5/8 wavelength antenna?
Dan Jacobson
BNC Wire Antenna
# Gerald S. Santomassimo, KB0VOF
# kb0...@juno.com
For VHF/UHF/800, measure a quarter-, half-, 5/8-, (or whatever)
wavelength piece.
Is it true one can use 5/8 wavelength antennas? I thought only 1, 1/2,
1/4, 1/8 ... could be used. By the way, can 2, 4, 8 also be used?
(Antenna longer than wavelength.)
--
http://jidanni.org/ Taiwan(04)25854780
'Doc
Dan,
Any length of antenna can be used, 1/4w, 5/8w, or
any other fraction or multiple of a wave length. Certain
lengths have advantages such as a radiation pattern of a
particular shape or direction, or an easily matched input
impedance. In genral, longer antennas do 'better' than
shorter antennas. But, as with any generalization, there
are exceptions...
'Doc
Andy KC2SSB
wrote:
>
>Is it true one can use 5/8 wavelength antennas? I thought only 1, 1/2,
>1/4, 1/8 ... could be used. By the way, can 2, 4, 8 also be used?
>(Antenna longer than wavelength.)
Umm.. 5/8's are used all the time. I have a 5/8 mobile 2m antenna on
the car. As a matter of fact, most 2m mobile antennas are 5/8. Many
base antennas are listed as being multiples of wavelengths in height.
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
73! de Andy KC2SSB (ex: KF4KHC/HL9HCT)
Beachwood, NJ USA Grid FM29vw
http://vhfradiobuff.tripod.com
Richard Harrison
"Is it true one can use 5/8 wavelength antennas?"
Yes. It is not resonant so a reactance must be added to neutralize the
inherent reactance of the non-resonant length to get a large current in
the antenna. A coil can be used much as if the antenna were 1/8-wave.
Grounded vertical antennas longer than 5/8 wavelength are uncommon by
design unless phase inverters are used between sections to keep the
current flowing in the same direction in all sections. Current naturally
reverses every 1/2-wave of travel in an antenna.
A grounded vertical antenna produces increasing field strength
perpendicular to the vertical as its length grows until it reaches the
length of 5/8-wave. At that point, the out-of-phase radiatiation from
the segment beyond 1/2-wavelength starts to detract enough to reduce
field strength along the horizontal with increasing growth in the
antenna`s length.
Energy which is distributed along the horizontal by a grounded 5/8-wave
vertical antenna is redistributed at higher angles in longer similar
antennas. Higher angle radiation is of no use to line of sight
propsagation and becomes harmful to a medium-wave broadcacter at night
when it reflects from the ionosphere to interfere with his ground-wave
coverage.
You don`t see multiple-wavelength vertical broadcast antennas with phase
inverters every 1/2-wave because they would be too expensive and
1/2-wave and 5/8-wave antennas give high signal strengths and near 100%
radiation efficiencies when used with the FCC specified ground system of
radials. In fact, most broadcasters are satisfied with the cost/
performance trade-off of a 1/4-wave grounded vertical antenna.
Efficiency is about 96% , the antenna is naturally resonant, and the
impedance presented to the feed system is conveniently low.They are
willing to trade about 40% more field strength from a 5/8-wave antenna
over a 1/4-wave antenna for the lower cost and easier operation of the
1/4-wave antenna.
Best regards, Richard Harrison, KB5WZI
Mark Keith
> In http://strongsignals.net/access/content/tips.html
>
> BNC Wire Antenna
> # Gerald S. Santomassimo, KB0VOF
> # kb0...@juno.com
>
> For VHF/UHF/800, measure a quarter-, half-, 5/8-, (or whatever)
> wavelength piece.
>
> Is it true one can use 5/8 wavelength antennas? I thought only 1, 1/2,
> 1/4, 1/8 ... could be used. By the way, can 2, 4, 8 also be used?
> (Antenna longer than wavelength.)
Sure, but you use a loading coil to tune to a 3/4 wave resonance. Only
1/4 and 3/4 wave whips give a good match "low Z" to a coax feed. All
other types of verticals will need matching. A whip .64 wave, which is
basically the same as a 5/8 wave, gives the most gain on the horizon
in theory than any other single piece radiator. So anything taller,
such as a 3/4 wave up, starts to show less gain on the horizon in
comparison as the pattern starts to develop higher angles. So you
don't want to use tall radiators over .64 wave, unless you have a way
to keep the currents in phase. IE: phasing stubs on "twin" collinear
type verticals. That type of antenna use stacked 5/8 radiators, with a
phasing stub between the elements. The eznec demo can show the types
of patterns you will see on the various lengths of whips. Also I
modeled a few for 10m use on this page.
http://web.wt.net/~nm5k/acompari.htm
MK
OC_CAM's Razor
> Is it true one can use 5/8 wavelength antennas? I thought only 1, 1/2,
> 1/4, 1/8 ... could be used. By the way, can 2, 4, 8 also be used?
> (Antenna longer than wavelength.)
An antenna can be any length. Of course, the performance varies with
different lengths. A Larsen 5/8 monopole is a great 2m antenna.
--
cheers, CAM http://www.qsl.net/w5dxp
-----= Posted via Newsfeeds.Com, Uncensored Usenet News =-----
http://www.newsfeeds.com - The #1 Newsgroup Service in the World!
-----== Over 80,000 Newsgroups - 16 Different Servers! =-----
Jimmy
not actually using a 3/4 is that it has a higher radiation angle than the
5/8 and would send a larger portion of your signal into outer space. In
theory a 5/8 has little gain over an end fed half wave antenna. In actuality
when you consider the loss of the impedance matching circuit required with
the 1/2 wl antenna the difference is more substantial. As a matter of fact
some commercially availabe 1/2wl anteenas are actually worse than a 1/4 wl
antenna.
"OC_CAM's Razor" <Cecil....@ieee.org> wrote in message
news:3E36FC05...@ieee.org...
Mark Keith
>
> Sure, but you use a loading coil to tune to a 3/4 wave resonance. Only
> 1/4 and 3/4 wave whips give a good match "low Z" to a coax feed. All
> other types of verticals will need matching.
One caveat...A center fed 1/2 wave vertical has a low Z feedpoint and
can be fed directly with coax. But a base fed 1/2 wave has a high Z
feedpoint, and will require a matching device.
MK
'Doc
Jimmy,
You want to run through that 5/8 wave really being
a 6/8 wave part again? I think I may have missed something
there...
'Doc
Jimmy
. to make it ressonant the 5/8 is loaded to 3/4 which is resistive and then
the feed point is attached to the loading coil at the desired resistance,
usually the 50 ohm, point.
"'Doc" <w5...@cwis.net> wrote in message news:3E377E80...@cwis.net...
Jimmy
Assume you take a random length radiator and add lumped inductance to the
feedpoint end of it to make it resonant(purely resistive at a desired
frequency). This is known as "loading an antenna".What is the difference if
this random length radiator just happens to be 5/8 wl long at the desired
freq and you add inductance to make it resonant. IS this not also "loading
the antenna", making it electrically longer.
"Jimmy" <Gfe...@carolina.rr.com> wrote in message
news:2hTZ9.123884$Mb.54...@twister.southeast.rr.com...
Roy Lewallen
things, and you've got to be careful not to confuse the two.
If you top load a vertical, it looks "electrically longer" because the
top loading changes the current distribution to resemble a longer
antenna (but with some of the top portion still missing), and the
radiation resistance increases as it would if the antenna were actually
made longer. The reactance also decreases as for a longer antenna.
But base loading an antenna doesn't change either the current
distribution or the radiation resistance. Unlike top loading, it has no
effect at all on the characteristics of the antenna itself. Therefore,
it doesn't make the antenna "electrically longer" in the same sense that
top loading does. All it does is to modify the feedpoint reactance. Some
people might regard the reactance change as making the antenna appear
longer, but all it's really doing is modifying the reactance seen by the
source. Because the current distribution isn't modified, the pattern
shape will be unaltered by base loading.
Although it's slightly more complicated to match (if base fed), a 1/2
wave antenna will provide better efficiency than 5/8 wave if you have a
poor ground system, because of its high base impedance. It's a much
better choice for something like an HT, where you're stuck with a very
poor "ground system". Modeling comparisons with realistic ground
conductivity and ground system loss show that a 5/8 wave antenna often
doesn't deliver the expected gain over a shorter antenna, and there's no
reason to believe that objective tests won't confirm the modeling results.
Roy Lewallen, W7EL
Yuri Blanarovich
>But base loading an antenna doesn't change either the current
>distribution or the radiation resistance. Unlike top loading, it has no
>effect at all on the characteristics of the antenna itself. Therefore,
>it doesn't make the antenna "electrically longer" in the same sense that
>top loading does. All it does is to modify the feedpoint reactance. Some
>people might regard the reactance change as making the antenna appear
>longer, but all it's really doing is modifying the reactance seen by the
>source. Because the current distribution isn't modified, the pattern
>shape will be unaltered by base loading.
If I add, say 3 turn coil at the base of the 20m mobile antenna, I can see the
shift of resonant frequency down. Isn't the electrical length of the radiator
extended by the wire/tubing in the coil? It appears to me that we are
extending the radiator (making it electrically longer) and tapping onto the
higher impedance point. True, it doesn't change the pattern, but sure makes the
radiator appear longer. Same goes for Beta matches on Yagis (hairpin). Driven
element has to be shortened to compensate for the "loading" effect of the
hairpin, to maintain resonance.
Yuri, K3BU
OC_CAM's Razor
> True, it doesn't change the pattern, but sure makes the
> radiator appear longer.
What you have to be careful of, Yuri, is whether the loading
coil makes the radiator appear longer to the transmitter or
to the 377 ohm Æther. :-)
'Doc
Jimmy,
If the 'graphics' work correctly in this example;
\|/
| <--- antenna
|
----------------- |
Coax Feed line( ----\
----------------- \ <---Impedance matching
| \ coil
----------\
|
_____
___ <--- Ground
_
I make the following assumptions;
A. The antenna begins at the coax's center conductor's connection
with the top of the impedance matching coil.
B. The impedance matching coil is connected between the center and
shield conductors of the coaxial feed line.
c. The bottom of the coil and shield are connected to ground.
If the antenna is a 5/8 wave vertical and there is no 'loading coil'
in it's construction, how is it's length electrically changed to 3/4
wave length?
'Doc
Roy Lewallen
properties are concerned) is extended by the length of the coil itself,
not by the wire it contains.
By your sentence "True, it doesn't change the pattern, but sure makes
the radiator appear longer," I see that I've completely failed to
communicate the point I was trying to make. Maybe someone else can do a
better job.
Roy Lewallen, W7EL
Roy Lewallen
ohms, then it looks like an extension of free space, and the RF just
slips off the antenna like. . . well, you can supply your own
nasal-secretion similes.
Roy Lewallen, W7EL
Wes Stewart
wrote:
|No, the electrical length of the radiator (insofar as its radiation
|properties are concerned) is extended by the length of the coil itself,
|not by the wire it contains.
|
|By your sentence "True, it doesn't change the pattern, but sure makes
|the radiator appear longer," I see that I've completely failed to
|communicate the point I was trying to make. Maybe someone else can do a
|better job.
I doubt it but I'll take a shot.
Yuri,
I hate to pull a Cecil,<g> but add a half-wavelength of transmission
line between your radiator and the inductor. Now you've physically
separated the matching function from the radiation function. The
radiator radiates identically whether the inductor is in place or not.
All that changes is the impedance seen at the input to the inductor.
The length of the inductor is immaterial.
|
|Roy Lewallen, W7EL
|
|Yuri Blanarovich wrote:
|> Roy, W7EL wrote:
|>
|>>But base loading an antenna doesn't change either the current
|>>distribution or the radiation resistance. Unlike top loading, it has no
|>>effect at all on the characteristics of the antenna itself. Therefore,
|>>it doesn't make the antenna "electrically longer" in the same sense that
|>>top loading does. All it does is to modify the feedpoint reactance. Some
|>>people might regard the reactance change as making the antenna appear
|>>longer, but all it's really doing is modifying the reactance seen by the
|>>source. Because the current distribution isn't modified, the pattern
|>>shape will be unaltered by base loading.
|>
|>
|> If I add, say 3 turn coil at the base of the 20m mobile antenna, I can see the
|> shift of resonant frequency down. Isn't the electrical length of the radiator
|> extended by the wire/tubing in the coil? It appears to me that we are
|> extending the radiator (making it electrically longer) and tapping onto the
|> higher impedance point. True, it doesn't change the pattern, but sure makes the
|> radiator appear longer. Same goes for Beta matches on Yagis (hairpin). Driven
|> element has to be shortened to compensate for the "loading" effect of the
|> hairpin, to maintain resonance.
The hairpin match is a form of a distributed L-network. The hairpin
does not "load" the antenna. The real part of the feedpoint impedance
of most Yagis is below 50 ohm. By shortening the DE, this
lower-than-50-ohm resistance is put in series with a capacitive
reactance.
This series capacitive reactance in conjunction with the shunt
inductance of the hairpin forms the L-network. If the real part of
the feedpoint impedance is known, then the reactances are easily
calculated using the design equations for L-networks.
Since the real part of the impedance changes slightly with length
changes, tuning can be an interative process. On the other hand, my
3-element Yagi was built to a model and a beta match was calculated.
It took one change in DE length and one change in shorting bar
position to achieve almost exact conformance to the model.
73,
Wes N7WS
Yuri Blanarovich
>
>The hairpin match is a form of a distributed L-network. The hairpin
>does not "load" the antenna. The real part of the feedpoint impedance
>of most Yagis is below 50 ohm. By shortening the DE, this
>lower-than-50-ohm resistance is put in series with a capacitive
>reactance.
>
I gues I am having trouble seeing what is the difference between say, taking
dipole (or driven element in a beam), inserting hairpin in the middle. I see
this as loading (electrically extending) the radiator to achieve resonance,
just like placing loading coils or stubs/hairpins anywhere else on the element.
Along the hairpin we would have impedance from minimum to max corresponding to
the length of the coil/stub. What makes it different when we attach the
feedline? Isn't it the case when we are searching for impedance along the way
to match the feedline? K7GCO is feeding his Yagis off center at the 50 ohm
point on the driven element.
Slightly confucused :-)
Yuri, K3BU
Tarmo Tammaru
ground) is around 35 ohms resistive, whereas the 5/8 is about 70 - J300.
When you add an inductor to the base of a 5/8 you make the impedance 70 +
j0., which is not the same as the 3/4 wave. Adding the coil has no effect on
the radiation pattern. If you really want 50 ohms, you have to fudge the
length a little.
Tam/WB2TT
"Jimmy" <Gfe...@carolina.rr.com> wrote in message
news:2hTZ9.123884$Mb.54...@twister.southeast.rr.com...
Jimmy
for one condition and that is when the antenna is resonant to begin with but
at some undesirable impedance that must be transformed to say 50 ohms as is
in the case of a i/2 wl antenna were several thousand ohm s has to be
matched to a coax.I also understand what you and Roy are saying but only see
it as a different perpesctive. Adding a coil tunes out capacitive reactance
in a short antenna. I see no difference in calling this matching or loading.
I prefer to use the term matching when the radiator is already resonant but
you need to change the impedance to some other resistive value. I use the
term loading when the impedance of the antenna is complex and need s to be
made purely resistive. If I had a 15 ft whip and placed an inductor at the
bottom of it to make resonate on 40M would YOU call this loading or
impedance matching?
You CHOOSE not ot view the coil as part of the antenna, it would be equally
correct to view it as part of the antenna. just as much as it would be
correct to view the coil in the base loaded 40M antenna I described as
part of the antenna. They both do exactly the same thing. A s a matter of
act I could draw a diagram of a base loaded 3/4 wl antenna and it would be
exactly the same as your 5/8. Can you may a drawing of a base loaded 3/4
antenna and have it look any different from your 5/8?
Jimmy
load it to 3/4. If I did, would not the circuit be identical to that of a
5/8 wl antenna with matching circuit.
"Tarmo Tammaru" <t-ta...@home.com> wrote in message
news:rCednSQdXJM...@comcast.com...
'Doc
Jimmy,
Very simply, if the coil is 'above' the feed point
of the antenna, it's a loading coil. If it's below
the feed point of the antenna it's changing the antenna's
input impedance. There can also be a third option, where
a loding coil and impedance matching coil can be combined
at the base of an antenna. It may not be much of a distinction
to you, but where the coil is in relation to the feed point of
the antenna does make a difference. Can you see the distinction
I'm making? As to whether Roy agrees with me, or has the same
view of it, you'll have to ask him...
'Doc
Tarmo Tammaru
propagation characteristics of a 5/8 antenna. Base loading or impedance
matching does not make it 3/4 any more than 1/4 or 5/4. You don't want a 3/4
antenna anyhow, because a lot of the energy goes straight up.
BTW, I think one way they get the 5/8 to be 50 ohms is tu use a tapped
inductor.
Tam/WB2TT
"Jimmy" <Gfe...@carolina.rr.com> wrote in message
news:pS7_9.3721$hG6.1...@twister.southeast.rr.com...
Jimmy
Doc,
But for a base loaded antenna or a 5/8 there is no requirement for the the
feed point to be above the coil. Feed can be accomplished in shunt with th
coil or in series with the coil.To say that a 5/8 is not the same as a base
loaded 3/4 is to say that the base loaded 3/4 is impossible to build. Is
this what you are trying to tell me? Also the better part of the coil on a
5/8 designed for 50 ohm input is above the tap so i guess you are saying
this part is for loading and the other part is for impedance matching..Glad
we finally agree that is at least partially loaded. LOL. If you design a
base loaded 3/4 wl antenna with the radiator length somewhere between 1/2
and 3/4 wl will not the basic construction be the same as for the 5/8, just
a little more or less coil to conpensate for the length of the radiator.
"'Doc" <w5...@cwis.net> wrote in message news:3E391722...@cwis.net...
Wes Stewart
First you must remember that the feedpoint impedance is normally given
as a series equivalent. (We do call it an impedance rather than an
admittance). Boy, I would never feed a Yagi element off-center.
Placing a stub, in *parallel* with the feedpoint is not the same as
placing a loading inductance *in series* with the feedpoint.
To amplify what I stated previously about the function of stub
matching as applied to Yagis I offer the following real-world example.
Modeling my HB 3-element 20-meter Yagi without any matching I get the
following results:
Freq R X
14.000 27.75 -10.42
14.050 27.54 -6.89
14.100 27.16 -3.28
14.150 26.63 0.44
14.200 25.97 4.30
14.250 25.18 8.31
14.300 24.29 12.50
Since I want the match to be decent over the CW and lower Phone band,
I decide to match at 14.15 MHz. So I need an L-network that steps up
26.6 ohm to 50 ohm. To increase the 26.6 ohm (Rs) to 50 ohm (Rp), a
*series* reactance is required.
Using the equations for L-networks I first determine the required
matching Q thus:
Q = (Rp/Rs-1)^0.5
Q = (50/26.63-1)^0.5
Q = 0.96 or Q ~ 1.
The series matching reactance is found from:
Xs = Rs * Q
Xs = 26.63 * 0.96
Xs = 24.98 or ~ 25 ohm.
Since I am going to use a parallel inductive stub, I need to make the
series reactance capacitive. I can do this by shortening the driven
element to make the reactance = -j25.
Back at the modeling program (I use AC6LA's MultiNEC as a shell over
EZNEC which makes this easy since I can make the element tip length a
variable and sweep the length at the match frequency).
Once I find the new length I have the following:
Freq R X
14.000 25.50 -35.59
14.050 25.29 -32.16
14.100 24.93 -28.67
14.150 24.43 -25.06
14.200 23.81 -21.34
14.250 23.07 -17.47
14.300 22.25 -13.44
Note that because the element is a bit shorter, the real part is
slightly lower so the matching Q is now
Q = Xs / Rs = 25.06/24.43
Q = 1.03 or for practical purposes still, Q ~ 1.
The required shunt inductive reactance is found
Xp = 50 / Q
Xp = +j50
So I need a stub that presents +j50 in *parallel* with the feedpoint.
In my case I planned to use 1/2" diameter tubing for the stub and
because I built the antenna from material from a TH-6, the mounting
insulators defined the spacing of the tubing and the distance from the
boom. Using equations from Reference Data for Radio Engineers for a
balanced line above ground, I determined that the Zo of the stub was
approximately 150 ohm.
Using another of AC6LA's handy programs, TLDetails, I set up a 150 ohm
line with a short on one end and determine that a length of 42.7
inches gives +j50 at 14.15 MHz.
Back at the antenna modeling program I install a 42.7" long, 150 ohm
shorted transmission line at the feedpoint and calculate:
Freq R X SWR
14.000 73.99 9.29 1.522
14.050 65.93 4.15 1.331
14.100 57.88 0.75 1.158
14.150 50.12 -1.13 1.023
14.200 42.86 -1.74 1.172
14.250 36.25 -1.32 1.381
14.300 30.37 -0.10 1.647
I my case, I also use the stub for the balun. A couple of photos show
this:
http://users.triconet.org/wesandlinda/YagiFeed-1a.jpg
http://users.triconet.org/wesandlinda/YagiFeed-2a.jpg
As a tribute to modeling and Leeson's "Physical Design of Yagi
Antennas", it took exactly one adjustment of the DE length and one
adjustment of the stub length to meet the design goals.
73,
Wes N7WS
Jimmy
knowledge it is not required by a loading coil to alter the current
distribution of antenna though where you place it will determine how and if
the current pattern is altered hence the propagation.As for as the angle of
elevation that i choose for my antenna that is my choosing. As a matter of
fact I have used 1.25 wl verticals. It is not a matter of how far they talk
but where they talk. In my opion furtherest is not always best. When
conditions are right for skip they allows me to talk into areas that i would
normally overshoot. BTW: please answer my question, Would not a base loaded
3/4 wl antenna with the actual radiator length significantly longer than 1/2
wl and signifcantly shorter than 3/4 be identical in constuction to a 5/8.
Yes or no.
"Tarmo Tammaru" <t-ta...@home.com> wrote in message
news:W5qcnUUVvNo...@comcast.com...
'Doc
Jimmy,
If you're not willing to understand, I'm certainly
not going to waste my time trying to explain. Good
luck...
'Doc
Jimmy
of the good things about a half wave end fed vertical. Also are you saying
for a coil to be a loading coil it MUST be placed so as to alter the
antennas current distribution. I guess to be politically correct one should
discribe an antenna as being a "a/b wl antenna loaded to a x/y wl".rather
than as a loaded x/y antenna.
"Roy Lewallen" <w7...@eznec.com> wrote in message
news:v3gb4la...@corp.supernews.com...
Richard Harrison
"I fail to see the difference between impedance matching and loading
Consider the double-tuned r-f transformer. It can be made with any
amount of coupling and any impedance ratio. It can also be made with
adjustable coupling or with a selectable impedance ratio.
Coupling is another term for mutual impedance, or how much occurrences
in one circuit influence another circuit. Coupling between a radio and
its antenna may be treated exactly in many respects as in a double-tuned
r-f transformer. Loading is coupling. Impedance matching is controlling
the ratio between the radio and its antenna. For the conjugate match,
reactance must be eliminated in addition to the resistive match.
Loading and matching interact. Change the impedance ratio and expect to
retune and to readjust the coupling.
Radios used to have swinging links to adjust loading, and tapped coils
to adjust impedance match.
Jimmy also wrote:
"If I had a 15 ft whip and placed an inductor at the bottom of it to
make it resonate on 40M would you call this loading or impedance
matching?"
Yes.
Traditionally, this is called a loading coil. "Tuning-coil" would be
more descriptive of its function which is reactance nullification.
Best regards, Richard Harrison, KB5WZI
Jimmy
I asked. You didnt even try to answer.
"'Doc" <w5...@cwis.net> wrote in message news:3E395077...@cwis.net...
Dave Shrader
When an antenna is 'loaded' the current distribution[s] on the antenna
change. If I place an inductor 1/2 way along an antenna I change the
'self inductance' of the antenna and the current changes both above and
below the inductor. If I now move the inductor the currents change to
satisfy the conditions imposed by the new location.
Impedance matching requires, as a minimum, two different reactive
components; an inductor and a capacitor. This comprises an 'L' section,
or with three components a 'TEE' section, or a 'PI' section;
additionally you may use a tapped inductor, a split capacitor etc.
A 5/8 wavelength vertical antenna has an end point Load Impedance of
approximately 120-j400 Ohms. For most amateur radios this has to be
converted to a 50 ohm load. the purpose of the matching section is to
accomplish the impedance transformation.
Now it is possible to do both with a tapped inductor at the base of the
antenna. Remember the antenna has capacitance to 'ground'. So with the
antenna and coil inductance and the capacitor to ground effect it is
possible to achieve a 50 ohm load.
The impedance looking into the matching section is approximately:
R = Q*SQRT[L/C]
So, antenna Q, and suitable values of Inductance and Capacitance will
transform 120-j400 to 50 ohms.
Conclusion: 1) Loading changes the current distribution on the antenna
2) Matching transforms one impedance to another.
73, Deacon Dave, W1MCE
Fred Hambrecht Sr
anything they want them to (see Alice in wonderland) and no one is supposed
to point out that the world has agreed on certain principals. Why!, because
it may hurt their self-esteem to point out they lack knowledge.
Gee Jimmy, your really close 2+2 is almost 5 so you get a smiley face...
"'Doc" <w5...@cwis.net> wrote in message news:3E391722...@cwis.net...
Tarmo Tammaru
.................................... Would not a base loaded
> 3/4 wl antenna with the actual radiator length significantly longer than
1/2
> wl and signifcantly shorter than 3/4 be identical in constuction to a
5/8.
> Yes or no.
>
radiating element of 3/4, and be neither shorter nor longer than 3/4
wavelength.
Tam/WB2TT
Jimmy
the current distibution on an antenna . AS others have said and I already
knew, A coil located at the base of an antenna does not significantly
disturb the current distibution in the antenna. to me loading is making a
raditor that exhibts a complex impedance at a desire freq resistive at that
freq by adding reactance that cancels out the reactive componet of the
antennas impedace.Matching means coversion from one resistive impedance to
another. I realize that there is very little difference in these two
meanings and I may be totally wrong about them but it certainly seems like
over the years this is the way they have been used . This may be an
oversimplication. Perhaps I should look at loading as part of the matching
process. With loading taking care of the reactive component and matching
taking care of the resistive. Hmmm, kind of sounds like what I aleady said.
Thanks for yor input it certainly gave me a new way of looking at it.
"Dave Shrader" <david....@attbi.com> wrote in message
news:3E39718C...@attbi.com...
****** I 'm not so sure about this, at least not with base loading.
> 2) Matching transforms one impedance to another.
Yes, does this mean transforming complex impedances to resistive impedances
too. (ie making an antenna resonate at a freq it was not designed.
>
> 73, Deacon Dave, W1MCE
Jimmy
Can you do anything but make idle insults?. I guess this is what improves
your self esteem.
"Fred Hambrecht Sr" <w4...@charter.net> wrote in message
news:v3it5dc...@corp.supernews.com...
Dave Shrader
However, the ARRL Antenna book devotes an entire chapter, chapter 16
[18th edition], to the issue. They explicitly indicate that loading
changes the current distribution in the antenna. The same chapter also
discusses Impedance Matching.
Try your local library and see if you can read the chapter.
73, Deacon Dave, W1MCE
Dan
> First you must remember that the feedpoint impedance is normally given
> as a series equivalent. (We do call it an impedance rather than an
> admittance).
[snip lots]
Wes described one way to think about how a hairpin transforms the
source impedance. And BTW, his photos showed the neat trick of using
one-half of the hairpin tube as a sleeve balun. Very slick.
There's another way to think about hairpins. Since the hairpin is
placed *in parallel* with the antenna feed point, you can determine
the *parallel equivalent* of the antenna source impedance and then
work from there. Wes and I didn't plan this, but it just so happens
that one of the illustrated examples for MultiNEC walks through the
steps that are needed.
Not exactly light reading, but available at
www.qsl.net/ac6la/mnex5.html .
Roy Lewallen
current distribution. If so, I'd appreciate it very much if you'd email
me the exact quote and where it appears, so I can send it to the editor
for correction. A quick look at the 17th Edition shows correct
explanations -- that the current distribution is changed if the loading
is done other than at the base, but not that it's changed by base loading.
Roy Lewallen, W7EL
Dave Shrader
Yuri Blanarovich
I have not received explanation or correction to what I was trying to say, or
question Roy's statement that
"base loading ... Unlike top loading, it has no
it doesn't make the antenna "electrically longer" in the same sense that
top loading does."
Forgive me, but let me try again and to break it into steps, to make sure we
are on the same (electrical :-) wavelength:
Lets take the quarter wave vertical radiator (Cecil is familiar with mobile
stuff and I measured it) so we have the constant - ground plane car body or
radial field. If that radiator is connected directly to ground, it has certain
impedance (low) and resonant frequency. Can we call the length of the radiator
(besides physical) electrical length (corresponds to resonant frequency)?
Now I will break the connection between the radiator and car body and I insert
a coil and connect it between the bottom of the radiator and car body. Can we
say we loaded the radiator? (If not what do we call it?) Doesn't the resonant
frequency of the radiator change, get lower? (I found it does.) Doesn't that
change the electrical length of the radiator? (As reflected in change in
resonant frequency?) I am not arguing by how much, length of the coil, length
of the wire, length of ....
Now for the purpose of matching, I can find point on the coil, where between
the ground of the coil and and few turns up, I can find 50 ohm point for the
coax. I know when I do that, say on 20m mobile, my resonant frequency drops,
doesn't that mean that electrical length of the radiator changed?
Are we not changing electrical length of conductors by inserting inductance or
capacitance somewhere in the conductor? Isn't "loading" loading regardles where
and how it is applied?
Where am I wrong?
Yuri, K3BU
ps
In AC6LU example in Multinec I see that resonant frequency (dip in SWR curve)
with hairpin match dropped, indicating shift in resonant frequency = change in
electrical length?
Roy, W7EL wrote:
>But base loading an antenna doesn't change either the current
>distribution or the radiation resistance. Unlike top loading, it has no
>effect at all on the characteristics of the antenna itself. Therefore,
>it doesn't make the antenna "electrically longer" in the same sense that
>top loading does. All it does is to modify the feedpoint reactance. Some
>people might regard the reactance change as making the antenna appear
>longer, but all it's really doing is modifying the reactance seen by the
>source. Because the current distribution isn't modified, the pattern
>shape will be unaltered by base loading.
If I add, say 3 turn coil at the base of the 20m mobile antenna, I can see the
shift of resonant frequency down. Isn't the electrical length of the radiator
extended by the wire/tubing in the coil? It appears to me that we are
extending the radiator (making it electrically longer) and tapping onto the
higher impedance point. True, it doesn't change the pattern, but sure makes the
radiator appear longer. Same goes for Beta matches on Yagis (hairpin). Driven
element has to be shortened to compensate for the "loading" effect of the
hairpin, to maintain resonance.
Yuri, K3BU
Mark Keith
> I hope the ARRL Antenna book doesn't say that base loading changes the
> current distribution. If so, I'd appreciate it very much if you'd email
> me the exact quote and where it appears, so I can send it to the editor
> for correction. A quick look at the 17th Edition shows correct
> explanations -- that the current distribution is changed if the loading
> is done other than at the base, but not that it's changed by base loading.
>
> Roy Lewallen, W7EL
I have the 16th edition. "1992". It seems correct also. In that
chapter, I see no discussion of current distribution for 5/8 wave
antennas, unless I missed it quickly reading through it. Only a couple
of 5/8 GP antenna projects. The only mention of loading coils
changing current distribution is as it applies to short loaded
verticals. IE: mobile antennas. They mention to improve the current
distribution, you must raise the coil up the whip, or add a
capacitance hat. Seems my version is about the same as yours. MK
Mark Keith
> I am not so sure that part of the definition of loading is that it changes
> the current distibution on an antenna . AS others have said and I already
> knew, A coil located at the base of an antenna does not significantly
> disturb the current distibution in the antenna. to me loading is making a
> raditor that exhibts a complex impedance at a desire freq resistive at that
> freq by adding reactance that cancels out the reactive componet of the
> antennas impedace.Matching means coversion from one resistive impedance to
> another. I realize that there is very little difference in these two
> meanings and I may be totally wrong about them but it certainly seems like
> over the years this is the way they have been used . This may be an
> oversimplication. Perhaps I should look at loading as part of the matching
> process. With loading taking care of the reactive component and matching
> taking care of the resistive. Hmmm, kind of sounds like what I aleady said.
> Thanks for yor input it certainly gave me a new way of looking at it.
As far as the loading coil on a 5/8 whip, I think it should really be
called a matching coil. In fact, I used to call it a matching coil,
but changed to be "politically correct". Maybe I should change
back...:/ For that type of coil to change the pattern, or current
distribution, it would have to actually make the element longer. But,
this is not the case as most of these coils are small and short.
Changing current distribution on a short whip is a totally different
matter the way I see it. Why? Because maximum current is always at the
base on a shorter than 1/4 wave whip. The only way you can change this
is to either add top capacitance which will give you a fairly linear
current distribution through the whip, or to raise the coil up from
the base to "draw" current up through the coil. The current at both
ends of the coil are appx the same. So this means the current
distribution improves from the base up to the point of the top of the
coil. It then diminishes when it gets to the capacitance, IE: whip or
hat or both. Using the top hat is the preferred way to go. Coil
position matters little in a case like that. Current distribution is
fairly linear no matter where the coil is. In that case, it would make
more sense to have the coil at the base to have the least number of
turns.
But, if you use the placement of the coil to improve current
distribution, the current will not be linear up the whip, and reduces
sharply once you get past the coil. To see why the current raises off
the base, note that the current in such a short whip with no hat is
max at the base. ALWAYS!!! If you have a ten ft whip base loaded with
a 80m coil with say 50 turns that took 50 ft, remember that the max is
at the base. Where does it go next? Through 50 ft of wire. Thats where
your current max remains, and it's basically unchanged from a unloaded
whip. Remember, it diminishes fairly fast as it sees the capacitive
whip. Now of you raise the coil up 5 ft, current is still max at the
base, but what does it see first? 5 ft of mast. Only then does it see
the 50 ft of wire in the coil. So being that current is fairly equal
at the bottom and top of the coil, you have fairly steady distribution
from the base, up the mast, and through the coil. Only then does it
see capacitance and start to diminish.
All the best mobile antennas use a large top hat. It's not really to
lower the number of turns needed in the coil, although that is one
bonus. It's mainly to give a fairly linear current distribution
through the whip. Very important on the low bands for the best
results. Your radiation resistance also increases greatly with good
top loading. Hopefully some of this makes sense...:/ MK
Fred Hambrecht Sr
idiots. Sorry Jimmy but your teachers did you no favors trying to increase
your self-esteem.
"Jimmy" <Gfe...@carolina.rr.com> wrote in message
news:Ehg_9.5628$hG6.1...@twister.southeast.rr.com...
Mark Keith
> I am not so sure that part of the definition of loading is that it changes
> the current distibution on an antenna . AS others have said and I already
> knew, A coil located at the base of an antenna does not significantly
> disturb the current distibution in the antenna. to me loading is making a
> raditor that exhibts a complex impedance at a desire freq resistive at that
> freq by adding reactance that cancels out the reactive componet of the
> antennas impedace.Matching means coversion from one resistive impedance to
> another. I realize that there is very little difference in these two
> meanings and I may be totally wrong about them but it certainly seems like
> over the years this is the way they have been used . This may be an
> oversimplication. Perhaps I should look at loading as part of the matching
> process. With loading taking care of the reactive component and matching
> taking care of the resistive. Hmmm, kind of sounds like what I aleady said.
> Thanks for yor input it certainly gave me a new way of looking at it.
More thoughts on current distribution. And how it can have a big
effect on a mobile antenna. When you look at a graph of current
distribution for a 1/4 wave, it's naturally max at the base, and
decreases in an orderly military manner until it's appx zero at the
tip. The current distribution of a short whip acts much the same, and
it all starts at the base. According to a graph in the antenna
handbook, on a 1/4 wave whip, 50 % current is at the 60 degree point
up the whip. "90% total height". I'm not sure how this changes on say
a 10 ft whip for 80m, but would be easily modeled... But anyway, the
current normally is fairly strong even halfway up the whip. "about 65%
for a 1/4 wave". So if you have a base loaded 10 ft whip, and the
first 50 ft of the antenna is loading coil in a 1 ft or so form
height, thats where the first 50 ft of strong current distribution
will be. In the coil...Bummer overall... :(
This is why I went to the trouble of raising my mobile coil from appx
25 inches from the base to now about 64 inches. That first 5 ft of
lower whip is where the strongest current is starting from the base.
And it's fairly uniform from the base until you get to the top of the
coil, where it starts to diminish as it see's the capacity. Remember
the graph and how current gradually decreases up the whip starting at
the base...
It's much better having that first 5 ft being whip, than it is to be
at the base of a loading coil consisting of a few turns of wire. Which
sounds like it would radiate the best? I'll take the mast any day...
BTW, the change did seem to be worthwhile. My performance on 80m shot
up quite a bit. And naturally, better on all the other bands too. MK
Roy Lewallen
> . . .
> Forgive me, but let me try again and to break it into steps, to make sure we
> are on the same (electrical :-) wavelength:
>
> Lets take the quarter wave vertical radiator (Cecil is familiar with mobile
> stuff and I measured it) so we have the constant - ground plane car body or
> radial field. If that radiator is connected directly to ground, it has certain
> impedance (low) and resonant frequency. Can we call the length of the radiator
> (besides physical) electrical length (corresponds to resonant frequency)?
That's the whole problem. I can't find "electrical length" in the
indexes of any of the several antenna textbooks I have, in the IEEE
dictionary, or even the ARRL Antenna Book. It means different things to
different people, and can mean just about anything you want it to. If
all it means to you is a way of describing feedpoint reactance, then
yes, you're right on track.
"Electrical length" is used with transmission lines to describe the
effect of different propagation velocities. Transmission lines of the
same electrical length will have the same current distribution, and
changing the electrical length has exactly the same effect as changing
the physical length. So an antenna effect more (but not entirely)
analagous to the transmission line usage is the effect of top loading,
where you actually change the current distribution and therefore make
the antenna behave like a longer radiator. The radiation resistance also
increases, just like it would if you made the antenna physically longer.
So some people tend to regard this as changing the "electrical length".
Base loading, by contrast, doesn't change the antenna at all. It doesn't
change the current distribution, radiation pattern, effective length,
gain, or anything else. All it does is add reactance to the feedpoint
impedance.
You're certainly free to use whatever terms you like in any way you
like. But it would be a lot less ambiguous if you just said that by
adding a base inductor you were changing the feedpoint reactance. That
statement is correct and not subject to misinterpretation. If your
objective is to communicate, the listener is a lot more likely to
understand what you mean if you use this terminology to describe the
effect of base loading, rather than saying you've made the antenna
"electrically longer".
> Now I will break the connection between the radiator and car body and I insert
> a coil and connect it between the bottom of the radiator and car body. Can we
> say we loaded the radiator? (If not what do we call it?)
Yes, that's common and well understood terminology for what you've done.
> Doesn't the resonant
> frequency of the radiator change, get lower? (I found it does.)
Yes.
Doesn't that
> change the electrical length of the radiator? (As reflected in change in
> resonant frequency?) I am not arguing by how much, length of the coil, length
> of the wire, length of ....
The problem is in the parentheses, as I elaborated on above.
> Now for the purpose of matching, I can find point on the coil, where between
> the ground of the coil and and few turns up, I can find 50 ohm point for the
> coax. I know when I do that, say on 20m mobile, my resonant frequency drops,
> doesn't that mean that electrical length of the radiator changed?
> Are we not changing electrical length of conductors by inserting inductance or
> capacitance somewhere in the conductor?
See above.
Isn't "loading" loading regardles where
> and how it is applied?
Yes.
> Where am I wrong?
I wouldn't say you're wrong, just using terminology that has more than
one meaning. Again, it's your choice. But if you really want to
communicate, unambiguous terminology is better.
Roy Lewallen, W7EL
Jimmy
I saw no mention of where this is a mandatory condition of loading. It does
say that the effect is more pronounce as you move the coil up the antenna. I
also know that it is the least at the base which is the most effective place
to load an antenna.Hmmm, interesting. Seems like current redistribution is
more of a by product of loading. Isnt this why 5/8 antennas have there coil
at the base, So as not to disturb the distribution of current on the length
of the radiator. Other wise you could place it anywhere on the antenna to
tune out the reactive componet of the antenna.
"Dave Shrader" <david....@attbi.com> wrote in message
news:3E39912F...@attbi.com...
Jimmy
"Fred Hambrecht Sr" <w4...@charter.net> wrote in message
news:v3k6krk...@corp.supernews.com...
Wes Stewart
|Interesting how discussions twist and threads wonder around.
Welcome to usenet <g>
|In AC6LU example in Multinec I see that resonant frequency (dip in SWR curve)
|with hairpin match dropped, indicating shift in resonant frequency = change in
|electrical length?
I guess there are a lot of ways to look at this, but remember that an
additional reactance (the stub) has been added. The point of lowest
SWR no longer necessarily indictes the "resonant frequency" of the
element.
Furthermore, the stub is not a lumped inductance but a transmission
line that has its own frequency sensitive characteristics.
Yuri Blanarovich
>
>"Electrical length" is used with transmission lines to describe the
>effect of different propagation velocities. Transmission lines of the
>same electrical length will have the same current distribution, and
>changing the electrical length has exactly the same effect as changing
>the physical length. So an antenna effect more (but not entirely)
>analagous to the transmission line usage is the effect of top loading,
>where you actually change the current distribution and therefore make
>the antenna behave like a longer radiator. The radiation resistance also
>increases, just like it would if you made the antenna physically longer.
>So some people tend to regard this as changing the "electrical length".
>Base loading, by contrast, doesn't change the antenna at all. It doesn't
>change the current distribution, radiation pattern, effective length,
>gain, or anything else. All it does is add reactance to the feedpoint
>impedance.
>
>You're certainly free to use whatever terms you like in any way you
>like. But it would be a lot less ambiguous if you just said that by
>adding a base inductor you were changing the feedpoint reactance. That
>statement is correct and not subject to misinterpretation. If your
>objective is to communicate, the listener is a lot more likely to
>understand what you mean if you use this terminology to describe the
>effect of base loading, rather than saying you've made the antenna
>"electrically longer".
>
Thanks Roy, looks like we are getting closer :-)
But... I am kind of surprised (or confused) by what you are saying, sounds to
me like trying to define what IS is.
We might be nit picking or perhaps looking at the problem through different
glasses -. I have a problem with flat statement that "electrical length of
antenna doesn't change" by loading.
I understand situation with coax and electrical vs. physical length, but I fail
to see the difference if the same thing is applied to the antenna radiator.
Bill Orr in his Radio Handbook, 20th edition, page 24.8 explains doublet (half
wave dipole), resonant frequency, electrical length, physical length and their
relations. So neglecting matching, feeding, patterns, etc. if we want to
maintain the resonant frequency of a dipole, and we change the thickness of the
conductor or insert capacitance or inductance in the wire, we change the
resonant frequency, which I still believe changes the electrical length and if
we want to maintain resonance on design frequency we have to compensate by
changing physical length.
"All it does is add reactance to the feedpoint impedance." - but that also
changes the resonant frequency of the radiator, therefore it's electrical
length. So seems to me it is not "all". Yes, it may not be significant in
radiation properties of the antenna, but if the resonant frequency is important
to me, I better know about it and compensate for it.
The electrical length of antenna element is important especially in parasitic
arrays. We can change it by changing the thickness of conductor, using
insulated wire, proximity of ground or objects or inserting reactances
(hairpin, capacitance, inductance, top hat, fractal kinks). I am pleased that
at least "some" people see it that way, but what is the problem with the rest
of them? :-)
Another statement that "base loading doesn't change the antenna at all," seems
to me a bit questionable. If inserting coil in the base of vertical changes the
resonant frequency down, then it makes it electrically longer and that is the
CHANGE. Or are we into "fuzzy logics" or making Johnny feel good - "it's close
enough"? I know that when I insert few turns into the base of my 20m mobile
whip, it shifts the resonant frequency below the band edge. It is perhaps
"close enough" for Cecil and his doublet and open wire feedline/matching, but
for parasitic array design it matters if there is a change in electrical length
in a piece of wire.
For one, I am eternally grateful for antenna modeling software, because allows
me to find the resonant frequencies of parasitic elements out of the array,
build them, check the resonant frequency out of array, trim them (tune their
electrical length) if needed and be able to replicate or scale the design as
close as possible. This is helpful in situations when (my) modeling software
cannot accommodate precisely all variables (ignoring Q and space impedance :-).
I guess my "problem" might be that I learned from Bill Orr that resonant
frequency is tied to the electrical length of the adiator.
Yuri, K3BU
Yuri Blanarovich
>
>I guess there are a lot of ways to look at this, but remember that an
>additional reactance (the stub) has been added. The point of lowest
>SWR no longer necessarily indictes the "resonant frequency" of the
>element.
>
True, there are other factors too, but I believe some of the electrical
"lengthening" is reflected in it.
>Furthermore, the stub is not a lumped inductance but a transmission
>line that has its own frequency sensitive characteristics.
>
Yea, but doesn't shorted "hairpin part" of the stub look to the radiator like
an inductance of the hairpin, and therefore lengthens the electrical length of
the radiator (as in Beta match)?
I know when I was P40A and tried to use sloping dipole from the hotel roof, had
no match boxes and my IC706 wasn't happy with low impedance, I simply made the
Beta match or hairpin out of the wire at the feedpoint and in order to maintain
the resonance within the band, I had to shorten the wires of the dipole. So to
me, it changed the electrical length of the radiator (it "loaded it") and it
acted like I inserted whatever inductance into the radiator. If you use Beta
match, you have to compensate for it by shortening the element (like in 402BA)
so the effect is we are "loading" or lengthening the element besides matching
it to coax.
Is this something new that "some" people don't know?
Yuri, K3BU
Wes Stewart
|Wes N7WS writes:
|>
|>I guess there are a lot of ways to look at this, but remember that an
|>additional reactance (the stub) has been added. The point of lowest
|>SWR no longer necessarily indictes the "resonant frequency" of the
|>element.
|>
|
|True, there are other factors too, but I believe some of the electrical
|"lengthening" is reflected in it.
|
|>Furthermore, the stub is not a lumped inductance but a transmission
|>line that has its own frequency sensitive characteristics.
|>
|
|Yea, but doesn't shorted "hairpin part" of the stub look to the radiator like
|an inductance of the hairpin, and therefore lengthens the electrical length of
|the radiator (as in Beta match)?
You're missing my point. Yes, the stub looks link an inductor;
however, it is a frequency sensitive inductor. An inductor made from a
coil of wire that is small in terms of wavelength has a fixed value of
inductance. If it is 1 microhenry at 1 MHz, it is still 1 microhenry
at 10 MHz.
A shorted stub (< 90 deg) is a distributed inductor; it is not small
in terms of wavelength. As the frequency changes, so does the
inductive reactance. The reactance of a shorted stub is found from:
Xl = Zo * tan(L)
where L is the length in electrical degrees
Clearly, with a fixed physical length, as the frequency changes the
electrical length changes, hence the reactance changes.
|
|I know when I was P40A and tried to use sloping dipole from the hotel roof, had
|no match boxes and my IC706 wasn't happy with low impedance, I simply made the
|Beta match or hairpin out of the wire at the feedpoint and in order to maintain
|the resonance within the band, I had to shorten the wires of the dipole. So to
|me, it changed the electrical length of the radiator (it "loaded it") and it
|acted like I inserted whatever inductance into the radiator. If you use Beta
|match, you have to compensate for it by shortening the element (like in 402BA)
|so the effect is we are "loading" or lengthening the element besides matching
|it to coax.
|
|Is this something new that "some" people don't know?
Let me try one more time to convince you that the matching function is
not "loading" the antenna.
Let's go back to my three-element Yagi example and assume for the
moment that I have adjusted the DE length make the feedpoint impedance
25 +j0 at Fo.
I split the DE in the middle and connect a zero-length perfect balun
and a half-wavelength of lossless transmission line to the element.
At the other end of the line I add a weatherproof box that has two
connectors. One connects to the line going to the antenna, the other
to a 50-ohm line going to the transmitter.
In the box I add two components. A capacitor with reactance -j25 is
connected between the two connectors. An inductor with reactance +j50
is shunted across the connector that connects to the transmitter.
You should agree that at Fo, this is nothing more than an impedance
repeating line and an L transmission line-matching network. None of
this is affecting the resonant frequency of the antenna. A
measurement of impedance at the input connector yields 50 +j0.
Now if I remove the half-wavelength line and put the box at the
feedpoint, *nothing* changes. The components in the box are still
just performing an impedance matching function. The DE is still
naturally resonant at Fo and radiates accordingly. A measurement of
impedance at the input connector yields 50 +j0.
Now I say to myself, "Self, you could get rid of that expensive
capacitor by shortening the DE to present the same capacitive
reactance." So by analysis or cut-and-try, I shorten the element to
make the impedance 25 -j25. (I know R will change too but bear with
me) The "natural" resonant frequency has increased by some factor (k)
and the radiation properties have changed slightly.
I install a jumper wire in the box to replace the capacitor and again,
at a Fo, *nothing* is different. A measurement of impedance at the
input connector yields 50 +j0.
Now I say to myself, "You could get rid of that box and inductor and
replace it with a stub that doesn't need waterproofing." So I do that
too and *nothing* changes. If I'm clever enough (ahem) I can run the
transmission line through one of the stub tubes and perform the balun
function without adding any components. A measurement of impedance at
the feedpoint at Fo yields 50 +j0. The "natural" resonant frequency
is still k * Fo and the radiation properties remain the same.
So three components and a box have been eliminated and there is no
measurable difference. The feedpoint is matched at Fo and the element
is resonant at k * Fo. No loading has taken place.
73,
Wes N7WS
'Doc
Mark,
And it seems to me that you definitely have made
a case for 'where' to put the coil in the body of
the antenna. It also seems to me that the fact that
the position of the coil doesn't make it an impedance
matching coil anymore, but a 'loading' coil, changing
the 'electrical' length of the antenna. If the 'physical'
length of the antenna isn't changed to change it's
electrical properties, then I would think that it would
be proper to say that it's 'electrical' length has changed
to have those properties that are desired. Yes, or no?
'Doc
Roy Lewallen
> Roy W7EL wrote:
> . . .
> I guess my "problem" might be that I learned from Bill Orr that resonant
> frequency is tied to the electrical length of the adiator.
>
> Yuri, K3BU
Careful readers of Orr's work will also learn that folding a unipole
increases efficiency because of the muliplied radiation resistance. I
pointed out this error to him many years ago, and got a nice postcard of
acknowledgement in return. Then saw it again in his column not long after.
I see absolutely no point in arguing about the definition of a term that
really has no definition. I don't personally care at all what
interpretation you or others want to make of "electrical length", or
whether people understand what you're trying to say. For myself, I'm
generally interested in using my words to communicate. So I try to
choose terms that aren't as easily misunderstood. And even so, I still
fail with disappointing frequency to communicate.
Roy Lewallen, W7EL
Wes Stewart
wrote:
|Yuri Blanarovich wrote:
|> Roy W7EL wrote:
|> . . .
|> I guess my "problem" might be that I learned from Bill Orr that resonant
|> frequency is tied to the electrical length of the adiator.
|>
|> Yuri, K3BU
|
|Careful readers of Orr's work will also learn that folding a unipole
|increases efficiency because of the muliplied radiation resistance. I
|pointed out this error to him many years ago, and got a nice postcard of
|acknowledgement in return. Then saw it again in his column not long after.
Yep. Almost anyone (even me <g>) can get something published. That
doesn't necessarily make it correct.
|
[snip]
| And even so, I still fail with disappointing frequency to communicate.
If I fail to understand something you wrote, I figure it's my lack of
comprehension, not your failure to communicate.
http://www.destgulch.com/movies/luke/luke18.wav
73,
Wes Stewart, N7WS
Wes Stewart
I meant to say, "the stub looks 'like' an inductor..."
Roy Lewallen
> . . .
> If I fail to understand something you wrote, I figure it's my lack of
> comprehension, not your failure to communicate.
Thanks, Wes. But when you actually get down to it, they amount to the
same thing.
Roy Lewallen, W7EL
Mark Keith
Well, I guess yes and no. To me, when it comes to short loaded
verticals,
both base and center loaded coils increase the electrical length of
the whip I suppose. They both load the whip to where the input
impedance is purely resistive at a lower frequency than without the
coils. But just increasing the electrical length as you refer to it is
not the mechanism that improves current distribution. If you have a
base load, and a center load, and the coil is tapped to have each
resonant at the same frequency, I consider them equal lengths
electrically as far as the point of pure resistance. "resonance" They
have different current distributions though. But as Roy noted, only
the center or top loaded whip actually "acts" like it's a longer
radiator than it is. I can see how people view that adding coil turns,
adds electrical length. But it's a different thing than the
"electrical length" Roy is talking about.
Also, myself, I still consider a base mounted coil on a short
vertical as a "loading coil", not an impedance matching coil. I don't
think current distribution should be an issue whether or not a coil
qualifies as a "loading coil" or not. The only job of the coil on a
short vertical is to tune out capacitive reactance. Base or center
mounted, they are doing the same thing. A base loaded short whip will
still require impedance matching, as does the center loaded version.
On the 5/8's antenna, the base coil is the impedance matching coil. A
totally different animal...
Now, when you talk about "electrically lengthening" the radiator as
Roy talks about, improved current distribution and increased radiation
resistance will be required to qualify as such. Dunno...I think I'm
starting to chase my tail. I'll be good and confused if I keep this
up....I don't want to get to the point where I start spewing BS to try
to explain myself. :( MK
'Doc
Mark,
I don't question the fact that an antenna may require
impedance matching, and after 'filtering' all the rest
about current distribution, then your answer to my question
would seem to be, yes. Thank you.
'Doc
Mark Keith
I'm still not exactly sure which improved property it is you refer to
though.
Do you mean having the radiator be resonant, or the improved current
distribution? If you filtered the part about current distribution,
then I assume you mean the act of tuning out the capacitive reactance.
I guess you could say yes. You would need to electrically lengthen the
antenna per say in order for it to be resonant at a lower frequency.
But I would say no for the improved current part. I see the main gist
of Roy's post about this. He is using the term to mean another thing
than you are in this case. I guess his initial posts about it were to
warn not to confuse the two uses of the term. His use describes a case
say where you have a top loaded antenna. The current distribution is
linear, and the radiation resistance can rise as high as 4 times the
amount of the base load. The antenna "acts" like it's a taller
radiator than it really is as far as the current distribution. This is
a case as the term as he is using it describes.
I think anyway...I see what he's getting at anyway... MK
Richard Clark
wrote:Thanx Wes,
For the classic cultural touchstone. Strother Martin has always been
a most interesting character actor for me.
73's
Richard Clark, KB7QHC
Tarmo Tammaru
I think "Electrical Length" is advertizing hype that is being taken out of
context, if there, in fact, is a context for it.
Tam/WB2TT
"Roy Lewallen" <w7...@eznec.com> wrote in message
news:v3k7hvj...@corp.supernews.com...
Mark Keith
If the 'physical'
> length of the antenna isn't changed to change it's
> electrical properties, then I would think that it would
> be proper to say that it's 'electrical' length has changed
> to have those properties that are desired. Yes, or no?
> 'Doc
After thinking about this a while, I would have to say yes, the
electrical length would have changed if say you used a coil or top hat
for loading. The structure is resonant at a lower freq than if the hat
were not there. It's kind of hard to avoid really if you add a
capacitance hat to the top of a vertical. And this would qualify as
"electrical lengthening" as far as Roy used the term in the case of
the hat. I guess what I was trying to get at, was that loading, or
"electrical lengthening" if some want to call it that ,or call it just
plain tuning out reactance, will not improve current distribution
alone. It's the act of placing it towards or at the top or end, that
is really doing the trick the way I see it. Maybe this will clarify my
answer to your question... MK