Importance of coax length on HF A myth?
James Pollock
Hi.
I have a friend who swears up and down that the length of the
coaxial cable to maintain resonance at a given frequency is important
for reducing the SWR. He feels that it acts in a similar principal to
the fact that the elements of an antenna must be cut to the correct
length for the antenna to be resonant at a given frequency.
It seems to me I have also read somewhere that this is a myth. I
need the true scoop on this. I'm preparing to put up a pretty good
antenna system on my roof (a homebrew 10 meter antenna cut to exactly
the middle of the band), and don't want to purchase an extra 6 ft of
Belden 9913 coax only to have it looped up and losing signal, when in
reality I could of used 20 feet instead of 26 feet of cable.
Please carbon copy any responses to this post to jp...@redrose.net
so I don't miss any good info.
Thanks
Jim
K1BQT
Hi Jim--
In your inquiery, you wrote:
I'm preparing to put up a pretty good
antenna system on my roof (a homebrew 10 meter antenna cut to exactly
the middle of the band), and don't want to purchase an extra 6 ft of
Belden 9913 coax only to have it looped up and losing signal, when in
reality I could of used 20 feet instead of 26 feet of cable.
In response:
As long as the antenna's feedpoint impedance somewhat matches the
impedance of the line, it doesn't really matter how long your cable is. It
is true that--as the antenna impedance changes (which it
will--constantly--across the band), the coax may take on the properties of
an impedance transformer. That is, it may take a 40-Ohm load presented by
the antenna and make it look like 60 Ohms at the transmitter--or whatever.
As a practical matter, this isn't a problem to lose sleep over unless the
mis-match becomes extremely large.
You could cut your feedline to a multiple of an electrical
half-wavelength. This will ensure that the transmitter sees exactly the
SAME impedance that is occuring at the antenna feedpoint. But, once
again, what's the point? The antenna is going to change impedance anyway
across its range--and your transmitter can cope. Normally, the only time
someone would use a multiple of a half-wavelength of feedline is when they
want to measure the feedpoint characteristics of the antenna from the
ground for documentation purposes.
By the same token, you'll be wasting money to use 9913 at 30 MHz over a
20-foot run. Go down to Radio Shack and get some RG8 mini. The
difference in loss will be barely measurable (measured in hundredths of a
dB) and meaningless.
My advice would be--don't sweat the small stuff. Instead, pay attention
to the things that will really matter--like radiation pattern on the
horizon, radiator efficiency, gain, and separation from lossy or
interfering media (ie other antennas--or roof flashing, aluminum-backed
insulation, and house wiring).
Good luck with it!
Rick K1BQT
Gary Coffman
In article <54nc7a$s...@lily.redrose.net> jp...@redrose.net (James Pollock) writes:
> I have a friend who swears up and down that the length of the
>coaxial cable to maintain resonance at a given frequency is important
>for reducing the SWR. He feels that it acts in a similar principal to
>the fact that the elements of an antenna must be cut to the correct
>length for the antenna to be resonant at a given frequency.
>
> It seems to me I have also read somewhere that this is a myth. I
>the middle of the band), and don't want to purchase an extra 6 ft of
>Belden 9913 coax only to have it looped up and losing signal, when in
>reality I could of used 20 feet instead of 26 feet of cable.
Ok, this can get complex because there are several interrelated
things happening here. Lets take them one at a time.
The only thing that determines SWR on a transmission line is
the ratio of the characteristic impedance of the line to the
impedance presented by *the load*. Nothing you can do at the
generator end can affect SWR, and changing the length of the
cable won't change SWR either (but see below).
However, changing the length of the line will change the impedance
presented to the generator. That's because a transmission line with
any SWR other than 1:1 acts as a transmission line transformer. By
changing the length of the line, you get an effect similar to, but
not quite the same as, changing the turns ratio of a transformer.
The SWR doesn't change, but the impedance presented to the generator
does. This can be useful, and is employed frequently for impedance
matching purposes. The most common such is the quarter wave matching
section.
In general, the impedance transformation will be complex, IE it will
be in the form of R+jX, where X can be capacitive or inductive depending
on cable length. The Smith Chart can be used to determine the transformation
effect by following the constant SWR circle around the chart for different
cable lengths (measured in wavelengths at the frequency of interest) and
reading out the R and X values. Note that the SWR is constant, but the
impedance presented to the generator changes with varying cable length.
Now something else can happen too, and this is where some of the
myths have arisen. If you feed a balanced antenna, such as a dipole,
with an unbalanced transmission line, such as coax, the *outside* of
the outer conductor of the transmission line can become part of one
arm of the dipole, and antenna currents can flow down the outside
of the transmission line.
Picture the dipole as a 1/4-wave section connected to the coax inner
and an L shaped element consisting of the other arm of the dipole and
the *outside* of the coax outer conductor. This can change the *load
impedance* presented to the transmission line because the antenna geometry
has been altered from a physically balanced dipole configuration, and
thus can change the SWR. Changing the coax length changes the effective
length of one leg of the dipole, and that changes the impedance presented
at the load end of the coax, and that changes SWR in the coax.
Coax transmission line currents flow on the outside of the inner
conductor and the inside of the outer conductor. This is because
of skin effect. The physics of coax forces the currents to be
equal in magnitude and opposite in direction. The opposite currents
cancel any radiation fields, and the coax doesn't radiate. But that
doesn't affect currents flowing on the outside of the outer conductor.
Those currents are independent of what's happening inside the coax.
Their radiation fields aren't canceled, and the coax becomes a radiating
part of the antenna.
Generally, it is undesirable to have antenna currents flowing on
the outside of the coax because it will alter the radiation pattern
of the antenna, alter its feedpoint impedance, and because it can
cause "hot shack" problems in some cases. The cure is to use a choke
balun at the antenna feedpoint to decouple the outside of the coax
from the antenna element to which the inside of the outer conductor
is connected. (A choke balun can be simply a few turns of the coax
tightly coiled just before connection to the feedpoint. Or it can
consist of a number of ferrite beads slipped over the coax near
the feedpoint. Its purpose is to act as a high impedance to currents
trying to flow down the *outside* of the coax outer conductor.)
When you use such a balun, or if you are feeding an inherently
unbalanced antenna such as a groundplane mounted vertical, then
the coax doesn't become part of the antenna, so it can't change
the load impedance presented to the coax, and thus can't change
SWR. Any length coax can now be used without affecting SWR, or
antenna radiation pattern.
So you can see, there is some truth to the myth that coax length
can alter SWR, but it isn't quite the way many amateurs picture
it. The reason it can happen is because the coax has become an
unintended part of the antenna due to a failure to properly deal
with the unbalanced to balanced connection at the antenna feedpoint
with a balun.
Gary
--
Gary Coffman KE4ZV | You make it, | Due to provider problems
Destructive Testing Systems | we break it. | with previous uucp addresses
534 Shannon Way | Guaranteed! | Email to ke...@radio.org
Lawrenceville, GA 30244 | |
Jim Potter
jp...@redrose.net (James Pollock) wrote:
>Hi.
>
> I have a friend who swears up and down that the length of the
>coaxial cable to maintain resonance at a given frequency is important
>for reducing the SWR. He feels that it acts in a similar principal to
>the fact that the elements of an antenna must be cut to the correct
>length for the antenna to be resonant at a given frequency.
>
> It seems to me I have also read somewhere that this is a myth. I
>need the true scoop on this. I'm preparing to put up a pretty good
>antenna system on my roof (a homebrew 10 meter antenna cut to exactly
>the middle of the band), and don't want to purchase an extra 6 ft of
>Belden 9913 coax only to have it looped up and losing signal, when in
>reality I could of used 20 feet instead of 26 feet of cable.
>
> Please carbon copy any responses to this post to jp...@redrose.net
>so I don't miss any good info.
>
> Thanks
>
> Jim
>
The length of coax doesn't change the VSWR, assuming it's all the same
impedance. However, the impedance at the transmitter output will be a
function of cable length if the VSWR is not 1:1. Ultimately what
matters is not the VSWR, but the impedance match to the transmitter.
All that is required is a conjugate match so that the transmitter sees
a resistive load of the right resistance at the output device.
There is one way that the coax length affects VSWT. If the attenuation
is high the VSWR gets better with length. Someone once said a good low
power load for 144 MHz is 100 ft of RG-58. It doesn't matter too much
what's on the far end.
de K9GXC, Jim
James M. Potter, President TEL: (505) 662-5804
JP Accelerator Works, Inc. FAX: (505) 662-5210
2245 47th Street EMAIL: jpo...@jpaw.com
Los Alamos, NM 87544-1604 URL: http://www.jpaw.com
Dick Hughes
On Thu, 24 Oct 1996 14:07:27 GMT, ga...@ke4zv.atl.ga.us (Gary Coffman)
wrote:
>In article <54nc7a$s...@lily.redrose.net> jp...@redrose.net (James Pollock) writes:
>> I have a friend who swears up and down that the length of the
>>coaxial cable to maintain resonance at a given frequency is important
>>for reducing the SWR. He feels that it acts in a similar principal to
>>the fact that the elements of an antenna must be cut to the correct
>>length for the antenna to be resonant at a given frequency.
>>
>> It seems to me I have also read somewhere that this is a myth. I
>>need the true scoop on this. I'm preparing to put up a pretty good
>>antenna system on my roof (a homebrew 10 meter antenna cut to exactly
>>the middle of the band), and don't want to purchase an extra 6 ft of
>>Belden 9913 coax only to have it looped up and losing signal, when in
>>reality I could of used 20 feet instead of 26 feet of cable.
>
>Ok, this can get complex because there are several interrelated
>things happening here. Lets take them one at a time.
>
>The only thing that determines SWR on a transmission line is
>the ratio of the characteristic impedance of the line to the
>impedance presented by *the load*. Nothing you can do at the
>generator end can affect SWR, and changing the length of the
>cable won't change SWR either (but see below).
<huge snip>
>Gary
In general, the correct length of coax for an HF antenna is the
distance it takes to get from the transmitter to the antenna. Don't
worry about length.
Dick Hughes - W6CCD
Steve Iezzi
> >coaxial cable to maintain resonance at a given frequency is important
> >for reducing the SWR. He feels that it acts in a similar principal to
> >the fact that the elements of an antenna must be cut to the correct
> >length for the antenna to be resonant at a given frequency.
> >
> > It seems to me I have also read somewhere that this is a myth. I
> >need the true scoop on this. I'm preparing to put up a pretty good
> >antenna system on my roof (a homebrew 10 meter antenna cut to exactly
> >the middle of the band), and don't want to purchase an extra 6 ft of
> >Belden 9913 coax only to have it looped up and losing signal, when in
> >reality I could of used 20 feet instead of 26 feet of cable.
> >
> >so I don't miss any good info.
> >
> > Thanks
> >
> > Jim
> >
> The length of coax doesn't change the VSWR, assuming it's all the same
> impedance. However, the impedance at the transmitter output will be a
> function of cable length if the VSWR is not 1:1. Ultimately what
> matters is not the VSWR, but the impedance match to the transmitter.
> All that is required is a conjugate match so that the transmitter sees
> a resistive load of the right resistance at the output device.
>
> There is one way that the coax length affects VSWT. If the attenuation
> is high the VSWR gets better with length. Someone once said a good low
> power load for 144 MHz is 100 ft of RG-58. It doesn't matter too much
> what's on the far end.
>
> de K9GXC, Jim
>
> James M. Potter, President TEL: (505) 662-5804
> JP Accelerator Works, Inc. FAX: (505) 662-5210
> 2245 47th Street EMAIL: jpo...@jpaw.com
> Los Alamos, NM 87544-1604 URL: http://www.jpaw.com
It's possible to use a cable with an impedance not
equal to the characteristic impedance of your
antenna and rig. For example, a half wave length
of 75 ohm cable will work between a 50 ohm antenna
and a 50 ohm transmitter. You can also use 75 ohm
cable to match a 100 ohm antenna to a 50 ohm rig
by cutting it to the right length. Unfortunately,
these tricks generally only work at one frequency.
In this case, the length of the cable is critical
to a given frequency.
Steve KT4FY
K9SQG
Let me put it this way, the length of coax does make a difference but one
must understand what that difference is. The length of the coax has NO
effect on the performance of the antenna itself BUT it may affect the
transciever it is connected to. The SWR of the antenna proper does NOT
change ragardless of the coax length. BUT the rig DOES see SWR changes as
a function of the coax length. How to keep the rig happy? Adjust the
length of the coax or use an "antenna" tuner which in reality is only a
"feed line tuner" in most cases, it will NOT affect the performance of the
antenna proper.
Anybody win any bets on this one?
Dave Hockaday
James Pollock wrote:
> I have a friend who swears up and down that the length of the
> coaxial cable to maintain resonance at a given frequency is important
> for reducing the SWR. He feels that it acts in a similar principal to
> the fact that the elements of an antenna must be cut to the correct
> length for the antenna to be resonant at a given frequency.
Hi Jim. This is true if the feedline impedance is different than that of
the antenna/transmitter. If you use 50 ohm feedline with a 50 ohm
antenna/transmitter, any length is acceptable. If you substituted say, 75
ohm feedline, you would need to cut the feedline in 1/2 wl multiples (the
impedance is repeated every 1/2 wl).
Dave Hockaday WB4IUY
wb4...@ipass.net
http://www.ipass.net/~hockaday/
http://www.ipass.net/~wb4iuy/
http://www.ipass.net/~teara/
http://www.geocities.com/TheTropics/3349/
http://www.RTPnet.org/~fcarc/
http://www.RTPnet.org/~rdrc/
Roy Lewallen
In article <326f9582...@news.santafe.edu>,
jpo...@jpaw.com (Jim Potter) wrote:
>The length of coax doesn't change the VSWR, assuming it's all the same
>impedance. However, the impedance at the transmitter output will be a
>function of cable length if the VSWR is not 1:1. Ultimately what
>matters is not the VSWR, but the impedance match to the transmitter.
>All that is required is a conjugate match so that the transmitter sees
>a resistive load of the right resistance at the output device.
>
>There is one way that the coax length affects VSWT. If the attenuation
>is high the VSWR gets better with length. Someone once said a good low
>power load for 144 MHz is 100 ft of RG-58. It doesn't matter too much
>what's on the far end.
There's one more way that cable length can affect VSWR. If common-mode
current is present (that is, current is flowing on the outside of coax, or
the currents in twinlead aren't equal and opposite), the feedline becomes
part of the antenna. In that situation, changing the length of the feedline
changes the antenna itself, which will affect VSWR.
VSWR can seem to change with cable length if the characteristic impedance
of the VSWR meter isn't the same as that of the cable. I've measured RG-58
at over 60 ohms Z0, and a couple of cheap SWR meters I've looked at were
off also, so I imagine this is a fairly common occurrence. In this case,
the SWR doesn't really change with feedline length, but the SWR meter says
it does.
Roy Lewallen, W7EL
Jim Potter
ga...@ke4zv.atl.ga.us (Gary Coffman) wrote:
>So you can see, there is some truth to the myth that coax length
>can alter SWR, but it isn't quite the way many amateurs picture
>it. The reason it can happen is because the coax has become an
>unintended part of the antenna due to a failure to properly deal
>with the unbalanced to balanced connection at the antenna feedpoint
>with a balun.
>
>Gary
Good point, Gary. I hadn't thought of that, but it makes a lot of
sense. Most of the systems I deal with have a resonant cavity load fed
with rigid coax where there is no chance of any rf currents on the
outside. (A good thing too, at 250 kW peak and 425 MHz.)
or...@capital.net
ga...@ke4zv.atl.ga.us (Gary Coffman) wrote:
>(partly deleted)
>When you use such a balun, or if you are feeding an inherently
>unbalanced antenna such as a groundplane mounted vertical, then
>the coax doesn't become part of the antenna, so it can't change
>the load impedance presented to the coax, and thus can't change
>SWR. Any length coax can now be used without affecting SWR, or
>antenna radiation pattern.
>
>So you can see, there is some truth to the myth that coax length
>can alter SWR, but it isn't quite the way many amateurs picture
>it. The reason it can happen is because the coax has become an
>unintended part of the antenna due to a failure to properly deal
>with the unbalanced to balanced connection at the antenna feedpoint
>with a balun.
>
>Gary
>--
>Gary Coffman KE4ZV | You make it, | Due to provider problems
>Destructive Testing Systems | we break it. | with previous uucp addresses
>534 Shannon Way | Guaranteed! | Email to ke...@radio.org
>Lawrenceville, GA 30244 | |
Thanks for this explanation, Gary. I have always been confused on this
coax length thing. Your explanation is quite easily understood.
73,
Butch N2YMJ
Maude Schyffert
> In general, the correct length of coax for an HF antenna is the
> distance it takes to get from the transmitter to the antenna. Don't
> worry about length.
>
> Dick Hughes - W6CCD
That is a damn good answer and works in 99.99%, the rest is fiction.
42 years of hamming SM0BKZ
Joel Gamble
James Pollock wrote:
>
> Hi.
>
> I have a friend who swears up and down that the length of the
> coaxial cable to maintain resonance at a given frequency is important
> for reducing the SWR. He feels that it acts in a similar principal to
> the fact that the elements of an antenna must be cut to the correct
> length for the antenna to be resonant at a given frequency.
>
> need the true scoop on this. I'm preparing to put up a pretty good
> antenna system on my roof (a homebrew 10 meter antenna cut to exactly
> the middle of the band), and don't want to purchase an extra 6 ft of
> Belden 9913 coax only to have it looped up and losing signal, when in
> reality I could of used 20 feet instead of 26 feet of cable.
>
> Please carbon copy any responses to this post to jp...@redrose.net
> so I don't miss any good info.
>
> Thanks
>
talking about!
The ONLY time coax length makes a difference is when it is radiating
rf,thus it becomes an active part of the antenna,instead of the passive
conduit it is designed to be.
Ko4qC
Dick Hughes
Anyone who wants to really get into this should read, "My Feed Line
Tunes My Antenna!" by Byron Goodman, W1DX. It was printed in QST in
March 1956, April 1977 and November 1991.
Dick Hughes - W6CCD
Steve Bramham
Maude Schyffert wrote:
>
> > In general, the correct length of coax for an HF antenna is the
> > distance it takes to get from the transmitter to the antenna. Don't
> > worry about length.
> >
> > Dick Hughes - W6CCD
Sometimes we spend way too much time performing analysis on a problem
rather than using common sense. I've been a licensed amateur radio
operator for over 30 years and have worked in commercial broadcasting
and two-way radio for almost as long. The correct length of coax cable
IS "the distance it takes to get from the transmitter to the antenna."
While others will argue the technical merits or matching this an that,
in the "real-world" it makes very little difference.
Oh well, sit back and wait for all the techno-wizards to dispute this
discussion but I know what works and it works in 99% of the cases
involved.....
Jim Devenport
James Pollock wrote:
<snippety>
> It seems to me I have also read somewhere that this is a myth. I
> need the true scoop on this. I'm preparing to put up a pretty good
> antenna system on my roof (a homebrew 10 meter antenna cut to exactly
> the middle of the band), and don't want to purchase an extra 6 ft of
> Belden 9913 coax only to have it looped up and losing signal, when in
> reality I could of used 20 feet instead of 26 feet of cable.
Jim, in addition to the responses you've already received I want to add
the certainty that even with RG-58 the "26 feet of cable" would result
in essentially NO extra noticeable loss than "20 feet of cable". And
with Belden 9913 the additional loss would be even more miniscule.
According to my charts, the losses of these cables are: (per 100 feet @
30 MHZ)
RG-58 2.5 dB
Belden 9913 0.75 dB (!)
RG-8 1.0 dB
So, even if you were using the dreaded RG58 the loss would be less than
a fraction of a dB for the additional 6 feet or so of coax. Even with a
high SWR (where the reflections cause more than one trip "up and down
the coax" and thus increase the effective loss) 26 feet is not a big
deal at all. 3 dB is generally regarded as a barely discernable
difference on the receiving end without accurate test equipment to
"help" notice the difference.
As your previous respondees have noted, if the SWR is high you CAN
"fool" the transmitter into "seeing" a lower SWR by messing with coax
length but far more will be gained by trying to lower the SWR up at the
antenna FEEDPOINT which will not only make your transmitter happier but
increase apparent antenna effectiveness too, both transmit and receive.
"SWR" is SO overhyped as important anyway. I fondly recall in the '70's
working all kinds of DX on 10 meters with a homebrew 2 (TWO!) meter J
pole since I was a poor college student and didn't have the nerve to
sneak HF wires onto the roof of my college housing.....
I suppose it worked so well because I didn't own an SWR meter, just
heard the band was open and tried the J pole since that was all I had on
the roof and had a blast with it, who knows what the SWR was, but I'd
have had to QRT had I had a meter to scare myself with.....
--
|-------------------------------------------------------|
|Jim Devenport WB5AOX |
|All Standard Disclaimers Disclaimed |
|My views rarely (if ever) reflect those of my employers|
|http://nis-www.lanl.gov/~jdport/ |
|-------------------------------------------------------|
Cecil Moore
Joel Gamble wrote:
> The ONLY time coax length makes a difference is when it is radiating
> rf,thus it becomes an active part of the antenna,instead of the passive
> conduit it is designed to be.
Hi Joel, got to nitpick a tad.
1. 100ft of RG-58 on 440MHz with an SWR of 1:1 and *no feedline radiation*
does make a difference.
2. 100ft of RG-58 on 28MHz with an SWR of 20:1 and *no feedline radiation*
does make a difference.
73, Cecil, W6RCA, OOTC (not speaking for my employer)
mac...@ibm.net
In <326f9582...@news.santafe.edu>, jpo...@jpaw.com (Jim Potter) writes:
>jp...@redrose.net (James Pollock) wrote:
>
>>Hi.
>>
>> I have a friend who swears up and down that the length of the
>>coaxial cable to maintain resonance at a given frequency is important
>>for reducing the SWR. He feels that it acts in a similar principal to
>>the fact that the elements of an antenna must be cut to the correct
>>length for the antenna to be resonant at a given frequency.
>>
>>need the true scoop on this. I'm preparing to put up a pretty good
>>antenna system on my roof (a homebrew 10 meter antenna cut to exactly
>>the middle of the band), and don't want to purchase an extra 6 ft of
>>Belden 9913 coax only to have it looped up and losing signal, when in
>>reality I could of used 20 feet instead of 26 feet of cable.
>>
>>so I don't miss any good info.
>>
>> Thanks
>>
>> Jim
>>
>impedance. However, the impedance at the transmitter output will be a
>function of cable length if the VSWR is not 1:1. Ultimately what
>matters is not the VSWR, but the impedance match to the transmitter.
>All that is required is a conjugate match so that the transmitter sees
>a resistive load of the right resistance at the output device.
>
>There is one way that the coax length affects VSWT. If the attenuation
>is high the VSWR gets better with length. Someone once said a good low
>power load for 144 MHz is 100 ft of RG-58. It doesn't matter too much
>what's on the far end.
>
>
>
>James M. Potter, President TEL: (505) 662-5804
>JP Accelerator Works, Inc. FAX: (505) 662-5210
>2245 47th Street EMAIL: jpo...@jpaw.com
>Los Alamos, NM 87544-1604 URL: http://www.jpaw.com
>
Well,
Let's if I can do this reply. Yes, It's a myth! The only thing that's changing is the
APPARENT XL-XC that the METER is seeing. We all know that the Electrical
wavelength recurrs at a different distance than the phusical wavelength. The old
velocity factor trick affects that (cheaper coax = lower VF). So, the loss is
cumulative to the length, the SWR associated with the coax = myth.
If you want to play with the numbers of length, coax type, swr, ant gain, and
effective power. Download TEEREV.ZIP from the ARRL BBS, or it's on the latest
level of the QRZ CD-ROM.
Jim WD9AHF
P.S. I agree with the guy who said the correct length is what reaches! Just make
sure that you look at what it costs. I wrote that TEEREV program to see what
the tradeoff was between good coax vs an amplifier. You will be surprised at
what the cost of 'cheap' coax is.
Richard Hosking
jp...@redrose.net (James Pollock) wrote:
>Hi.
> I have a friend who swears up and down that the length of the
>coaxial cable to maintain resonance at a given frequency is important
>for reducing the SWR. He feels that it acts in a similar principal to
>the fact that the elements of an antenna must be cut to the correct
>length for the antenna to be resonant at a given frequency.
> It seems to me I have also read somewhere that this is a myth. I
>need the true scoop on this. I'm preparing to put up a pretty good
>antenna system on my roof (a homebrew 10 meter antenna cut to exactly
>the middle of the band), and don't want to purchase an extra 6 ft of
>Belden 9913 coax only to have it looped up and losing signal, when in
>reality I could of used 20 feet instead of 26 feet of cable.
Jim
If the coax is matched (ie the load presents a resistive, 50 Ohm
impedance) then the length of the coax is unimportant, except that it
has loss depending on how good it is and the operating frequency. If
the load is not matched properly then there will be different
impedances at different points aliong the coax and you will get a
different SWR with a 50 ohm source, depending on where you feed the
coax. So your friend is correct if the load is not m,atched
accurately.
Richard Hosking
VK6BRO
rich...@iinet.net.au
http://www.iinet.net.au/~richardh
Cecil Moore
Steve Bramham wrote:
> I know what works and it works in 99% of the cases involved.....
Hey Steve, be fair - tell us about that other 1%.
CHARLES J. MICHAELS
There is a common situation in which an SWR meter indicates
differing SWR with length. This occurs when currents on the outside
of the coax which will differ wtih length are getting into the meter
and mixing with the normal coax currents to produce an erroneous
SWR reading that then appears to vary with length.
Charlie, W7XC
--
William E. Sabin
If the SWR meter is suitably shielded the outside currents will bypass
the SWR-measuring circuitry. A good SWR meter should be well shielded for
this reason.
Bill W0IYH
Tom Sefranek
Cecil Moore <Cecil_...@ccm.ch.intel.com> wrote:
>Joel Gamble wrote:
>> The ONLY time coax length makes a difference is when it is radiating
>> rf,thus it becomes an active part of the antenna,instead of the passive
>> conduit it is designed to be.
>Hi Joel, got to nitpick a tad.
>1. 100ft of RG-58 on 440MHz with an SWR of 1:1 and *no feedline radiation*
>does make a difference.
Your right, I'd much rather have 75 ohm 1" hardline WITH VSWR than 50
ohm RG-58 at 450 Mhz.
>2. 100ft of RG-58 on 28MHz with an SWR of 20:1 and *no feedline radiation*
>does make a difference.
Again I'd rather have 75 ohm cable TV hardline .5" than RG-58.
Cecil is right, the general rule is get what you can for the lowest
loss. Hgh VSWR adds a LOT of loss to an already losssy cable.
>73, Cecil, W6RCA, OOTC (not speaking for my employer)
Tom
WA1RHP
Net Elmer
I am my own employer!
Anthony Severdia
In <JLKznCW.c...@delphi.com> Cecil Moore <cecil...@delphi.com>
writes:
>
>Jim Potter <jpo...@jpaw.com> writes:
>
>>to see that the ham radio newsgroup generally seems to be populated
by
>>considerate people and not the nutcakes that wase bandwidth on a
>>couple of other newsgroups I follow.
>
>Hi Jim, just don't ask "Where does reflected power go?"
>That one turned almost everyone (including me) into a
>nutcake.
>
>73, Cecil, W6RCA, OOTC
Cecil:
And the answer is ......(?) :)
-=Tony=- W6ANV
W8JI Tom
In article <54rg26$s...@opera.iinet.net.au>, rich...@iinet.net.au (Richard
Hosking) writes:
>If
>the load is not matched properly then there will be different
>impedances at different points aliong the coax and you will get a
>different SWR with a 50 ohm source, depending on where you feed the
>coax. So your friend is correct if the load is not m,atched
>accurately.
>
>
There will indeed be different impedances, BUT the SWR will remain the
same.
Any change is caused by:
1.) Common mode currents that affect antenna tuning. This indicates the
system needs a choke or current balun.
2.) The line impedance being different than the SWR meter impedance. This
indicates a cheap line or poorly dedsigned or adjusted meter.
3.) Line attenuation (this only makes SWR get better as the line is
longer). This indicates a poor line at the operating frequency.
If the system is good, even if the SWR is high, there is absolutely NO
detectable change in SWR with line length.
73 Tom
CHARLES J. MICHAELS
Richard Hoskings said -
If the coax is matched (ie the load presents a resistive, 50 Ohm
impedance) then the length of the coax is unimportant, except that it
has loss depending on how good it is and the operating frequency. If
the load is not matched properly then there will be different
impedances at different points aliong the coax and you will get a
different SWR with a 50 ohm source, depending on where you feed the
coax. So your friend is correct if the load is not m,atched
accurately.
Richard Hosking
Richard,
Nonsense.
Charlie, W7XC
--
ha...@pop3.worldaccess.nl
K9SQG wrote in response to the subject question:
> Let me put it this way, the length of coax does make a difference but one
> must understand what that difference is. The length of the coax has NO
> effect on the performance of the antenna itself BUT it may affect the
> transciever it is connected to. The SWR of the antenna proper does NOT
> change ragardless of the coax length. BUT the rig DOES see SWR changes as
> a function of the coax length. How to keep the rig happy? Adjust the
> length of the coax or use an "antenna" tuner which in reality is only a
> "feed line tuner" in most cases, it will NOT affect the performance of the
> antenna proper.
>
> Anybody win any bets on this one?
The part I disagree with is:
> ... or use an "antenna" tuner which in reality is only a
> "feed line tuner" in most cases, it will NOT affect the performance of the
> antenna proper.
Maybe I get the true meaning of the statement wrong, but I read sometimes
remarks like this and then the feeling comes to me that most HAMS believe that
reflected power is LOSS at all times. And it is this belief I have tried to
beat for years now. Some statements:
USING A TUNER AT THE GENERATOR SIDE
The use of an antenna tuner at the generator (=transmitter side) WILL change
the performance of the system. Agree: the bad VSWR between antenna and feedline
still exists, but using a tuning device WILL affect the system performance.
The antennatuner redirects the initial reflected power back to the antenna in
phase with the initial forward power. This 'second wave' will also be partially
reflected because of VSWR<>1, but the rest is transmitted. This game of
bouncing between tuner and antenna goes on until all power is either dissipated
in the cable due to loss or radiated.
EXAMPLE
So in case we have a 100 Watt transmitter and a VSWR of 1:3 it means that 25
percent of the initial power is reflected back to the generator. With the use
of an adjusted tuner we redirect this energy again to the antenna, so 25W goes
to the antenna if we consider a lossless line. Of this secundairy power 6.25
Watts are reflected to the transmitter, and 18.75 Watts are radiated into free
space.
The 6.25W is again redirected at the tuner and 75% is radiated this third pass
(about 4.7W) and 25% is reflected (about 1.55).
(make sure you have set TABs)
To antenna Radiated
100 ---> 100 --> 75 W
<-- 25
--> 25 --> 18.75W
<-- 6.25
--> 6.25 --> 4.7W
<-- 1.55
--> 1.55 --> 1.17W
<-- 0.39
--> 0.39 --> 0.3W
-------- +
and so on 99.92 Watts are transmitted already!!!
And NOT only 75 Watts
NOW WITH ATTENUATION IN THE FEEDLINE
The story is about the same, but every foot or meter the signal has to travel,
the signal will be attenuated due to the non-perfect cable.
The result is a faster die-out of the sequence
EXAMPLE WITH LOSS (3 dB) AND VSWR=3
To antenna Arriving Radiated
100 ---> 50 --> 37.5 W
6.25 <-- 12.5
6.25 --> 3.125 --> 2.344W
0.391 <-- 0.781
0.391 --> 0.195 --> 0.146W
0.17 <-- 0.34
0.17 --> 0.085 --> 0.06W
0.03 <-- 0.06 -------------- +
and so on 40.05 W radiated totally
Due to the 'bad' VSWR the feedline just appears to have a higher loss than can
be achieved by a VSWR of 1:1.
THE BOTTOM LINE
The best thing to do is a good match between antenna and a good (=lowloss)
feedline. The second best thing is: use a tuner to keep the re-reflected power
in phase with the original wave when the VSWR<>1. Sometimes you cannot (easily)
change the match between antenna and feedline (because it is high in the sky or
out there in space or whatever reason).
As long as you choose your feedline wisely (=lowcost) the drawback of tuning at
the transmitted side in stead of tuning the antenna to the feedline is extra
loss. Of course always examples are thinkable to proof that some exeptions
exist.
With an existing antenna system a transmitter side tuner is a good solution!
If you want to respond to this piece of theory, please Carbon Copy to my
emailaddress (ha...@worldaccess.nl) since I go out sailing tomorrow and do not
know when I will be able to read any reactions to this).
'73's
----
+-----------------------------------+
| Henk |
| 'Hank Panthouse' |
| Broekhuizen |
| P A 3 B L P |
+-----------------------------------+
| email: ha...@worldaccess.nl |
+-----------------------------------+
+-----------------------------------+
| It's better to burn up, |
| than to fade away |
+-----------------------------------+
Merv Stump
ARRL currently has a program under development which analyzes transmission
line. I'm fortunate to have been asked to play with an early copy.
Running some quick examples shows the following:
At 28 MHz an antenna with a purely resistive load of 50 ohms fed with 100
feet of RG213 has a loss of 1.181 dB. There is no additional loss due to
SWR.
The same antenna fed with 100 feed of 1/2 inch 75 ohm hard line has a loss
of .420 dB due to the cable and an additional .032 dB due to the SWR for a
total loss of .452 dB.
An antenna with a purely resistive load of 500 ohms (10/1 SWR) fed with
RG213 has a loss of 1.181 due to the cable and an additional 2.702 dB due
to the SWR for a total loss of 3.883 dB.
The same antenna fed with 1/2 inch 75 ohm hard line has a loss of .420 DB
due to the cable and an additional .032 dB due to SWR for a total loss of
452 dB.
Incidentally, I chose purely resistive loads just to simplify things. The
program works with complex loads as well. If anyone would like a specific
calculation you can eMail me. All that is needed is the type cable, the
length of the cable, the frequency and the load, (resistive and
inductive).
Regards, Merv
Dick Hughes
On 26 Oct 1996 20:36:08 GMT, ha...@aztec.asu.edu (CHARLES J. MICHAELS)
wrote:
Right on Charlie. The SWR is determined by the load! It does not
change just because you change the length of the feed line. Not sure
if we will ever get this across.
Dick Hughes - W6CCD
Jim Potter
w7...@teleport.com (Roy Lewallen) wrote:
>There's one more way that cable length can affect VSWR. If common-mode
>current is present (that is, current is flowing on the outside of coax, or
>the currents in twinlead aren't equal and opposite), the feedline becomes
>part of the antenna. In that situation, changing the length of the feedline
>changes the antenna itself, which will affect VSWR.
>
>VSWR can seem to change with cable length if the characteristic impedance
>of the VSWR meter isn't the same as that of the cable. I've measured RG-58
>at over 60 ohms Z0, and a couple of cheap SWR meters I've looked at were
>off also, so I imagine this is a fairly common occurrence. In this case,
>the SWR doesn't really change with feedline length, but the SWR meter says
>it does.
>
>Roy Lewallen, W7EL
I agree with your comment on common mode currents. Also, with your
comment about the directional coupler. What it really amounts to is
that if the coupler has poor directivity (equivalent to not being
balanced for the impedance of the cable being used), some of the
forward wave is coupled into the reflected port. The resultant
apparent reflected signal depends on the relative phases of the
forward and reflected power and thus on the position of the coupler in
the standing wave. This phenomenon can also make the VSWR look better
than it really is if the two waves tend to cancel.
This discussion is a good example of the Internet at work. It's nice
to see that the ham radio newsgroup generally seems to be populated by
considerate people and not the nutcakes that wase bandwidth on a
couple of other newsgroups I follow.
73 de K9GXC, Jim
Cecil Moore
Jim Potter <jpo...@jpaw.com> writes:
>to see that the ham radio newsgroup generally seems to be populated by
>considerate people and not the nutcakes that wase bandwidth on a
>couple of other newsgroups I follow.
altavoz
James Pollock wrote:
>
> Hi.
>
> I have a friend who swears up and down that the length of the
> coaxial cable to maintain resonance at a given frequency is important
> for reducing the SWR. He feels that it acts in a similar principal to
> the fact that the elements of an antenna must be cut to the correct
> length for the antenna to be resonant at a given frequency.
>
> It seems to me I have also read somewhere that this is a myth. I
> need the true scoop on this. I'm preparing to put up a pretty good
> antenna system on my roof (a homebrew 10 meter antenna cut to exactly
> the middle of the band), and don't want to purchase an extra 6 ft of
> Belden 9913 coax only to have it looped up and losing signal, when in
> reality I could of used 20 feet instead of 26 feet of cable. Jim
altavoz: 9913 !!! on 30 MHZ NOT !! RG8FOAM
You'll see a .7 db/100' to .9db/100' increase in attenuation
going to RG8foam . WOW , BIG LOSS ( NOT)
Stick to radiation pattern adjustments , not splitting hairs
over coax.
Read "amatuer radio vert' antenna handbook" By CAPT Paul H.Lee
deceased (N6PL) . Its the greatest.
______End of text from altavoz___________
Gary Coffman
In article <54rg26$s...@opera.iinet.net.au> rich...@iinet.net.au writes:
>Jim
>If the coax is matched (ie the load presents a resistive, 50 Ohm
>impedance) then the length of the coax is unimportant, except that it
>has loss depending on how good it is and the operating frequency. If
>the load is not matched properly then there will be different
>impedances at different points aliong the coax and you will get a
>different SWR with a 50 ohm source, depending on where you feed the
>coax. So your friend is correct if the load is not m,atched
>accurately.
No, the SWR is constant, and is a function of the mismatch at the
load end only. That constant SWR is what causes the presented
impedance variation along the line (due to transmission line
transformer action). If you insert the SWR meter anywhere along
the line, it will read the same SWR (discounting line loss). A
SWR meter reads SWR, not impedance.
Internally, a SWR meter is a 50 ohm line section, so it merely
acts as a short extension to the cable being measured. (If you
try to measure 75 ohm line with a 50 ohm SWR meter, however,
you'll be in for some surprises.)
Conversely, if you have an operating impedance bridge, you can
measure the impedance at any point along the line, and given
that, the cable length from the load to that point, and a Smith
Chart, you can determine the SWR on the line (discounting line
loss).
Now if the line has loss, and all real lines do, then you have
to add a fourth variable, and the system becomes more complex.
Instead of a constant SWR circle on the Smith Chart, now you have
to follow a SWR spiral which approaches 1:1 as cable length tends
toward infinity. The approach is asymptotic, and if cable loss is
high, will quickly approach 1:1, but never quite reaches 1:1. If
loss is low, the approach will be very gradual, and for moderate
line lengths can be assumed to be a constant SWR circle.
(Incidently, this is a convienent way to measure line loss,
just place a short or open at the load end of the cable and
read the SWR at the source end, the closer to 1:1 it is, the
higher the cable loss. There's a formula in the Handbook which
will give you the exact loss in db for a given SWR and line
length.)
For moderate lengths of good cable at HF (good cable, not RG-58),
cable loss can be discounted and the SWR circle can be assumed
nearly constant. At VHF+ however, even with good cables loss may
become significant when the length exceeds several wavelengths
and must be considered when analyzing line behavior.
This means that a low SWR is not necessarily a good thing. It
can indicate a lossy system rather than indicating a good match.
One way to differentiate is to look at the SWR curve over a band
of frequencies. If it is broad and flat, the cable is probably
lossy (or you have a dummy load for a termination). Real antennas
are typically only close to design impedance over a narrow frequency
range (typically less than 5% around the design frequency), so the
SWR curve should quickly rise as it moves away from the center
frequency of that range. Anyone who says he has a broad flat SWR
is probably really telling you he has a pretty good dummy load
for an antenna system.
(Note there are exceptions, antennas such as discones and log
periodics present a near constant impedance over a large frequency
excursion. But in general a low antenna system Q is indicative of
low efficiency and high loss, IE the behavior of a dummy load.)
altavoz
Jim Devenport wrote:
>
> James Pollock wrote:
> <snippety>
> > It seems to me I have also read somewhere that this is a myth. I
> > need the true scoop on this. I'm preparing to put up a pretty good
> > antenna system on my roof (a homebrew 10 meter antenna cut to exactly
> > the middle of the band), and don't want to purchase an extra 6 ft of
> > Belden 9913 coax only to have it looped up and losing signal, when in
> > reality I could of used 20 feet instead of 26 feet of cable.
>
> Jim, in addition to the responses you've already received I want to add
> the certainty that even with RG-58 the "26 feet of cable" would result
> in essentially NO extra noticeable loss than "20 feet of cable". And
> with Belden 9913 the additional loss would be even more miniscule.
> According to my charts, the losses of these cables are: (per 100 feet @
> 30 MHZ)
> RG-58 2.5 dB
> Belden 9913 0.75 dB (!)
> RG-8 1.0 dB
>
> So, even if you were using the dreaded RG58 the loss would be less than
> a fraction of a dB for the additional 6 feet or so of coax. Even with a
> high SWR (where the reflections cause more than one trip "up and down
> the coax" and thus increase the effective loss) 26 feet is not a big
> deal at all. 3 dB is generally regarded as a barely discernable
> difference on the receiving end without accurate test equipment to
> "help" notice the difference.
> As your previous respondees have noted, if the SWR is high you CAN
> "fool" the transmitter into "seeing" a lower SWR by messing with coax
> length
altavoz: No, you cant fool the transmitter, the SWR will not change
on changing coax length.
but far more will be gained by trying to lower the SWR up at the
> antenna FEEDPOINT which will not only make your transmitter happier but
> increase apparent antenna effectiveness too, both transmit and receive.
> "SWR" is SO overhyped as important anyway. I fondly recall in the '70's
> working all kinds of DX on 10 meters with a homebrew 2 (TWO!) meter J
> pole since I was a poor college student and didn't have the nerve to
> sneak HF wires onto the roof of my college housing.....
> I suppose it worked so well because I didn't own an SWR meter, just
> heard the band was open and tried the J pole since that was all I had on
> the roof and had a blast with it, who knows what the SWR was, but I'd
> have had to QRT had I had a meter to scare myself with.....Jim Devenport WB5AOX
altavoz
> It's possible to use a cable with an impedance not
> equal to the characteristic impedance of your
> antenna and rig. For example, a half wave length
> of 75 ohm cable will work between a 50 ohm antenna
> and a 50 ohm transmitter. You can also use 75 ohm
> cable to match a 100 ohm antenna to a 50 ohm rig
> by cutting it to the right length. Unfortunately,
> these tricks generally only work at one frequency.
>
> In this case, the length of the cable is critical
> to a given frequency.
>
> Steve KT4FY
altavoz: No, a 50 ohm ant' feeding a 75ohm coax will
cause a SWR into coax and towards the radio and an exact
length of coax will not reduce the SWR except as
attenuation factor over that length.
altavoz
K1BQT wrote:
>
> Hi Jim--
>
> In your inquiery, you wrote:
>
> I'm preparing to put up a pretty good
> antenna system on my roof (a homebrew 10 meter antenna cut to exactly
> the middle of the band), and don't want to purchase an extra 6 ft of
> Belden 9913 coax only to have it looped up and losing signal, when in
> reality I could of used 20 feet instead of 26 feet of cable.
>
>
> As long as the antenna's feedpoint impedance somewhat matches the
> impedance of the line, it doesn't really matter how long your cable is. It
> is true that--as the antenna impedance changes (which it
> will--constantly--across the band), the coax may take on the properties of
> an impedance transformer. That is, it may take a 40-Ohm load presented by
> the antenna and make it look like 60 Ohms at the transmitter--or whatever.
> As a practical matter, this isn't a problem to lose sleep over unless the
> mis-match becomes extremely large.
>
> You could cut your feedline to a multiple of an electrical
> half-wavelength. This will ensure that the transmitter sees exactly the
> SAME impedance that is occuring at the antenna feedpoint. But, once
> again, what's the point? The antenna is going to change impedance anyway
> across its range--and your transmitter can cope. Normally, the only time
> someone would use a multiple of a half-wavelength of feedline is when they
> want to measure the feedpoint characteristics of the antenna from the
> ground for documentation purposes.
>
> By the same token, you'll be wasting money to use 9913 at 30 MHz over a
> 20-foot run. Go down to Radio Shack and get some RG8 mini. The
> difference in loss will be barely measurable (measured in hundredths of a
> dB) and meaningless.
>
> My advice would be--don't sweat the small stuff. Instead, pay attention
> to the things that will really matter--like radiation pattern on the
> horizon, radiator efficiency, gain, and separation from lossy or
> interfering media (ie other antennas--or roof flashing, aluminum-backed
> insulation, and house wiring).
>
> Good luck with it!
>
> Rick K1BQT
altavoz: well said, N6PL Paul H. Lee had a book on verticals
that shows how to "wrap" a 2:1 SWR at 80 meters to cover
the whole band . It will work at any frequency . Passive
components only .
altavoz
Dave Hockaday wrote:
>
> James Pollock wrote:
> > I have a friend who swears up and down that the length of the
> > coaxial cable to maintain resonance at a given frequency is important
> > for reducing the SWR. He feels that it acts in a similar principal to
> > the fact that the elements of an antenna must be cut to the correct
> > length for the antenna to be resonant at a given frequency.
>
> the antenna/transmitter. If you use 50 ohm feedline with a 50 ohm
> antenna/transmitter, any length is acceptable. If you substituted say, 75
> ohm feedline, you would need to cut the feedline in 1/2 wl multiples (the
> impedance is repeated every 1/2 wl).
>
> Dave Hockaday WB4IUY
altavoz: You dont need to cut in 1/2 wave multiples
since it wont change the SWR except thru a very small
increase due to decreasing attenuation over length.
H.F. has very low attenuation on RG8
mac...@ibm.net
In <327156...@ccm.ch.intel.com>, Cecil Moore <Cecil_...@ccm.ch.intel.com> writes:
>Joel Gamble wrote:
>> The ONLY time coax length makes a difference is when it is radiating
>> rf,thus it becomes an active part of the antenna,instead of the passive
>> conduit it is designed to be.
>
>Hi Joel, got to nitpick a tad.
>
>1. 100ft of RG-58 on 440MHz with an SWR of 1:1 and *no feedline radiation*
>does make a difference.
>
>does make a difference.
>
Cecil,
Have they just recently started calling 440 Mhz HF? :-))) <- beard
Jim WD9AHF
Merv Stump
of coax question:
At 28 MHz an antenna with an impedance of 70R +4J fed with 11.597 feet of
RG213(1/2 wave length at 28 MHz) has an impedance at the input of 69.28R
+3.76J. Virtually identical to the load, but even a halfwave of cable
does have some transforming characteristics due to it's loses. TLA
computes the SWR at load as 1.41/1 and at the input as 1.40/1.
The same antenna fed with 5.798 feet of RG213 (1/4 wave length at 28MHz)
has an impedance at the load of 35.77R -2.35J. An obvious transformation
of the impedance, however, the SWR is still 1.41/1.
If you feed the antenna with 1/2 wave of PERFECT COAX,(NO LOSSES) TLA
computes the impedance at input as 70.02R + 3.79J (The difference between
this and 70R and 4J can be attributed to rounding.)
Regards, Merv
Cecil Moore
in his statement above.
73, Cecil, W6RCA, OOTC
K1BQT
altavoz: well said, N6PL Paul H. Lee had a book on verticals
that shows how to "wrap" a 2:1 SWR at 80 meters to cover
the whole band . It will work at any frequency . Passive
components only .
You can do this--but with significant reduction in efficiency as you move
away from Fr (see Frank Witt's work with 80-meter dipoles in the ARRL
Antenna Handbook for more on this). The point I wanted to make is that
impedance may change with line length, but true VSWR will not (save for
the influence of line loss). That is to say, 25 Ohms may be transformed
to 100 Ohms--or to any complex impedance on the Smith Chart in between--by
the line, but the VSWR will remain 2:1 and the transmitter will probably
handle it.
The only consideration that could make a difference is--if the
transmitter's ALC uses a diode voltage-detectior referenced to a desired
power ouput into a 50 Ohm load, the transmitter will try to deliver more
power into the 25 Ohm load than into the 100 Ohm load.
Rick K1BQT
altavoz
Cecil Moore wrote:
>
> altavoz <alt...@worldnet.att.net> writes:
>
> >altavoz: No, a 50 ohm ant' feeding a 75ohm coax will
> >cause a SWR into coax and towards the radio and an exact
> >length of coax will not reduce the SWR except as
> >attenuation factor over that length.
>
> trying to say is if one has a 50 ohm antenna and feeds
> it with a multiple of a halfwavelength of 75 ohm coax,
> then the 50 ohm SWR meter at the transmitter will read
> 1:1 and the transmitter will see 50 ohms. There will
> be a conjugate match at the 75 to 50 ohm transition
> point and all the reflected energy will be re-reflected
> back towards the antenna at that point. With no tuner
> in the circuit, any length of coax other than a multiple
> of halfwavelengths will upset the conjugate match and
> allow reflected energy to be dissipated in the protection
> circuitry or the final amp itself. So there is a magic
> length that the 75 ohm coax can be trimmed to to achieve
> the desired conjugate match.
>
> This can be visualized as two quarter wave transformers.
> The first one transforms the 50 ohm antenna impedance
> to 100 ohms in the middle of the halfwavelength of 75
> ohm coax and the second one transforms that 100 ohms
> back to 50 ohms, making the transmitter happy.
>
> 73, Cecil, W6RCA, OOTC
altavoz: Beatiful ! But absolutely wrong !
A TRANSMISSION LINE ,REGARDLESS OF LENGTH, TERMINATED IN A
RESISTANCE NOT EQUAL TO ITS Z0 WILL HAVE A UNIFORM SWR ALONG
ITS LENGTH WHICH DECREASES SLIGTHLY THRU ATTENUATION OF
THE LINE. Half wave sections dont match anything, only
1/4w sections match unequal sections.
altavoz
Cecil Moore wrote:
>
> Joel Gamble wrote:
> > The ONLY time coax length makes a difference is when it is radiating
> > conduit it is designed to be.
>
> Hi Joel, got to nitpick a tad.
>
> 1. 100ft of RG-58 on 440MHz with an SWR of 1:1 and *no feedline radiation*
> does make a difference.
altavoz: I get 271 "1/4 wavelengths" and that makes it an ODD multiple
of 1/4w and therefore it will radiate cause the open end of the coax
will see a short on the outside end of the coax at the radio end.
.66 VF RG58 , assuming a high SWR at load and 1:1 at radio.
> 2. 100ft of RG-58 on 28MHz with an SWR of 20:1 and *no feedline radiation*
> does make a difference.
altavoz: The diff we're talking of is trimming the line and in your
case trimming the line will increase the RF ( feedline radiation)
on the outside of the coax By pulling down the Z seen on the outside
by the antenna . Trimming 1/4w will peak the FEEDLINE RADIATION.
> 73, Cecil, W6RCA, OOTC (not speaking for my employer)
--
Cecil Moore
CHARLES J. MICHAELS
Re this discussion about SWR and feeder length:
While it is indeed true that SWR is fairly constant with
feeder length except for a decrease with length due to attenuation,
the impedance seen at the line input differs wtih each length and
some impedances may be more acceptable to the transmitter output
circuits than others. This is particularly true of pi output
circuits found in most tube amplifiers.
Charlie, W7XC
--
Gary Coffman
In article <3272E1...@worldnet.att.net> altavoz <alt...@worldnet.att.net> writes:
>> It's possible to use a cable with an impedance not
>> equal to the characteristic impedance of your
>> antenna and rig. For example, a half wave length
>> of 75 ohm cable will work between a 50 ohm antenna
>> and a 50 ohm transmitter. You can also use 75 ohm
>> cable to match a 100 ohm antenna to a 50 ohm rig
>> by cutting it to the right length. Unfortunately,
>> these tricks generally only work at one frequency.
>>
>> In this case, the length of the cable is critical
>> to a given frequency.
>>
>> Steve KT4FY
>
>cause a SWR into coax and towards the radio and an exact
>length of coax will not reduce the SWR except as
>attenuation factor over that length.
You two are talking about different things. Sure, the SWR on
the cable will not be unity, but if you cut the cable to an
electrical halfwave (or multiple), you *don't care* because
the impedance seen by the source is a repetition of the
impedance presented by the load. The source doesn't care
about SWR on the cable, it only cares that it sees the
correct impedance presented to it.
In the case of a 100 ohm load and a 50 ohm source, an electrical
1/4-wave of 75 ohm cable will transform the load impedance to
50 ohms at the source end of the cable. The source will be happy,
the load will be happy, and we'll be real happy there is SWR on
the cable, because it is this SWR which makes the transformation
happen. A "high" SWR isn't always bad, in fact it can be downright
useful at times.
Mandy Wright
In article <54trpv$p...@sjx-ixn9.ix.netcom.com>, Anthony Severdia
<anth...@ix.netcom.com> writes
>
> And the answer is ......(?) :)
>
> -=Tony=- W6ANV
If the transmitter impedence equals the transmission line impedence then
the returning wave, seeing a perfect match, is disippated, warming up
the amplifier.
I suggest further reading
William Orr - Radio Handbook Section on Antennas and Matching Systems.
This will explain how a conjugate match at the transmitter end of the
feeder can be used to return the standing wave to the antenna, the
transmitter then seeing a perfect load.
_Warning_ The VSWR on the feeder remains the same.
Gross mis-matching of the load in high power equipment will cause very
high standing waves, which at alternating 1/4 wave intervals along the
line, will lead to high current and local heating, or high voltage and
insulation breakdown ;)
Mandy
(Replying on behalf of my husband, Tim (G1BCR / G9BZW), who hates computers
despite (or maybe because of) being a practising RF Engineer.)
(He's going to *have* to learn to type!) Sussex UK
A N Thompson
Cecil was right when he stated that a half-wavelength
of 75 ohm coax would produce a 1:1 match for a 50
ohm source when the coax is connected to a 50 ohm
load. See the ARRL Antenna Book, 17th ed., p. 24-12:
"When the line length is an even multiple of 90º (that
is, a multiple of ½ wavelength), the input resistance
is equal to the load resistance, regardless of the
line Zo... It does not matter whether the impedence
...is resistive, reactive, or a combination of both."
( I actually tested this experimentally tonight with a
resistive load and the results confirmed the theory).
Altavoz's assertion that "only 1/4 wave sections match
unequal sections" is incorrect. Quarter-wavelength
matching sections are a special case of series-section
transformers. Read "Series-Section Transformers" in
the same ARRL Antenna Book, pp. 26-15.
73,
Arliss
W7XU
>Date: Sun, 27 Oct 1996 14:10:35 -0800
>From: altavoz <alt...@worldnet.att.net>
>Subject: Importance of coax length on HF A myth?
>> altavoz <alt...@worldnet.att.net> writes:
>>
>> >altavoz: No, a 50 ohm ant' feeding a 75ohm coax will
>> >cause a SWR into coax and towards the radio and an exact
>> >length of coax will not reduce the SWR except as
>> >attenuation factor over that length.
>>
>> Most people know this is true already. What Steve is
>> trying to say is if one has a 50 ohm antenna and feeds
>> it with a multiple of a halfwavelength of 75 ohm coax,
>> then the 50 ohm SWR meter at the transmitter will read
>> 1:1 and the transmitter will see 50 ohms.
(portions deleted)
Cecil Moore
altavoz wrote:
> altavoz: Beatiful ! But absolutely wrong !
Hi Altavoz, Sorry to have to bust your bubble but a half-
wavelength of any impedance transmission line faithfully
reproduces the impedance of the load (minus losses). With
a 50 ohm antenna and no losses in the transmission line,
the following is true.
Transmission line SWR Impedance seen at the transmitter
1/2 WL of 50 ohm 1:1 50 ohms
1/2 WL of 75 ohm 1.5:1 50 ohms
1/2 WL of 300 ohm 6:1 50 ohms
1/2 WL of 450 ohm 9:1 50 ohms
So you see a half-wave section matches the impedance of the
antenna regardless of the magnitude of the SWR. If the SWR
is anything other than 1:1, a half-wavelength (or multiple)
is the *only* length that will match the 50 ohms of the
antenna.
altavoz
altavoz: My mistake . YOU ARE RIGHT . The Z from a 50 ohm
load will go up as you move towards the transmiter on coax
thats higher than 50ohm and will go lower on coax thats less
then 50ohms. At 1/4w it will be highest and resistive ( geometric
mean) and at the half wave point ,AS YOU HAVE VERY WELL SHOWN,
it is the same as the load since a half wave looks like a series
resanant circuit .
Kok Chen
altavoz wrote:
>Cecil Moore wrote:
>>
>> Most people know this is true already. What Steve is
>> trying to say is if one has a 50 ohm antenna and feeds
>> it with a multiple of a halfwavelength of 75 ohm coax,
>> then the 50 ohm SWR meter at the transmitter will read
>> 1:1 and the transmitter will see 50 ohms....
>> So there is a magic
>> length that the 75 ohm coax can be trimmed to to achieve
>> the desired conjugate match.
>
>altavoz: Beatiful ! But absolutely wrong !
>
>A TRANSMISSION LINE ,REGARDLESS OF LENGTH, TERMINATED IN A
>RESISTANCE NOT EQUAL TO ITS Z0 WILL HAVE A UNIFORM SWR ALONG
>ITS LENGTH WHICH DECREASES SLIGTHLY THRU ATTENUATION OF
>THE LINE. Half wave sections dont match anything, only
>1/4w sections match unequal sections.
Cecil is (again) correct.
I did not see Cecil make any claim that the VSWR in the 75-ohm LINE
is 1:1. All he said was that if you terminate one end of a 1/2
wavelength 75-ohm transmission line with 50+i0 ohm load that you will
see 50+i0 ohm at the other end.
Indeed, you can use a 1/2 wavelength line of any impedance.
Just visualise going around a Smith chart in a concentric circle, eh?
73
Kok Chen, AA6TY kc...@apple.com
Apple Computer, Inc.
Cecil Moore
altavoz wrote:
> ... therefore it will radiate cause the open end of the coax
> will see a short on the outside end of the coax at the radio end.
How is the RF current going to get past the choke?
Cecil Moore
altavoz wrote:
> No, you can't fool the transmitter...
and later altavoz wrote:
> The 1/4w 70 ohm coax converts the 100 (ohm) antenna to
> 50 ohms to please the radio.
So Altavoz, please tell us the difference between "fooling
the transmitter" and "pleasing the radio"?
Cecil Moore
Cecil Moore wrote:
> So Altavoz, please tell us the difference between "fooling
> the transmitter" and "pleasing the radio"?
My apologies, I posted this before I received the following:
>altavoz: My mistake . YOU ARE RIGHT . The Z from a 50 ohm
>load will go up as you move towards the transmiter on coax
>thats higher than 50ohm and will go lower on coax thats less
>then 50ohms.
Please ignore the previous posting. I'll be glad when netnews
is real-time.
Dave Hand
altavoz wrote:
>
> > It's possible to use a cable with an impedance not
> > equal to the characteristic impedance of your
> > antenna and rig. For example, a half wave length
> > of 75 ohm cable will work between a 50 ohm antenna
> > and a 50 ohm transmitter. You can also use 75 ohm
> > cable to match a 100 ohm antenna to a 50 ohm rig
> > by cutting it to the right length. Unfortunately,
> > these tricks generally only work at one frequency.
> >
> > In this case, the length of the cable is critical
> > to a given frequency.
> >
> > Steve KT4FY
>
> cause a SWR into coax and towards the radio and an exact
> length of coax will not reduce the SWR except as
> attenuation factor over that length.
>
>
Wrong O buddy boy.
An electrical 1/2 of coax is a tried and true method of mesuring
the SWR at the antenna feed point remotely for exactly that reason.
Its totally invisible to the remote test gear. If you doubt it try it!
Dave Hand WB4HYP
altavoz
Cecil Moore wrote:
>
> altavoz wrote:
> > ... therefore it will radiate cause the open end of the coax
> > will see a short on the outside end of the coax at the radio end.
> > ...Trimming 1/4w will peak the FEEDLINE RADIATION.
>
> How is the RF current going to get past the choke?
>
altavoz: You're assuming a choke . The thread makes more sense
if you dont assume a choke ( i went all the way back to JOEL
GAMBLEs post)
I think he's saying a 440 signal into RG58 is a 1:1 NO MATTER
WHAT cause it's 100 feet long and that means it could be an
open circuit . I think he tried to get one past us by making
us assume that there was a termination .
Anyway the way to kill it is BALUN,steel wool, ferrite,
cut coax to a particular length ( narrow BW), or offset feed
the antenna ( bal ant has an unbal' point where you can hook
coax, and pattern does not suffer ).
What i find amasing is so many novices worry about the
match , but not the radiation pattern . You can match
anything, even a 1/2w by stretching it a little to bring
down the R and that 2nd lobe is too small to hurt .
Radiation pattern on a J pole is bad but has a nice
stub to match it....so build a 5/8w using that stub to match
it. Problem is the standard way ( "L" )is just as simple.
Thanks for correcting my sloppy thinking, 73's
altavoz
altavoz: The reduction in eff' is in the caps/coils at the
feed point ( except for pattern change) and that was as
good as you could do in those days . Today , an AUTO
TUNER is a smaller part of a persons
paycheck compared to 20 years ago . Thats the way
i do it since i found a $150 ICOM auto tune, then found
a $40 one at swap meet. With cap hat, 160 thru 10 .
However , i dont do much HF ,as i find very few want
to talk tech on HF.
I think IC735 actually reads SWR ?
Not talking at you , just broadcasting to the group....
HF can handle tuner at either end of RG8 ( FOAM is not required)
VHF absolutely needs RG8 FOAM and tuner at the antenna. A VHF
1/4w vert' with radials bent down about 45 deg is a perfect
50 OHM match to coax, but pattern or GAIN is better if you bend
down all the way . And RINGO RANGER II is the best ant for the
money ( flea mkt $30, 2 5/8 waves colinear)
altavoz
Kok Chen wrote:
>
> altavoz wrote:
>
> >Cecil Moore wrote:
> >>
> >> Most people know this is true already. What Steve is
> >> trying to say is if one has a 50 ohm antenna and feeds
> >> it with a multiple of a halfwavelength of 75 ohm coax,
> >> then the 50 ohm SWR meter at the transmitter will read
> >> 1:1 and the transmitter will see 50 ohms....
> >> So there is a magic
> >> length that the 75 ohm coax can be trimmed to to achieve
> >> the desired conjugate match.
altavoz: No , its not a conjugate match . A 1/2w is not a conjugate
match . Conjugate means to change the sign of the REACTANCE.
There's no sign change in a 1/2w .
> Just visualise going around a Smith chart in a concentric circle, eh?
> Kok Chen, AA6TY kc...@apple.com
> Apple Computer, Inc.
altavoz: Yes i use my Smith chart often thats how i corrected
myself on the 1/2w and notified him that i had made a mistake.
macino@ibm.net@smtp-news2.ibm.net
In <19961028.0753...@juno.com>, w7...@juno.COM (A N Thompson) writes:
>Cecil was right when he stated that a half-wavelength
>of 75 ohm coax would produce a 1:1 match for a 50
>ohm source when the coax is connected to a 50 ohm
>load. See the ARRL Antenna Book, 17th ed., p. 24-12:
> "When the line length is an even multiple of 90º (that
> is, a multiple of ½ wavelength), the input resistance
> is equal to the load resistance, regardless of the
> line Zo... It does not matter whether the impedence
> ...is resistive, reactive, or a combination of both."
>( I actually tested this experimentally tonight with a
>resistive load and the results confirmed the theory).
>
>Altavoz's assertion that "only 1/4 wave sections match
>unequal sections" is incorrect. Quarter-wavelength
>matching sections are a special case of series-section
>transformers. Read "Series-Section Transformers" in
>the same ARRL Antenna Book, pp. 26-15.
>
>73,
>
>Arliss
>W7XU
>
>>Date: Sun, 27 Oct 1996 14:10:35 -0800
>>From: altavoz <alt...@worldnet.att.net>
>>Subject: Importance of coax length on HF A myth?
>
>>> altavoz <alt...@worldnet.att.net> writes:
>>>
>>> >cause a SWR into coax and towards the radio and an exact
>>> >length of coax will not reduce the SWR except as
>>> >attenuation factor over that length.
>>>
>>> trying to say is if one has a 50 ohm antenna and feeds
>>> it with a multiple of a halfwavelength of 75 ohm coax,
>>> then the 50 ohm SWR meter at the transmitter will read
>
> (portions deleted)
>
>>> 73, Cecil, W6RCA, OOTC
>>
>>
>>
>>A TRANSMISSION LINE ,REGARDLESS OF LENGTH, TERMINATED IN A
>>RESISTANCE NOT EQUAL TO ITS Z0 WILL HAVE A UNIFORM SWR ALONG
>>ITS LENGTH WHICH DECREASES SLIGTHLY THRU ATTENUATION OF
>>THE LINE. Half wave sections dont match anything, only
>>1/4w sections match unequal sections.
Cecil is correct on this one. Experiment with an electrical halfwave length and
you will see. Remember, that bandwidth over which this occurs is pretty narrow.
I use a 3/4 inch 75 ohm feedline between a 4-1K and my Mosely yagi some 150'
feet away. SWR=flat, but I don't use that feed more than 5 KC up or down from
14.183. It works very well, well enough that I don't use an antenna tuner at all
on that setup.
- Jim
Cecil Moore
Mandy Wright <Ma...@mrwolf.demon.co.uk> writes:
>If the transmitter impedence equals the transmission line impedence then
>the returning wave, seeing a perfect match, is disippated, warming up
>the amplifier.
said this was not the case in "Reflections". Mr Maxwell wrote
me a nice letter explaining that in his book, he was always
referring to tube type transmitters with pi-net outputs
that had achieved a Zg-match. For transmitters with fixed
50 ohm outputs, it would be a Z0-match. For either a Zg or
Z0-match, there is a change in impedance. It does seem safe
to say that if there's no change in impedance, reflected
energy will not be re-reflected. Of course, some of the
reflected energy is often used by the protection circuitry
to decrease the output of the amplifier, thus reducing the
reflected energy.
If one understands what happens to the reflected energy in
the following example, one has a handle on the physics.
----50 ohm coax----+----0.5WL 300 ohm line----+--50 ohm load
A B
Reflections occur at point B. When the reflected energy
gets back to point A, it sees a Z0-match and is 100%
re-reflected at that point. Physically, it is exactly
the same thing that happens to light in a reflection-
less picture frame. No reflected energy flows in the
50 ohm coax in the above example.
73, Cecil, W6RCA, OOTC
W8JI Tom
In article <3274F6...@worldnet.att.net>, altavoz
<alt...@worldnet.att.net> writes:
> Anyway the way to kill it is BALUN,steel wool, ferrite,
>cut coax to a particular length ( narrow BW), or offset feed
>the antenna ( bal ant has an unbal' point where you can hook
>coax, and pattern does not suffer ).
Did you say a steel wool balun? Now we are talking LOW frequency, like
below audio!
Did you say a balanced antenna has a unbal point where you can hook it up
with no pattern distortion? Now we are talking an impossible to find
point.
73 Tom
W8JI Tom
In article <327527...@microdes.com>, Dave Hand <dh...@microdes.com>
writes:
>
>Wrong O buddy boy.
>
>An electrical 1/2 of coax is a tried and true method of mesuring
>the SWR at the antenna feed point remotely for exactly that reason.
>Its totally invisible to the remote test gear. If you doubt it try it!
>
>Dave Hand WB4HYP
Sorry Dave, but he's right.
The length of line does NOT change the SWR along the line, except by
attenuation and various equipment shortcomings.
73 Tom
altavoz
Cecil Moore wrote:
>
> altavoz wrote:
> > match . Conjugate means to change the sign of the REACTANCE.
> > There's no sign change in a 1/2w .
>
>
> ---50ohm coax---A---1/2WL 75ohm coax---B---50ohm load
>
> Are there reflections in the 75 ohm coax? yes
> Are there reflections in the 50 ohm coax? no
> Sure looks like there is a Z0 match at point A
> which forces a conjugate match in the rest of
> the system. Remember the conjugate of 50+j0
> is 50-j0. :-)
>
> 73, Cecil, W6RCA, OOTC (not speaking for my employer)
altavoz: page 16-11 ARRl " Conjugate matching is obtained
thru a controlled wave interference between 2 sets of
reflected waves.."
In your case there is no intertference and NO CONJUGATE MATCH.
--
Dick Hughes
I think all of you are right. It's just how you are expressing it.
Dave is merely saying that an electrical 1/2 wave of coax will mirror
what it see's at one end to the other end.
To quote the good book, "... a line an exact multiple of 1/2 wave in
length (disregarding line losses) simply repeats, at its input or
sending end, whatever impedance exists at its output or receiving end.
It does not matter whether the impedance at the receiving end is
resistive, reactive, or a combination of both. Sections of line
having such length can be cut in or out without changing any of the
operating conditions..." 15th Addition of The ARRL Antenna Book.
Dick Hughes - W6CCD
Cecil Moore
miker in Tigard, OR.
Dick Hughes wrote:
>
> On Thu, 24 Oct 1996 14:07:27 GMT, ga...@ke4zv.atl.ga.us (Gary Coffman)
> wrote:
>
> >In article <54nc7a$s...@lily.redrose.net> jp...@redrose.net (James Pollock) writes:
> >> I have a friend who swears up and down that the length of the
> >>coaxial cable to maintain resonance at a given frequency is important
> >>for reducing the SWR. He feels that it acts in a similar principal to
> >>the fact that the elements of an antenna must be cut to the correct
> >>length for the antenna to be resonant at a given frequency.
> >>
> >> It seems to me I have also read somewhere that this is a myth. I
> >>need the true scoop on this. I'm preparing to put up a pretty good
> >>antenna system on my roof (a homebrew 10 meter antenna cut to exactly
> >>the middle of the band), and don't want to purchase an extra 6 ft of
> >>Belden 9913 coax only to have it looped up and losing signal, when in
> >>reality I could of used 20 feet instead of 26 feet of cable.
> >
> >Ok, this can get complex because there are several interrelated
> >things happening here. Lets take them one at a time.
> >
> >The only thing that determines SWR on a transmission line is
> >the ratio of the characteristic impedance of the line to the
> >impedance presented by *the load*. Nothing you can do at the
> >generator end can affect SWR, and changing the length of the
> >cable won't change SWR either (but see below).
>
> <huge snip>
>
> >Gary
>
> In general, the correct length of coax for an HF antenna is the
> distance it takes to get from the transmitter to the antenna. Don't
> worry about length.
>
> Dick Hughes - W6CCD
There's an important issue that has not been mentioned.
If your SWR is 1:1, coax length is unimportant.
If it's not, then the impedance looking into the coax is a function
of the length. Draw a (constant SWR) circle on a Smith Chart. As you
change length,
you move around that circle. As long as your transmiter can match
any impedance on the circle, you're OK. With a high SWR or with a
modern no-tune rig, you might find that the rig behaves significantly
differently depending on the coax length. SWR doesn't change, but
the matching network required to drive the coax sure does.
Having said that, I suggest you put your effort into making the antenna
1:1 rather than messing with the coax length.
miker
Cecil Moore
altavoz <alt...@worldnet.att.net> writes:
>altavoz: page 16-11 ARRl " Conjugate matching is obtained
>thru a controlled wave interference between 2 sets of
>reflected waves.."
example there is a controlled wave interference between
two sets of reflected waves in the 75 ohm coax. One set
sees the 50 ohm load impedance and the other set sees
a virtual open or short circuit at the impedance bump
a half-wavelength away. That, my friend, is a conjugate
match in anybody's book. All it takes for a conjugate
match is to see conjugates looking both directions up
and down a transmission line. 50+j0 and 50-j0 satisfies
that requirement.
73, Cecil, W6RCA, OOTC
altavoz
altavoz: Impossible ? Sorry to hear that. Just look at the way
the J pole matches and you'll grasp it.
If your 2meter yagi put a dime sized spot on a point
10 miles down the road and one day you had PATTERN DISTORTION,
you could justifiably be upset ( gain from 100db to zero db) ,
but in antennas we dont get upset on a small pattern
change that can be compensated for.
It will still be the familiar doughnut with very minor changes.
A .6 wl ant must have a .6 wl radial system.
mor...@ipers1.e-technik.uni-stuttgart.de
In article <326fc364...@news.efn.org>,
Dick Hughes <dhu...@efn.org> wrote:
>In general, the correct length of coax for an HF antenna is the
>distance it takes to get from the transmitter to the antenna. Don't
>worry about length.
Please do not disregard situations where the line can be used for
matching. Take a quad loop, with 100 Ohms input impedance.
A quaterwave of 75 Ohms line, with a few coiled turns for a balun
will save you an antenna tuner. And there a other situations
where matching is simplified (less losses) when using a defined
length of line for tranformation.
73, Moritz DL5UH
macino@ibm.net@smtp-news2.ibm.net
Excellent example here. I fed a triband quad with this method for several years.
It was the only way I could get a GEM Quad to play with a single 50 ohm coax to
three tuned 75 ohm quarter wave stubs.
Jim WD9AHF
OS/2 Warp V4 Java Enabled Operating System.
Roy Lewallen
In article <327527...@microdes.com>,
Dave Hand <dh...@microdes.com> wrote:
>An electrical 1/2 of coax is a tried and true method of mesuring
>the SWR at the antenna feed point remotely for exactly that reason.
>Its totally invisible to the remote test gear. If you doubt it try it!
You probably mean "measuring the impedance at the antenna feed point". If
used for that purpose, even a small amount of coax loss can make a
suprising amount of difference in the measurement. It's particularly bad if
the impedance to be measured is very different from the Z0 of the half
wavelength cable. Loss will cause the value looking into the line to be
skewed toward the Z0 of the cable.
As for SWR measurement, consider an antenna which has exactly a 50 ohm
feedpoint resistance and no reactance. Connect it to your rig with a half
wavelength of 75 ohm cable, and put an SWR meter at the rig end. A 50 ohm
SWR meter will read 1:1. The SWR on the cable is 1.5:1. A 75 ohm SWR meter,
if you can find one, will read 1.5:1. (Actually, the meter will read a
little less due to inevitable cable loss.)
How about an antenna which has a 75 ohm feedpoint resistance and no
reactance, connected to the rig with a 50 ohm cable? A 50 ohm SWR meter at
the rig will measure 1.5:1, regardless of the length of the coax. (Again,
the actual reading will be a little less due to loss, and will decrease
as the coax gets longer because then the loss gets greater.)
Roy Lewallen, W7EL
J. Fred Riley
w7...@teleport.com (Roy Lewallen) wrote:
>How about an antenna which has a 75 ohm feedpoint resistance and no
>reactance, connected to the rig with a 50 ohm cable? A 50 ohm SWR meter at
>the rig will measure 1.5:1, regardless of the length of the coax.
I think you meant to say, "...connected to the rig with 75 ohm coax."
Fred
JFR...@AIRMAIL.NET
FREDW...@AOL.COM
7066...@COMPUSERVE.COM
Roy Lewallen
In article <55ejg1$6...@library.airnews.net>,
jfr...@iadfw.net (J. Fred Riley) wrote:
>w7...@teleport.com (Roy Lewallen) wrote:
>>How about an antenna which has a 75 ohm feedpoint resistance and no
>>reactance, connected to the rig with a 50 ohm cable? A 50 ohm SWR meter
at
>>the rig will measure 1.5:1, regardless of the length of the coax.
>
>I think you meant to say, "...connected to the rig with 75 ohm coax."
Nope. Often as I screw up, I managed to say what I meant this time. But the
50 ohm SWR meter would also read 1.5:1 for any length of cable if the cable
were 75 ohms. (Of course the SWR on the cable is 1:1 in this case, not
1.5:1 as it would be if the cable were 50 ohms.)
Roy Lewallen, W7EL
k7...@aol.com
>w7...@teleport.com (Roy Lewallen) wrote:
>>How about an antenna which has a 75 ohm feedpoint resistance and no
>>reactance, connected to the rig with a 50 ohm cable? A 50 ohm SWR meter
at
>>the rig will measure 1.5:1, regardless of the length of the coax.
>
>I think you meant to say, "...connected to the rig with 75 ohm coax."
Yes, in the case of 50 ohm line connected to a 75 ohm load, the SWR in the
line is 1.5:1 only at the load, given that any practical line has
attenuation. The SWR at the transmitter end will be less than 1.5:1, and
indeed the lenght of the line does make a difference. But with a 75 ohm
line and 75 ohm load, the SWR on the line is 1:1, independent of length,
presenting "exactly" a 75 ohm load to the SWR meter, and if it's a 50 ohm
meter and accurate, it will read 1.5:1 SWR.
73, K7ITM
Cheers,
Tom
Please email to: to...@lsid.hp.com.
The account from which this was posted is seldom checked for email.