR, then the tuning is
as good as possible.
When the field strength is at a maximum, you would find that the
antenna is resonant. Whether it is 50 ohms resistive at that point
or 25 ohms makes essentially no difference.
: > it would be ballpark at best, like a francis.
: > as for measuring at the feedpoint, that works the same as the 1/2 wave
: > stub.
:
: Exactly, that's why I test at the feedpoint. The difference
: is you can use shorter coax runs and increase field strength
: by not worrying about resonating the coax. And whether or
Exactly.
--
Bill Nelson (bi...@peak.org)
Bill Eitner
Bill Nelson wrote:
>
> Bill Eitner (kd6...@earthlink.net) wrote:
>
> : even over large chunks of spectrum. In my experience SWR and
> : field strength track together regardless of whether the coax
> : is a resonant length or not. That's why I don't worry about
>
> This is true if the antenna is 50 ohms resistive, at resonance. Many are
> not.
>
> : it. The results simply don't justify the change from a "use
> : the shortest possible length" philosophy to a "use only reso-
> : nant lengths" philosophy. If you doubt an SWR measurement
> : simply tune for highest field strength and leave it at that.
>
> I never worry about SWR - I always tune for maximum field strength.
> As long as my radio can handle any resulting SWR, then the tuning is
> as good as possible.
of self-protection that lowers their output in the event
of high SWR (reflected power). With that in mind if the
SWR gets too high the field strength will be reduced due
to the lower level of transmitter output. That makes
high field strength with a high SWR impossible. That's
why I say SWR and field strength track together when
using modern equipment regardless of line length.
> When the field strength is at a maximum, you would find that the
> antenna is resonant. Whether it is 50 ohms resistive at that point
> or 25 ohms makes essentially no difference.
I have trouble agreeing with that unless you're assuming
the use of a tube output circuit. Remember, this is a
CB newsgroup. I think it's appropriate in this forum to
restrict our conclusions to what's applicable using typical,
modern, solid-state CB equipment.
> : > it would be ballpark at best, like a francis.
> : > as for measuring at the feedpoint, that works the same as the 1/2 wave
> : > stub.
> :
> : Exactly, that's why I test at the feedpoint. The difference
> : is you can use shorter coax runs and increase field strength
> : by not worrying about resonating the coax. And whether or
>
> Exactly.
>
> --
> Bill Nelson (bi...@peak.org)
--
-=[Bill Eitner]=-
Amor Powers
SEAN FOLEY spewed forth:
> please hook this circuit up exactly as
> described and see for yourself.
> theory and hands on learning are sometimes
> different.
Ok, Sean, I took your challenge and tried your experiment today.
1. Antenna: 27.000 MHz 1/4 wave Ground Plane with 3 radials on a 16ft
self supporting mast. Center of coax directly to whip, shield of coax to
junction of radials. Adjusted radial angles (approx 45 degrees) for 50
ohm resistive match at 27.000 MHz resonance.
Used a screw-on PL259, and type N adapter, started with 100 ft of RG-58
(cutting at random intervals). I also made up 4 resonant lengths, 3/4
wave, 1.25 wave, 1.5 wave(three half waves), 2.5 wave (five half
waves).
SWR curve measured over frequency sweep 25 to 29 MHz at 1601points.
Each time a new coax length was tried, SWR curve was stored on disk.
My HP 8752A Network Analyzer says:
SWR remains the same whether coax is resonant 1/2 waves (odd integers)
or any other coax length.
27.000 MHz impedance is 50 ohms (close to resistive) is right in the
middle of the Smith Chart, SWR 1.02:1 and stays there with all coax
lengths...
The SWR at 27.500 MHz is 1.27:1 and stays the same regardless of coax
length.
However: the measured impedance at 27.500 MHz varied widely above and
below 50 ohms, but never got any closer to the center of the Smith Chart
at 50 ohms resistive. It was a reactive match at some coax lengths. But
the SWR was rock solid at 1.27:1
Next, I de-tuned the resonance by shortening the antenna to 28.200 MHz.
This caused a reactive mis-match at 27.000 MHz.
Same result.
Using the 28.200 MHz antenna... If I add a lot more coax, (a 100 ft
length) the reflected power measured at 27.000 gets better! :-)
But this is due solely to loss in the coax (heat).
Just goes to show: better SWR does not always mean more radiated power.
> i'm sure that most of you have seen rf
> do things that are technically impossible.
Actually, I haven't so far, but my mind is open. (probably due to being
an RF Engineer)
May I respectfully suggest you continue your studies, Sean, expanding on
your knowledge, and delve into the difference between impedance and
reactance. You seem pretty sharp, keep up the work. Thanks for starting
a good thread. I like experiments, and double-dog-dares.
> P.S. the fat lady isn't singing yet.
Do-re-me-fa-so-la-ti-do!
I'm starting my finale (but I lost 20 pounds since I've been dieting).
> keep the crow on the back burner for whomever gets
> to eat it.
> nomatter how long it cooks, it still tastes the same.
Hope you enjoy the main course.
It's been roasting on my linear. ?:-)
Sincerely,
Amor Powers
aka Big Bertha
aka Kim, Unit 224 (West Coast Portable)
One of The-Powers-That-Be
Lots of hot air
Thanks go to my son, aka Tower of Power, who helped with the soldering
and antenna support.
Equipment used:
HP 8752A Network Analyzer (Hewlett Packing Company)
calibration traceable to NIST
keywords:
Coax Length Network Analyzer CB Radio RF linear amplifier eating crow
resonance antenna reactive mis-match 27 MHz Coax Length Network Analyzer
CB Radio RF linear amplifier eating crow
____________________________________________________
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CB Radio RF linear amplifier eating crow
resonance antenna reactive mis-match 27 MHz Coax Length Network Analyzer
CB Radio RF linear amplifier eating crow
resonance antenna reactive mis-match 27 MHz Coax Length Network Analyzer
CB Radio RF linear amplifier eating crow
resonance antenna reactive mis-match 27 MHz Coax Length Network Analyzer
CB Radio RF linear amplifier eating crow
resonance antenna reactive mis-match 27 MHz Coax Length Network Analyzer
CB Radio RF linear amplifier eating crow
resonance antenna reactive mis-match 27 MHz
Bill Eitner
You need to re-examine the test circuit. In Sean's circuit
you need to insert a 50 ohm, non-inductive resistor in series
with the center conductor. Then you need to terminate the coax
with a pin (short). As you change the location of the termination
short the SWR should change. Sean's original test set out to
prove that the radio-end SWR would change when moving a short
along a piece of coax when there was a 50 ohm, non-inductive
resistor placed in series with the center conductor. His
test also set out to prove that when the coax length from
resistor to short equaled a half-wavelength (adjusted for
velocity factor) the SWR would be 1:1.
I think you, your attitude, your network analyser, and all the
bullshit at the end of your posts should be able to verify
those results--don't you?
Good luck.
Bill
Amor Powers
SEAN FOLEY spewed forth:
> in order to to present a formidable case for
> using electrically correct lengths of coaxial cable,
THE FORMIDABLE CASE FOR COAX LENGTHS?
What exactly do you intend to use the correct electrical lengths of
coaxial cable for in the formidable case?
Or are you just varying the length of your coaxial feedline from the
antenna to the transmitter?
Or just having a joke with the gang? :-)
THE REAL ISSUES (CUT TO THE CHASE)
The real transmitting issues of importance to many CBers are:
Maximum Effective Radiated Power!
or Best Match Between Antenna/Coax/Transmitter
(regardless of tricking the SWR and Power meters)
THE RIGHT TOOL FOR MEASUREMENT OF SWR
Any measurement of this sort can easily be made by a Network Analyzer
such as the common Hewlett Packing Company units. I have one in my lab
that I use all the time (300kHz to 1.3GHz). It's great for looking at CB
Antenna systems (slight overkill). It displays Smith Chart format s11
reflected phase and magnitude and rectangular format SWR vs Frequency.
Common SWR or Antenna Analyzers using diode detectors (I have several
brands and types) are often not conclusive or downright wrong for
accurate measurement of mismatch vectors.
ANTENNA TO FEEDLINE MATCH IS WAY MORE IMPORTANT
If the antenna presents a good match to the characteristic impedance of
the line, i.e., 50 ohms for CB, the impedance presented at any point on
the coax where you cut it and put a connector on it will be 50 ohms.
That means a low SWR. Assuming your transmitter wants to see a 50 ohm
load for maximum power transfer, there is no benefit for any specific
length, other than the shortest needed from the antenna location to the
transmitter location.
THE CB COAX LINE LENGTH LEGEND (BURN ME AT THE STAKE!)
If the antenna cannot be made to be close to a 50 ohm match, it is
helpful to know what the complex impedance actually is. Adding or
subtracting 50 ohm coax feedline length will do nothing to improve the
antenna-to-feedline match, and some power will be lost to heat, instead
of radiated by the antenna. The mismatch will be the same (the length of
the vector from the center of the Smith Chart), whether it is rotated
around the chart in phase by more or less coax or resonant lengths or
not. You can cut coax lengths until you are blue in the face. The real
problem is getting the antenna to match the 50 ohm impedance of the
coax.
COAX LENGTHS AS MATCHING DEVICES
Various coaxial sections of different characteristic impedance (not 50
ohm coax) in series or as parallel stubs (including 50 ohm coax), can be
used to make matching section transformers in the event that the load
(antenna feedpoint) impedance cannot be made to be 50 ohms. It is
imperative that this matching be done at the antenna feedpoint, to match
it to the 50 ohm feedline. By the time the mismatch gets to the
transmitter end of the coax, it's too late to have a beneficial effect
on the antenna to feedline mismatch.
IN LAYPERSON'S TERMS
The Smith Chart is the common means of prediction of the effects of
coax line length. A Network Analyzer is the correct tool to measure it
in the field. But this stuff is common RF knowledge. You already know
all that. I'm just explaining it in the layperson's terms so everyone
else will follow our discourse.
Amor Powers
One of The-Powers-That-Be
Lots of hot air...
2KW Mobile: Waste more in reflected power and heat
than most stations use forward.
keywords:
Smith Chart transmission line length coaxial cable Network Analyzer RF
CB Radio Antenna SWR Smith Chart transmission line length coaxial cable
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length coaxial cable Network Analyzer RF CB Radio Antenna SWR Smith
Chart transmission line length coaxial cable Network Analyzer RF CB
Radio Antenna SWR Smith Chart transmission line length coaxial cable
Network Analyzer RF CB Radio Antenna SWR
Bill Eitner
Amor Powers wrote:
> However, the SWR curve over frequency is unchanged by the resonant coax
> length. Some SWR meters can be slightly fooled by various reactive
> loads. Maybe Sean's meter or "antenna analyzer" is one of those. The
> common SWR meter is really not a true VSWR meter. It is just a
> reflectometer. A directional coupler with diode detectors. It's a pretty
> good instrument for the general purpose, but not the be-all end-all for
> deriving match information.
This part says it all. Other regulars to this group don't
believe that SWR meters can be fooled whatsoever. I'm glad
someone with an RF engineering background finally verified
this inaccuracy that many SWR meters suffer from.
The next timeless question that needs answering is: will
using a resonant length of coax cancel the reactance and
cause a more correct SWR reading when using a meter that
is effected by reactive loads?
Amor Powers
Bill Eitner wrote:
>
> You need to re-examine the test circuit. In Sean's circuit
> you need to insert a 50 ohm, non-inductive resistor in series
> with the center conductor.
That is just Sean's method for finding resonant lengths of coax.
Sean's method for doing this works correctly. (So does the formula:
velocity factor X wavelength= coax length) Or just hook it up to the
Network Analyzer reflected polar mode, and pin it or cut it to resonance
at the desired frequency.
I believe he tried to take it a step further by pointing out that the
antenna impedance repeats itself at the end of these resonant lengths of
coax. Sean is correct about this also.
But then I believe he drew the conclusion that the same impedance
repeated at the end of the coax equates to the ability to effect or
measure match at the antenna feedpoint in some extraordinarily special
manner while sitting in the easy chair in the shack... At the TX end of
the coax (without a remote directional coupler!) [Am I correct about
this, Sean?]
However, the SWR curve over frequency is unchanged by the resonant coax
length. Some SWR meters can be slightly fooled by various reactive
loads. Maybe Sean's meter or "antenna analyzer" is one of those. The
common SWR meter is really not a true VSWR meter. It is just a
reflectometer. A directional coupler with diode detectors. It's a pretty
good instrument for the general purpose, but not the be-all end-all for
deriving match information. 30 to 40 db of directivity is nice for the
directional coupler in a Network Analyzer, used with a tracking phase
and magnitude receiver.
Sean just forgot to figure in *reactance* to the match vectors. The
reactive part of the mismatch is just as important as the resistive
part.
Don't get too down on good ol' Sean.
This is a common error, repeated over the centuries by many a sage
thinker; most not nearly as bright as Sean.
Sincerely,
Amor
Amor Powers
aka Big Bertha
aka Kim, Unit 224 (West Coast Portable)
One of The-Powers-That-Be
Lots of hot air
SEAN FOLEY
Amor Powers <po...@bootlegger.net> wrote in article
<33E5C0...@bootlegger.net>...
> Bill Eitner wrote:
> >
> > You need to re-examine the test circuit. In Sean's circuit
> > you need to insert a 50 ohm, non-inductive resistor in series
> > with the center conductor.
>
> That is just Sean's method for finding resonant lengths of coax.
> Sean's method for doing this works correctly. (So does the formula:
> velocity factor X wavelength= coax length) MY TEST ACTUALLY
GIVES YOU THE REAL LENGTH AS OPPOSED TO THE THEORETICAL LENGTH. AS STATED
EARLIER, I HAVE FOUND PRODUCTION VARIANCES ON THE SAME SPOOL OF CABLE.
THIS WILL GIVE YOU AN EXACT READING FOR YOUR SPECIFIC CABLE.
Or just hook it up to the
> Network Analyzer reflected polar mode, and pin it or cut it to resonance
> at the desired frequency.
>
> I believe he tried to take it a step further by pointing out that the
> antenna impedance repeats itself at the end of these resonant lengths of
> coax. Sean is correct about this also.
> MIRROR IMAGE READING FROM ONE END TO THE OTHER. NO ERRORS INTRODUCED
THROUGH THE CABLE ITSELF, OTHER THAN LOSS.
> But then I believe he drew the conclusion that the same impedance
> repeated at the end of the coax equates to the ability to effect or
> measure match at the antenna feedpoint in some extraordinarily special
> manner while sitting in the easy chair in the shack... At the TX end of
> the coax (without a remote directional coupler!) [Am I correct about
> this, Sean?]
> MY CONCLUSIONS HAVE BEEN THAT BY MEASURING THE FEEDPOINT IMPEDANCE VIA
RESONATED LENGTHS OF CABLE HELPS TO SEE THE EXACT LOAD THAT THE RADIO SEES.
THE COAX SHOULD BE APPR. 50 OHMS RADIATION RESISTANCE REGARDLESS OF
LENGTH. BY TRYING TO KEEP OUR ANTENNA AT APPR. 50 OHMS, WE SHOULD HAVE
GREATER POWER TRANSFER. THIS HAS SHOWN ME GREAT RESULTS IN REAL LIFE. AS
FOR THE DIRECTIONAL COUPLER, I DO HAVE BIRDS, BUT MOST CBER'S (LET ALONE
HAMS) DON'T. AS FOR THE RESULTS USING MY SWR ANALYZER AS COMPARED TO A
NETWORK ANALYZER, THE RESULTS HAVE BEEN VERY CLOSE. CONSIDERING THE PRICE
DIFFERENCE, THEY ARE DAMN GOOD. > However, the SWR curve over
frequency is unchanged by the resonant coax
> length. Some SWR meters can be slightly fooled by various reactive
> loads. Maybe Sean's meter or "antenna analyzer" is one of those.
THIS IS EXACTLY MY POINT. A
TYPICAL USERS METER IS NOT A PRECISION INSTRUMENT. BY USING THE RESONATED
LINE, IT TAKES AS MANY VARIABLES OUT OF THE CIRCUIT AS POSSIBLE. THIS
MINIMIZES POSSIBLE PROBLEMS THAT CAN BE A DIRECT LINK BACK TO THE TYPICAL
INACCURACIES OF THE METERS IN THE $20 TO $200 RANGE.
The
> common SWR meter is really not a true VSWR meter. It is just a
> reflectometer. A directional coupler with diode detectors. It's a pretty
> good instrument for the general purpose, but not the be-all end-all for
> deriving match information.
UNFORTUNATELY, THE SWR METER MAY BE ALL THAT THEY HAVE TO WORK WITH. A
COMMON THING THAT I SEE USING RANDOM LENGTHS IS HOW DIFFERENTLY THE RADIO
LOADS INTO THE ANTENNA. THE SUBJECT RADIO PRODUCES THE SAME CARRIER ON CH
1 AND CH 40 INTO A DUMMY LOAD. THE "S/RF" METER IS PRESET TO PRODUCE S9 ON
TX. WHEN WE TAKE THIS AND PUT IT ON AN ANTENNA THAT HAS BEEN TUNED WITH
RANDOM LENGTHS, THE S METER CAN SHOW VERY DIFFERENT LEVELS UPON TX'ING ON 1
AND 40. THE AMOUNT OF SIGNAL REQUIRED TO CALIBRATE THE FORWARD MOTION OF
THE METER ALSO VARIES FROM CH 1 TO 40. WHILE THE SWR MAY SHOW BALANCED ON
BOTH CHANNELS ACCORDING TO OUR SWR METER, THE RADIO ISN'T LOADING EVENLY
ACROSS THE BAND. BY TUNING THE ANTENNA WITH THE AID OF RESONATED CABLE,
THE S METER APPROXIMATES THE SAME LOAD AS THE DUMMY LOAD WHEN TX'ING (S9).
THE LEVELS REQUIRED TO CALIBRATE THE METER ARE ALSO ALMOST EXACTLY THE SAME
ON 1 AND 40 ONCE RESONANCE HAS BEEN ACHIEVED. THIS TELLS ME THAT THE
ANTENNA LOAD IS MORE LINEAR ACROSS THE BAND THAN THE SAME ANTENNA TUNED
WITH A RANDOM LENGTH. PERFORMANCE WILL ALSO BE BETTER BALANCED THROUGHOUT
THE BAND..
0 to 40 db of
directivity is nice for the
> directional coupler in a Network Analyzer, used with a tracking phase
> and magnitude receiver.
>
> Sean just forgot to figure in *reactance* to the match vectors. The
> reactive part of the mismatch is just as important as the resistive
> part.
> THIS MAY BE TRUE. I DON'T HAVE A NETWORK ANALYZER TO DO THIS STUFF. I
HAVE COME UP WITH MOST OF THIS BY READING ARRL MATERIALS AND VERY WELL
KNOWN AND ACCEPTED AUTHORS. WHAT I HAVE POSTED HAS BEEN MY EXPERIENCE
FIRST HAND THROUGH YEARS OF EXPERIMENTING AND READING. TO READ IS
EXCELLENT, AS HOPEFULLY IT WILL EDUCATE AND CHALLENGE YOU. I TOOK IT UPON
MYSELF TO VERIFY MUCH OF WHAT I READ TO SEE FOR MYSELF. DON'T TAKE SOMEONE
ELSE'S WORD FOR IT, AS MOST ARE ONLY REPEATING WHAT THEY HAVE BEEN TOLD.
THAT IS WHY I WANTED OTHERS TO PERFORM THE SIMPLE TEST AND "SEE FOR
YOURSELF". I ALSO WANTED OTHERS TO USE ONE MOBILE ANTENNA AS A REFERENCE
AND TRY VARIOUS LENGTHS OF CABLE, DOCUMENTING THE DIFFERENCES IN READINGS,
IF ANY. MORE TIMES THAN NOT, THEY WOULD SEE A DIFFERENT READING IF USING
COMMON SWR METERS. FEW HAVE TRIED THIS, OTHER THAN YOURSELF, AND THIS WAS
NOT USING A TYPICAL SWR METER. YOU HAVE LAB QUALITY EQUIPMENT AND
THEREFORE OBTAIN MORE ACCURATE RESULTS. UNFORTUNATELY, THE COST EFFECTIVE
METHOD OF MASS PRODUCTION AND MAKING THINGS MARKETABLE TO THE GENERAL
PUBLIC RESULTS IN LESS PRECISE EQUIPMENT WITH MORE ROOM FOR ERROR. MY
SUGGESTIONS TAKE THIS INTO ACCOUNT AND HELPS TO GIVE THE MOST ACCURATE
READINGS WITH A LIMITED BUDGET. IF YOU WENT THROUGH ALL OF THE TROUBLE TO
DO THIS USING A BASE SETUP, WHICH IS WAY MORE WORK, TRY MY SUGGESTION
USING A MOBILE ANTENNA THAT IS ELECTRICALLY SHORTENED THROUGH LOADING
COILS. TRY IT USING A COMMON CB/HAM TYPE SWR METER, LIKE A DOSY, RAT SHACK
OR BLACK CAT FOR EXAMPLE. I'M SURE THAT YOUR RESULTS WILL MIRROR MINE.
TUNE THE ANTENNA WITH A RANDOM LENGTH AND THEN COMPARE THE OTHERS. DO NOT
USE THE NETWORK ANALYZER TO FIRST TUNE THE ANTENNA, AS THIS IS NOT A COMMON
OR TYPICAL THING AND WOULD SKEW THE RESULTS FOR THE AVERAGE JOE.
> Don't get too down on good ol' Sean.
> This is a common error, repeated over the centuries by many a sage
> thinker; most not nearly as bright as Sean.
> I'LL HAVE TO TAKE THIS AS A COMPLIMENT. THANK YOU. SEAN AKA BIGFOOT
> Sincerely,
> Amor
>
Amor Powers
Bill Eitner spewed forth:
> The next timeless question that needs answering is: will
> using a resonant length of coax cancel the reactance and
> cause a more correct SWR reading when using a meter that
> is effected by reactive loads?
The answer, Bill, is: no and maybe.
Are we are getting into the statistical
prediction area of RF Engineering,
aka BMCA (Bastardized Monte Carlo Analysis)?
Roll the dice.
Most antenna loads by nature, are reactive at some desired frequencies
within the band of interest. Adding or subtracting coax length will
just rotate the phase of the various frequencies' match points, some
frequencies may become reactive loads at some lengths, moving other
frequencies to become more resistive, but there is no magic length which
will "cancel" the reactance over the entire band, making all cheap
reflectometers (SWR meters) work more accurately like Network Analyzers.
Phase of the signal cannot be measured on an SWR meter. Only magnitude.
So, rotating the phase (changing coax length) will do nothing for our
SWR measurement accuracy (unless the feedline is radiating a lot, which
is a real can of worms). However, with a Network Analyzer, phase can be
measured, so a known length of feedline can be used to some advantage!
Let's put the SWR meter accuracy thing in perspective, and use it for
what it is made for:
The important thing to use an SWR meter for is to determine the resonant
frequency of the antenna, and tune the antenna to resonance at the
desired frequency. Then, if the SWR is still high at resonance, the
antenna feedpoint impedance is suspect. Since antenna feedpoint
impedance matching adjustments should be made at the resonant
frequency, small amounts of reactive inaccuracy of the meter at other
frequencies is not a big issue.
The antenna will perform best when the resonance and best match in the
middle frequency in the desired band is achieved. This is from CB Course
101.
In standard CB antenna systems, such as groundplanes, beams, and whips,
antenna bandwidth cannot be changed by varying feedline length. (There
are some kinds of antennas [used by hams and other strange beings] which
can be built using radiating feedlines.)
For amusement, we should let the coax battle continue for at least
another few months, don't you think, Bill?
See the new thread "The Legend of the Great Coax Cutter".
Amor Powers
One of The-Powers-That-Be
Lots of hot air
END OF POST
___________________________________________________
Keywords: (viewing not advised for younger audiences)
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Bill Eitner
feedpoint SWR testing is the method of choice if one
is looking for the most accurate SWR data for antenna
tuning purposes when all that's available is a common
SWR meter. Do you agree?
> For amusement, we should let the coax battle continue for at least
> another few months, don't you think, Bill?
Personally, I'm sick of it. I've been through it 3
times in 10 months on this newsgroup. It gets old
quick when no one believes what anyone else says.
That's why I'm asking you specific questions.
Perhaps the majority out here will recognize your
credentials as adequate and your answers as accurate.
> See the new thread "The Legend of the Great Coax Cutter".
>
Also, lose the "spewed forth" at the beginning of
quoted text. Remember the "do onto others" adage.
In other words, would you want that kind of crap
added to your posts?
George Warner
> But this is due solely to loss in the coax (heat).
> Just goes to show: better SWR does not always mean more radiated power.
>
> > i'm sure that most of you have seen rf
> > do things that are technically impossible.
>
> Actually, I haven't so far, but my mind is open. (probably due to being
> an RF Engineer)
> May I respectfully suggest you continue your studies, Sean, expanding on
> your knowledge, and delve into the difference between impedance and
> reactance. You seem pretty sharp, keep up the work. Thanks for starting
> a good thread. I like experiments, and double-dog-dares.
>
> > P.S. the fat lady isn't singing yet.
>
> Do-re-me-fa-so-la-ti-do!
> I'm starting my finale (but I lost 20 pounds since I've been dieting).
>
> > keep the crow on the back burner for whomever gets
> > to eat it.
> > nomatter how long it cooks, it still tastes the same.
>
> Hope you enjoy the main course.
> It's been roasting on my linear. ?:-)
>
> Sincerely,
> Amor Powers
> aka Big Bertha
> aka Kim, Unit 224 (West Coast Portable)
>
> One of The-Powers-That-Be
> Lots of hot air
>
> Thanks go to my son, aka Tower of Power, who helped with the soldering
> and antenna support.
> Equipment used:
> HP 8752A Network Analyzer (Hewlett Packing Company)
> calibration traceable to NIST
Hey, Amor. Thanks for the good, relevant post. What took you
so long? You could have helped save me a lot of typing.
George Warner
warn...@loveboat.com
sergio prestipino
> 1. Antenna: 27.000 MHz 1/4 wave Ground Plane with 3 radials on a 16ft
> self supporting mast. Center of coax directly to whip, shield of coax to
> junction of radials. Adjusted radial angles (approx 45 degrees) for 50
> ohm resistive match at 27.000 MHz resonance.
> Used a screw-on PL259, and type N adapter, started with 100 ft of RG-58
> (cutting at random intervals). I also made up 4 resonant lengths, 3/4
> wave, 1.25 wave, 1.5 wave(three half waves), 2.5 wave (five half
> waves).
>
> SWR curve measured over frequency sweep 25 to 29 MHz at 1601points.
> Each time a new coax length was tried, SWR curve was stored on disk.
>
> My HP 8752A Network Analyzer says:
> SWR remains the same whether coax is resonant 1/2 waves (odd integers)
> or any other coax length.
>
Mr. Powers,
In reference to some other thread on coax length, I was in a cb
shop, and
read an interesting note on co-phasing. they recommend installing the
antannae,
and tuning seperately with 18ft. of RG58 (50ohm)for min SWR, on each
antenna,
and then connecting your rg59 (75ohm) 18ft coax rig.
Also, why use 75ohm cable, isn't there a better way, with a matching
transformer, and two 50ohm sections of cable? Why 18ft????
PS. I read this on the back package of a Wilson 2000.
Regards... ... Sugar Bear
George Warner
In article <01bca0f1$04af6fa0$65fe...@bigfoot.realworld.com>, "SEAN FOLEY"
<big...@xsite.net> wrote:
You concluded that you see the same impedance with a half
wavelength of coax. While this is correct, you use this fact to
somehow claim that 1/2 wavelength of coax is required to see the
correct SWR in your circuit. The fact is that this is the only
length that gives a 1:1 SWR in your circuit. All other non-1:1
SWRs are correct; that is what you get when your coax is not
1/2 wavelength long.
> THE COAX SHOULD BE APPR. 50 OHMS RADIATION RESISTANCE REGARDLESS OF
> LENGTH. BY TRYING TO KEEP OUR ANTENNA AT APPR. 50 OHMS, WE SHOULD HAVE
> GREATER POWER TRANSFER. THIS HAS SHOWN ME GREAT RESULTS IN REAL LIFE. AS
> FOR THE DIRECTIONAL COUPLER, I DO HAVE BIRDS, BUT MOST CBER'S (LET ALONE
> HAMS) DON'T. AS FOR THE RESULTS USING MY SWR ANALYZER AS COMPARED TO A
> NETWORK ANALYZER, THE RESULTS HAVE BEEN VERY CLOSE. CONSIDERING THE PRICE
> DIFFERENCE, THEY ARE DAMN GOOD.
By the way, I can't prove this but I'll go ahead and say it anyway.
I believe that there is a myth surrounding these Bird watt meters.
I use them and I like them but I'm not convinced that they are
much more accurate than other inexpensive watt meters.
They are well built but are they any more accurate? I'm not so sure.
This may be a good topic for another thread.
> > However, the SWR curve over
> > frequency is unchanged by the resonant coax
> > length. Some SWR meters can be slightly fooled by various reactive
> > loads. Maybe Sean's meter or "antenna analyzer" is one of those.
> THIS IS EXACTLY MY POINT. A
> TYPICAL USERS METER IS NOT A PRECISION INSTRUMENT. BY USING THE RESONATED
> LINE, IT TAKES AS MANY VARIABLES OUT OF THE CIRCUIT AS POSSIBLE. THIS
> MINIMIZES POSSIBLE PROBLEMS THAT CAN BE A DIRECT LINK BACK TO THE TYPICAL
> INACCURACIES OF THE METERS IN THE $20 TO $200 RANGE.
Amor's key word was "slightly" but I won't even make it an issue.
The problem is that your "point" still doesn't hold up here.
Even if an SWR meter were fooled ("slightly" or "greatly") by
reactive loads, a given length of coax does not necessarily
remove that reactance. In other words, what Amor is saying is
that you can put your SWR meter right at the antenna and it may
see some error due to the reactance of the antenna impedance.
A half wavelength of coax will do nothing to improve this reading
or make it maximally accurate.
> > The
> > common SWR meter is really not a true VSWR meter. It is just a
> > reflectometer. A directional coupler with diode detectors. It's a pretty
> > good instrument for the general purpose, but not the be-all end-all for
> > deriving match information.
I'm not sure how it is not a "true VSWR meter." What do you mean
"just a reflectometer?" Yes, it is a "directional coupler with diode
detectors," but what else do you need to get a relatively accurate
SWR reading?
These common SWR meters are really not a whole lot different than
a network analyzer when it comes to measuring SWR. Both contain
directional couplers with a means to measure the signal levels.
The network analyzer contains additional circuitry and computational
ability to sweep frequencies and measure phase but this is not
necessary for a simple SWR measurement at a given frequency.
> UNFORTUNATELY, THE SWR METER MAY BE ALL THAT THEY HAVE TO WORK WITH.
There is nothing unfortunate about it. It is all you really need.
> A COMMON THING THAT I SEE USING RANDOM LENGTHS IS HOW DIFFERENTLY THE RADIO
> LOADS INTO THE ANTENNA. THE SUBJECT RADIO PRODUCES THE SAME CARRIER ON CH
> 1 AND CH 40 INTO A DUMMY LOAD. THE "S/RF" METER IS PRESET TO PRODUCE S9 ON
> TX. WHEN WE TAKE THIS AND PUT IT ON AN ANTENNA THAT HAS BEEN TUNED WITH
> RANDOM LENGTHS, THE S METER CAN SHOW VERY DIFFERENT LEVELS UPON TX'ING ON 1
> AND 40. THE AMOUNT OF SIGNAL REQUIRED TO CALIBRATE THE FORWARD MOTION OF
> THE METER ALSO VARIES FROM CH 1 TO 40. WHILE THE SWR MAY SHOW BALANCED ON
> BOTH CHANNELS ACCORDING TO OUR SWR METER, THE RADIO ISN'T LOADING EVENLY
> ACROSS THE BAND. BY TUNING THE ANTENNA WITH THE AID OF RESONATED CABLE,
> THE S METER APPROXIMATES THE SAME LOAD AS THE DUMMY LOAD WHEN TX'ING (S9).
> THE LEVELS REQUIRED TO CALIBRATE THE METER ARE ALSO ALMOST EXACTLY THE SAME
> ON 1 AND 40 ONCE RESONANCE HAS BEEN ACHIEVED. THIS TELLS ME THAT THE
> ANTENNA LOAD IS MORE LINEAR ACROSS THE BAND THAN THE SAME ANTENNA TUNED
> WITH A RANDOM LENGTH. PERFORMANCE WILL ALSO BE BETTER BALANCED THROUGHOUT
> THE BAND..
Don't read into these "S/RF" meter readings too deeply.
There is not much that you can conclude from the CB "S/RF" meter
reading on transmit. These meters typically work off of a simple
diode detector circuit. There is no directional coupler used here;
thus, these are basically voltmeters and these readings are sensitive
to the impedance seen at the input to the coax. High impedances will
show high voltage readings and low impedances will show low voltage
readings.
While a half wavelength of coax will present the antenna impedance
at its input, this RF meter reading is still a relative reading
without a whole lot of meaning. A reading that varies across channels
could be an indication of how your radio is tuned or how your antenna
is tuned. Even your "resonant" coax is really only resonant at one
frequency, so even it is a factor in this reading.
George Warner
warn...@loveboat.com
The2X4
In article <warnergt-ya0240800...@news.ptd.net>,
warn...@loveboat.com (George Warner) writes:
The Lioness wrote:
>> > The
>> > common SWR meter is really not a true VSWR meter. It is just a
>> > reflectometer. A directional coupler with diode detectors. It's a
pretty
>> > good instrument for the general purpose, but not the be-all end-all
for
>> > deriving match information.
>
>I'm not sure how it is not a "true VSWR meter." What do you mean
>"just a reflectometer?" Yes, it is a "directional coupler with diode
>detectors," but what else do you need to get a relatively accurate
>SWR reading?
>
>These common SWR meters are really not a whole lot different than
>a network analyzer when it comes to measuring SWR. Both contain
>directional couplers with a means to measure the signal levels.
>The network analyzer contains additional circuitry and computational
>ability to sweep frequencies and measure phase but this is not
>necessary for a simple SWR measurement at a given frequency.
>
>
Thanks for the enlightenment here George. Seems to me Amor, by mere
mention of equipment at her disposal, really attempts to take a bite out
of other's posts.
Could be her gear has secret method of converting the RF into a visual
indication.
At the price of it I guess it should. Maybe she would see 1.15:1 where we
might have read 1.17:1 SWR. Her measurement could be taken as gospel
because her meter cost over $300.
I'm not impressed. I'll settle for 1.7:1. if necessary 'til I can get to
the antenna.
Carl
They call me everything in the book
Sumtimes theya call me 2x4
Sumtimes they just call me...when they need a good hand
Then they call me The Radioman
We're in the CB radio bidness!
Carl Motsinger representing EFJ Devices
N9EFJ
Amor Powers
George Warner at loveboat.com wrote:
> Hey, Amor. Thanks for the good, relevant post. What took you
> so long?
Sorry, George, I had to convince my son (aka Tower of Power) that
another antenna party would be fun!
> You could have helped save me a lot of typing.
You should try one of those speed-typing shareware programs... they
really helped me. I don't even think about it any more. Although some
people accuse me of typing faster than I can think. Sort of like copying
CW way faster than it's possible to write it down. :-) .... ..
Amor Powers
One of The-Powers-That-Be
Lots of hot air
END OF POST
keyword footer:
Amor Powers
sergio prestipino (Sugar Bear) spewed forth:
> Mr. Powers,
Ms...
But you can call me Amor :-)
> In reference to some other thread on coax length, I was in a cb
> shop, and
> read an interesting note on co-phasing. they recommend installing the
> antannae,
> and tuning seperately with 18ft. of RG58 (50ohm)for min SWR, on each
> antenna,
> and then connecting your rg59 (75ohm) 18ft coax rig.
They gave you good advice, Sergie baby.
Our objective is to get to 50 ohms at the transmitter. We start with two
50ohm antennas with 50ohm coax. Then we hook them up with a tee
connector at the transmitter and we get 25ohms impedance (50 divide by 2
= 25). Whoops! This is not a wonderful match for 50ohms!
> Also, why use 75ohm cable, isn't there a better way, with a matching
> transformer, and two 50ohm sections of cable? Why 18ft????
We need to have two antennas with 100ohm impedances hooked together to
make 50ohms. 100ohm mobile antennas and 100ohm coax are not very common.
It is probably possible to build ferrite balun transformers to do it,
but they are expensive and fragile and bulky and lossy. A real can of
worms :-)
So, one easy, cheap, dependable way to transform 50ohms up to a higher
impedance, lets say (roughly) 100 ohms, is to put a quarter wave 75ohm
section of coax at the feedpoint. Simply, the 75ohm coax is about
halfway in between 50 and 100 ohms. It is sort of like a gentle
stairstep in between the two impedances. Combining the two ~100ohm feeds
together gets us to 50ohms (100 divide by 2 = 50).
Voila' -- a lovely match!
But 6feet (quarter wave of coax for CB frequency with velocity factor of
..7) is too short for most CB installations, so the next longer piece of
75ohm coax which will still do the same job is the odd (third) multiple
of a quarter wave (18feet= 3quarter wave). Even multiples of quarter
waves (halfwave, fullwave, etc) won't work for the matching section.
They just don't have what it takes!
18 feet turns out to be a magic number for mobile CB antenna situations.
It is about a physical halfwave length on the outside of the coax, and
three electrical quarterwaves on the inside! That is the reason some
designs of magnetic mount, through-the-glass, and other antennas use it
also. (I love the through-the-glass antenna on my sports car).
If pressed, we could get into the RF theory (rotation of the phase with
inductive or capacitive reactance) behind different impedance matching
sections, but that is a complex thing to explain via text. This is not a
binary group, so all my handwaving and drawing with my air pencil I've
been doing during this thread seems to have somehow gotten lost in the
translation :-)
Amor Powers
One of The-Powers-That-Be
Lots of hot air
Soprano in the Finale (currently dieting)
END OF POST
Keyword footer: (audience participation not advised)
RF CB Radio coaxial cable coax cutting phase match mobile antenna RF CB
Radio coaxial cable coax cutting phase match mobile antenna RF CB Radio
coaxial cable coax cutting phase match mobile antenna RF CB Radio
coaxial cable coax cutting phase match mobile antenna RF CB Radio
coaxial cable coax cutting phase match mobile antenna RF CB Radio
coaxial cable coax cutting phase match mobile antenna RF CB Radio
coaxial cable coax cutting phase match mobile antenna RF CB Radio
coaxial cable coax cutting phase match mobile antenna RF CB Radio
coaxial cable coax cutting phase match mobile antenna RF CB Radio
coaxial cable coax cutting phase match mobile antenna RF CB Radio
coaxial cable coax cutting phase match mobile antenna RF CB Radio
coaxial cable coax cutting phase match mobile antenna RF CB Radio
coaxial cable coax cutting phase match mobile antenna RF CB Radio
coaxial cable coax cutting phase match mobile antenna RF CB Radio
coaxial cable coax cutting phase match mobile antenna RF CB Radio
coaxial cable coax cutting phase match mobile antenna RF CB Radio
coaxial cable coax cutting phase match mobile antenna RF CB Radio
coaxial cable coax cutting phase match mobile antenna RF CB Radio
coaxial cable coax cutting phase match mobile antenna RF CB Radio
coaxial cable coax cutting phase match mobile antenna RF CB Radio
coaxial cable coax cutting phase match mobile antenna RF CB Radio
coaxial cable coax cutting phase match mobile antenna RF CB Radio
coaxial cable coax cutting phase match mobile antenna RF CB Radio
coaxial cable coax cutting phase match mobile antenna RF CB Radio
coaxial cable coax cutting phase match mobile antenna RF CB Radio
coaxial cable coax cutting phase match mobile antenna
Bill Nelson
bvand...@nb.com wrote:
: >
: > This fact about line length not affecting SWR(excluding line loss
: > effects for long lengths) is common knowledge among anyone who has
: > studied Transmission Line theory.
: >
: > You better think twice before you question the laws of Physics using
: > data obtained from mediocre instrumentation.
: >
: > Kelly Flowers, P.E.
: > ARS NN8E
:
: No offense Kelly but, why does this happen to me????
:
: In a mobile setup with the same antenna and mount.....
: I use 18 feet of coax and get an SWR reading of 1.7:1
: then
: I use 12 feet of coax and get an SWR reading of 1.1:1
:
: WHY WHY WHY does this happen???
: The only thing that has changed is the coax length!!!!
If you use the original 12 feet of coax for both setups, you are also
adding several connectors. You may have some sort if impedance bump,
due to an improperly installed connector.
: I would love for someone to explain this to me since I have tested this
: premise manymany times. I would plainly state that COAX LENGTH INDEED
: DOES AFFECT SWR READINGS IN A CB MOBILE SETUP. Of course if someone can
: explain why the above happens, I will change my statement.
How are you measuring the SWR? With the meter in the radio?
The claim was that coax length does not affect the SWR of the system.
There was no claim that the "measured" SWR would not change. I can think
of a couple of reasons why this could occur - for poorly designed meters.
--
Bill Nelson (bi...@peak.org)
Bill Nelson
Bill Eitner (kd6...@earthlink.net) wrote:
: >
: > I never worry about SWR - I always tune for maximum field strength.
: > As long as my radio can handle any resulting SWR, then the tuning is
: > as good as possible.
:
: Most modern, solid-state transmitters incorporate a form
: of self-protection that lowers their output in the event
: of high SWR (reflected power). With that in mind if the
: SWR gets too high the field strength will be reduced due
: to the lower level of transmitter output. That makes
: high field strength with a high SWR impossible. That's
: why I say SWR and field strength track together when
: using modern equipment regardless of line length.
With current CB antennas, you will not find any that are a long
way from 50 ohms when resonant. At worst, they will be somewhere
between 35 ohms and 75 ohms. Even a 2:1 SWR will not reduce the
output of most modern CB rigs significantly. Nor is the additional
loss due to that SWR very high. For example, assume that you have
50 feet of RG-8/U coax - which would have a line loss of about
0.45 dB. With a 2:1 SWR, the additional loss would be about 0.2
dB, for a total loss of 0.65 dB. The indicated SWR at the input
would be about 1.8:1. Note that the SWR is actually 2:1, but the
line loss simply reduces the reflected power that gets back, so
the reading on a meter is lower (and the transmitter sees this
as a lower SWR). This has nothing to do with line length - a
line half as long, with twice the loss per unit length, would
produce the same results.
: > When the field strength is at a maximum, you would find that the
: > antenna is resonant. Whether it is 50 ohms resistive at that point
: > or 25 ohms makes essentially no difference.
:
: I have trouble agreeing with that unless you're assuming
: the use of a tube output circuit. Remember, this is a
: CB newsgroup. I think it's appropriate in this forum to
: restrict our conclusions to what's applicable using typical,
: modern, solid-state CB equipment.
See above. I use transistor gear, even stuff without the foldback
power protection circuitry, and have never had any problem with
relatively low SWR levels.
--
Bill Nelson (bi...@peak.org)
Amor Powers
SEAN spewed forth:
> MY CONCLUSIONS HAVE BEEN THAT BY MEASURING THE
> FEEDPOINT IMPEDANCE VIA RESONATED LENGTHS OF CABLE
> HELPS TO SEE THE EXACT LOAD THAT THE RADIO SEES.
Sean, now I'm curious how you are measuring the feedpoint impedance with
an SWR meter (reflectometer).
> THE COAX SHOULD BE APPR. 50 OHMS RADIATION RESISTANCE REGARDLESS OF
> LENGTH.
You might think it's nit-picky, but standard coaxial cable has 50 ohms
"characteristic impedance". This means that when terminated by 50 ohms
and sourced by 50 ohms, it appears to the source as neither inductive
nor capacitive. The coax by itself doesn't have radiation resistance.
"radiation resistance" is kind of a misnomer, and is used to refer to
certain aspects of the antenna itself.
Measurement of the reflected power in an antenna/transmission line
system using inexpensive (low directivity) SWR meters could be done by
averaging the SWR readings from several different inserted lengths of
feedline.
The *antenna* SWR is best measured at the feedpoint of the antenna.
Impedance measurement devices can benefit somewhat from known resonant
feedlines, however, to my knowledge, a common SWR meter (reflectometer)
does not directly read out impedance.
My mind is open. Please let me in on your method of measuring impedance
with a reflectometer! I love these little RF magic tricks.
The trick of bringing your hand up next to the antenna when checking
reflected power is a good one for antenna resonance.
Also, there's the one where a metal pole is hooked to the ground
connection at the base of the antenna, varied in angle to lower the
impedance, and deriving an impedance approximation from the SWR.... ..
Maybe we should start a new thread called
"CB Magic Tricks" .... .. ?:-)
Amor Powers
One of The-Powers-That-Be
Lots of hot air
END OF POST
__________________________________________________
keyword footer:(viewing not advised for younger audiences)
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Amor Powers
After careful consideration and thoughtful deliberation,
Bill entered via the user interface:
> So, given this, resonating coax is not necessary and
> feedpoint SWR testing is the method of choice if one
> is looking for the most accurate SWR data for antenna
> tuning purposes when all that's available is a common
> SWR meter. Do you agree?
Yes, antenna feedpoint is the best place to measure *antenna* match or
*antenna* impedance, no matter what gear you choose.
Antenna tuning (frequency of resonance with respect to 50ohms) can be
measured at either the TX output or the antenna feedpoint or anywhere in
between. It is possible for the some antenna feedpoints to not be 50ohms
resistive at resonance. But most CB antennas are not very adjustable for
impedance. The only adjustment on most of them is length (resonant
frequency, i.e., "channel with the lowest SWR").
System SWR is best measured at the TX output.
Do we need to *measure* all this stuff? Or do we simply need to optimize
the system for 50ohms?
The only time I find a need to measure impedance is when I'm designing a
new antenna.
On the practical side...
Personally, If all I have at the time is a cheap SWR meter, I just put
it in line with a short jumper to the TX; then adjust the tuning length
of the antenna, and call it done.
For some mobile installations, where the lower mounted whip runs close
to the body of the vehicle (semis, vans, jeeps, RVs, trucks), a parallel
inductor (hot to ground) is sometimes needed to be added at the antenna
feedpoint to match the low impedance of the whip to the 50ohm coax. But
I've found that this too is easily adjusted with the same placement of
SWR meter (reflectometer).
Amor Powers
One of The-Powers-That-Be
Lots of hot air
END OF POST
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keyword footer: (do not attempt to grok this)
CB radio antenna coax coaxial cable cutting SWR reflected power match
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Bill Eitner
Amor Powers wrote:
>
> After careful consideration and thoughtful deliberation,
> Bill entered via the user interface:
Thank you for not insulting me with your usual quote
message.
>
> > So, given this, resonating coax is not necessary and
> > feedpoint SWR testing is the method of choice if one
> > is looking for the most accurate SWR data for antenna
> > tuning purposes when all that's available is a common
> > SWR meter. Do you agree?
>
> Yes, antenna feedpoint is the best place to measure *antenna* match or
> *antenna* impedance, no matter what gear you choose.
Thank you for clearing that up.
> Antenna tuning (frequency of resonance with respect to 50ohms) can be
> measured at either the TX output or the antenna feedpoint or anywhere in
> between. It is possible for the some antenna feedpoints to not be 50ohms
> resistive at resonance. But most CB antennas are not very adjustable for
> impedance. The only adjustment on most of them is length (resonant
> frequency, i.e., "channel with the lowest SWR").
>
> System SWR is best measured at the TX output.
> Do we need to *measure* all this stuff? Or do we simply need to optimize
> the system for 50ohms?
In my opinion, optimizing the system for greatest field
strength covers all the bases.
> The only time I find a need to measure impedance is when I'm designing a
> new antennna.
> On the practical side...
> Personally, If all I have at the time is a cheap SWR meter, I just put
> it in line with a short jumper to the TX; then adjust the tuning length
> of the antenna, and call it done.
People are getting different SWR readings depending on
feedline length. That's what started this thread.
Trying to sort out why this happens as well as designing
an antenna optimization procedure that avoids the problem
(without using multi-thousand dollar equipment) is what
we're after. Your above precedure is where we started.
I think it's commonly thought that better results can
be obtained by using more involved procedures. Personally,
I think the procedure you outline above is good enough
for most CB applications. In my opinion, if one decides
to go beyond the above (normal) procedure feedpoint SWR
testing/adjustment and/or field strength optimization are
the most feasible options using common (cheap) equipment.
SEAN FOLEY
belden and a few others make 93 ohm cable that is best suited for co
phasing. Sean aka Bigfoot
SEAN FOLEY
i am using an swr analyzer that has both an swr meter and impedance bridge
in it. this works fine for most circumstances but there are a few
situations that it doesn't work well for. in these circumstances i will use
either a typical well calibrated swr meter or one of my Birds to measure
the reflected power. just like any other job, there are certain tools
that are more suited to specific jobs than others. Sean aka Bigfoot
Amor Powers
SEAN FOLEY spewed forth:
> i am using an swr analyzer that has both an swr meter
> and impedance bridge in it.
The impedance bridge is a useful instrument which will benefit from
known coaxial cable length.
Sugar Bear
....snipped most
>
>
Thanx Amor,
Correct me if I'm wrong but I think this means I can use 2 lengths of
6ft if
it's long enough for my installation? Had I known, it would've saved me
mucho
hassles. I've got a bunch left coiled up in a 1ft diameter under the
dash. Is
the velocity factor much different for various coax? I'm using RG59 if
the v
factor is .7, I need 6.3feet.
The other thing is, i don't beleive that I need 18ft of 50 ohm to
tune up
each antenna seperatley before i connect the 75ohm harness. Any 50 ohm
length
should do based on what I've read so far?
Lastly, someone told me I need 8ft (I think it should be 9ft.)
between the
two antennas (one free space quarter wave) between the two antennas or
else
I'm wasting my time. I've got two firesticks mounted on my pickup truck
mirors,
only 6ft apart. How bad is this? i used extra grounding straps to ground
the
brackets to the truck body, Do you think those ground plane gizmo's
help? The
tuning slugs are all the way out as far as they can go, A perfect match
on channel
40 but if the slugs were just a tad longer...
Regards... ..... Sugar Bear
Amor Powers
Sugar Bear spewed forth:
> Thanx Amor,
> Correct me if I'm wrong but I think this means I can use 2 lengths >of 6ft if it's long enough for my installation?
True, Sugar. You can cut the existing coax (from the T connector end) to
6 feet in length on each side, and solder on a new T connector.
> Is the velocity factor much different for various coax? I'm using
> RG59 if the v factor is .7, I need 6.3feet.
Correct. Standard foam (50 or 75ohm) coax V factor is .7
> The other thing is, I don't believe that I need
> 18ft of 50 ohm to tune up each antenna separately before I
> connect the 75ohm harness. Any 50 ohm length
> should do based on what I've read so far?
Correct. Any 50 ohm length can be used for the antenna resonance tuning.
> Lastly, someone told me I need 8ft
> (I think it should be 9ft.) between the
> two antennas (one free space quarter wave)
> between the two antennas or else I'm wasting my time.
> I've got two firesticks mounted on my pickup truck
> mirors, only 6ft apart. How bad is this?
Sugar, this is a really touchy subject amongst pickup owners :-)
But here goes:
The wider spacing is better, but you'll probably need a WIDE LOAD permit
much beyond 8 feet! :-) 8 feet is the recommended minimum spacing, if
you desire to transmit better (a couple dB better) towards the front and
rear of the truck, while taking away signal towards the left and right
directions. 6 feet apart will not give as much gain to the front and
rear, giving more signal to the sides. If you know which compass
direction the other CB station is, it's easy to test this beam pattern
by listening to the other CB station while driving around in cicles in
an empty parking lot or field. Just don't get arrested for "cutting
donuts"!
More radiation efficiency can be achieved by slanting each of the twin
antennas at 20 to 30 degree angles toward the side or front of the
truck, away from the body of the truck. (The rabbit-ears effect). It
looks strange, but it works.
The advantage of "trucker twins" is really more pronounced on a large
cabover semi tractor-trailer rig (18 wheeler), because of the following
reasons:
Twin Antennas on a semi tractor side mirrors:
1. Communication with other vehicles on the interstate
highway is enhanced by favoring the front and rear direction
and rejecting the side directions.
(co-phased beam)
2. Semi tractors (especially higher cabovers) are already
very tall (close to overpass clearances). 102inch whips are
impractical when mounted higher (exceed overhead clearances)
or ineffective when mounted lower on the body.
3. A single antenna mounted on the roof or side mirror tends
to have it's radiated signal blocked by the vehicle body
in some directions.
4. With twins, the antenna on the right side tends to make up for
the poor radiation pattern of the one on the left side, and
vice-versa.
5. The twin antennas give a more balanced appearance to the
beauty of the truck. Twin antennas look cool. :-)
Best Antenna for a pickup truck:
Many previous long distance "horse-races" over 20 years between mobile
units in pickup trucks have proven beyond a shadow of a doubt that the
102inch whip will beat out a pair of 4 foot twins in all directions by
at least one or two S units. So, if you are really looking for the most
signal, you'll have to put a long whip (or twin long whips) on your
pickup. Ask anyone who has single or twin 102inch whips on their pickup.
It's impressive how much better they are. A good compromise for less
height is single or twin 6 or 7 foot long whips.
> Do you think those ground plane gizmo's
> help? The tuning slugs are all the way out as far
> as they can go, A perfect match on channel
> 40 but if the slugs were just a tad longer...
Yes, Sugar.
An add-on metallic loop "ground plane gizmo" (known to radio engineers
as a capacitive hat) helps to lower the frequency of resonance, just the
same as adding a longer tuning slug (stinger).
Speaking of stingers, I hope I didn't stir up too much of a hornets'
nest this time... ?:-0
Amor Powers
One of The-Powers-That-Be
Lots of hot air
END OF POST
--------------------------------------------
keyword footer: (parental discretion advised)
CB Radio antenna coaxial cable transceiver HF 27 Megahertz MHz ground
plane capacitive hat tuning whip CB Radio antenna coaxial cable
transceiver HF 27 Megahertz MHz ground plane capacitive hat tuning whip
CB Radio antenna coaxial cable transceiver HF 27 Megahertz MHz ground
plane capacitive hat tuning whip CB Radio antenna coaxial cable
transceiver HF 27 Megahertz MHz ground plane capacitive hat tuning whip
CB Radio antenna coaxial cable transceiver HF 27 Megahertz MHz ground
plane capacitive hat tuning whip CB Radio antenna coaxial cable
transceiver HF 27 Megahertz MHz ground plane capacitive hat tuning whip
CB Radio antenna coaxial cable transceiver HF 27 Megahertz MHz ground
plane capacitive hat tuning whip CB Radio antenna coaxial cable
transceiver HF 27 Megahertz MHz ground plane capacitive hat tuning whip
CB Radio antenna coaxial cable transceiver HF 27 Megahertz MHz ground
plane capacitive hat tuning whip CB Radio antenna coaxial cable
transceiver HF 27 Megahertz MHz ground plane capacitive hat tuning whip
CB Radio antenna coaxial cable transceiver HF 27 Megahertz MHz ground
plane capacitive hat tuning whip CB Radio antenna coaxial cable
transceiver HF 27 Megahertz MHz ground plane capacitive hat tuning whip
CB Radio antenna coaxial cable transceiver HF 27 Megahertz MHz ground
plane capacitive hat tuning whip CB Radio antenna coaxial cable
transceiver HF 27 Megahertz MHz ground plane capacitive hat tuning whip
CB Radio antenna coaxial cable transceiver HF 27 Megahertz MHz ground
plane capacitive hat tuning whip CB Radio antenna coaxial cable
transceiver HF 27 Megahertz MHz ground plane capacitive hat tuning whip
CB Radio antenna coaxial cable transceiver HF 27 Megahertz MHz ground
plane capacitive hat tuning whip CB Radio antenna coaxial cable
transceiver HF 27 Megahertz MHz ground plane capacitive hat tuning whip
Amor Powers
Sugar Bear sez:
> I want to change my two 4ft. firesticks to two solaricon
> base loaded steel whips, to reduce the stress
> Do you think I'll lose any performance?
Yes. The base loaded whips will have reduced performance.
> Do you think a 102" whip mounted on my rear hitch will beat out my
> twin 4ft. firestick antenna's? I have a fiberglass cap on the truck.
Yes. 102inch whip will be much better! (The breakfast of skip-talkers).
Since you are considering the change, you might try it for yourself, and
compare the difference between the two antenna systems. It will make a
true believer out of you :-) .
> PS: Why are the firesticks 5/8" wave, I thought you needed a quarter
> wave..
It is really just a marketing gimmick, when it comes to short verticals
for mobile applications. There is really not much difference between the
performance of a 48inch wound fiberglass 1/4 wave and a 48inch wound
fiberglass 5/8 wave. Full size 5/8 wave antennas for 27MHz base
applications (about 20 feet long) have more gain than full size 1/4 wave
base antennas (about 9 feet long). But this difference (3db) is only
realized with nearly full size antennas.
But they have sold a lot of antennas with this marketing gimmick, and
have a lot of gullible operators believing it now!
You know what they say: perception is reality...
If the advertising executives could only get the radio waves to
cooperate!
Sugar Bear
Amor Powers wrote:
> Best Antenna for a pickup truck:
> Many previous long distance "horse-races" over 20 years between mobile
> units in pickup trucks have proven beyond a shadow of a doubt that the
> 102inch whip will beat out a pair of 4 foot twins in all directions by
> at least one or two S units. So, if you are really looking for the most
> signal, you'll have to put a long whip (or twin long whips) on your
> pickup. Ask anyone who has single or twin 102inch whips on their pickup.
> It's impressive how much better they are. A good compromise for less
> height is single or twin 6 or 7 foot long whips.
>
Dear Amor,
I want to change my two 4ft. firesticks to two solaricon base loaded
steel whips, to reduce the stress and prevent the inevitable cracking
off of
my rearview mirors one day when I bump into a tree. The solaricons are
52"
tunable steel whips and are base loaded CB antenna's. The firesticks are
5/8
wave fiberglass top loaded with a tuneable slug on top. Do you think
I'll lose
any performance?
Do you think a 102" whip mounted on my rear hitch will beat out my
twin
4ft. firestick antenna's? I have a fiberglass cap on the truck.
PS: Why are the firesticks 5/8" wave, I thought you needed a quarter
wave..
PSS: What is a "hybrid", or "mutant" cb antenna, (a longer 10M
antenna?)
Sugar Bear
Amor Powers wrote:
>
> Sugar Bear sez:
>
> > I want to change my two 4ft. firesticks to two solaricon
> > base loaded steel whips, to reduce the stress
>
Do you think it will be noticeable? Why is base loaded not as good?
> > PS: Why are the firesticks 5/8" wave, I thought you needed a quarter
> > wave..
>
> It is really just a marketing gimmick, when it comes to short verticals
> for mobile applications. There is really not much difference between the
> performance of a 48inch wound fiberglass 1/4 wave and a 48inch wound
> fiberglass 5/8 wave. Full size 5/8 wave antennas for 27MHz base
> applications (about 20 feet long) have more gain than full size 1/4 wave
> base antennas (about 9 feet long). But this difference (3db) is only
> realized with nearly full size antennas.
>
But why 5/8 wave as opposed to a half wave? And what is a "half breed"
CB
antenna.
Best Regards... .... Sugar Bear
Myra
...bits snipped
>
> Exactly, that's why I test at the feedpoint. The difference
> is you can use shorter coax runs and increase field strength
> by not worrying about resonating the coax. And whether or
> not one believes that coax influences SWR readings, you've
> done what it takes to make the argument meaningless in either
> case. That's what I like the most--if I test at the feedpoint
> I don't need to concern myself with either side of this debate.
>
> This debate has been going non-stop for as long as I've been
> in radio. I decided years ago that the smartest thing to
> do would be to incorporate testing and tuning techniques that
> would keep me from having to choose a side. Pre-tuning,
> measuring SWR at the feedpoint whenever possible, and putting
> more effort into increasing field strength than lowering SWR
> are the techniques that will keep you out of this never-ending
> argument. When a customer comes to me asking about all of this
> I teach him about these techniques rather than trying to push
> one philosophy over another. In the long run I think that's
> the best way to go.
>
>> as you said, been there, done that.
>
> Although you haven't transcended this worthless debate which
> can easily be worked around regardless of ones stand on this
> rather petty issue. You're here now--do it!
>--
>
> -=[Bill Eitner]=-
A slow clap (one hand).
Congrats, Bill, 10 out of 10, Yeah!
I agree 100%!!!!
Myra Howerd (who still reads these things...)