On Sunday, February 16, 2014 11:12:51 AM UTC-6, Jeff Liebermann wrote:
On Sun, 16 Feb 2014 06:32:38 -0800 (PST), W5DXP

wrote:



On Saturday, February 15, 2014 9:58:10 PM UTC-6, Jeff Liebermann wrote:

However, a J-pole (or Zepp) is not a 1/2 wave antenna. The driven

element is a 1/4 wavelength long, and therefore DOES require a ground

plane.



Sorry, that is not correct.



Agreed. See my followup to my posting where I noticed that I've been

building J-pole antennas with the coax feed connected to the wrong

element. Judging by some of the photos I've found, I'm not alone.


I never built J-poles myself. Never was that fond of the design
for some reason. I always preferred the "ringo" method of feeding
a base fed half wave. IE: a flat single turn tapped coil, and coax
capacitor. I used them on 10m mostly. They work decently well in
most cases with no decoupling section. But they work even better
with decoupling. I used a 1/4 wave length of coax to a union which
was attached to the mast, and had a set of three radials.

But... The 5/8 ground planes were always better than the 1/2 waves
on distant 10m local stations. Even the decoupled version.
And being low angle space wave stuff, it's a pretty good test.
Both antennas were at about 36 ft up at the base.

According to a modeling exercise I did once, the best way to
run a 5/8 GP is with 5/8 radials. I've also used 3/4 wave radials,
which seemed to work well. But according to the modeling, the 5/8
radials will give more gain. Starts to resemble a dual 5/8 collinear.
And naturally, you would want to use a decoupling section for the
best performance.
This is pretty old, but compares the different lengths for 10m
use.
http://home.comcast.net/~nm5k/acompari.htm

"Jeff Liebermann" wrote in message
...
On Sat, 15 Feb 2014 22:29:31 -0800, "Sal" salmonella@food
poisoning.org wrote:

I can agree with the need for preventing feedline radiation but one thing
you and Terry say may be erroneous. I believe the radiating element of a
J-pole to be a half wavelength long, not a quarter-wave.

You're correct. See my follow up posting. I've been building J-poles
driving the short 1/4 wave element instead of the long 1/2 wave
element.

I looked at
Terry's EZNEC wires list and observed the long side (the radiator or
driven
element) of the J is 57 inches and the short side (the stub) is 19 inches.
(These dimensions agree with my idea of a 2m J-pole. I've made a few.)

As I understand the action of the J-pole, net radiation is low or nil from
currents in the lower third of the antenna (bottom third of the radiator
and
the adjacent stub).

Correct. The original Zepp antenna was designed to be lowered from a
hydrogen filled Zeppelin dirigible. Any sparks or arcing caused by
high voltage from the antenna to the dirigible would be considered a
really bad idea. So, the antenna was designed to have zero voltage
and probably zero current at the closest point of the antenna to the
dirigible.

The desired radiation comes from the top two-thirds (38
inches) of the radiator, which is very nearly a half wave at 2m. Allowing
for so-called "end effect," it's almost exactly cut to 146 MHz.

Well, here we disagree slightly. I once made a similar comment in
this group about the radiation from an end fed collinear antenna came
mostly from the bottom of the antenna. I was corrected by Roy
Lewallen (W7EL) who pointed out that the current through the length of
such an antenna is the same and therefore the radiation from all parts
of the antenna are similarly identical. Unless I'm missing something
(a real possibility), the situation is the same with a J-pole.
https://groups.google.com/forum/?hl=en#!original/rec.radio.amateur.antenna/DREJnRznluQ/bZyCgwa0JvwJ

This is interesting:
http://www.qikzepp.com/QikZepp_technical_information.html
It shows a 1909 German patent for a Zepp antenna. However, the
accompanying description once again makes the mistake of feeding the
1/4 WL section and not the longer 1/2 WL section:

Early fixed installation Zepp Antennas were a half-wavelength
long(or multiple) and fed with a 1/4 WL (or multiple) open
wire feed line which uses only one of the wires. The feed
line provided a matching section for the transmitter.

Please check my reasoning and math Your comments are welcome. Thanks.

I hate being wrong, but do I seem to be getting used to it.

I'm used to it, too, so there's no chance you will monopolize it.

I saw your follow up posting. Several comments.

Hams who write construction articles sometimes support the notion of a
consensus about feeding a J-pole from the stub side. Consensus doesn't
carry any scientific weight. Some construction articles acknowledge such a
consensus but claim it doesn't really matter. I've been happy soldering a
small mounting tab to the short (stub) side, then bolting on a panel-mount
SO-239 and extending the center conductor across to the long side. I'm not
sure I care enough to experiment, A versus B. It would involve many hours'
work to get it right.

Maybe you weren't actually clashing with Roy (whom I admire). If he were
regarding the upper two-thirds of the structure as the antenna (and
regarding the lower two thirds only as a feed/matching-section/transformer),
it might only have been a failure to communicate about what is the
"antenna." I risk being wrong again by commenting on brief snatches of
somebody else's conversation.

With an end-fed half-wave antenna, I believe the current maximum is at the
midpoint of the radiating section, 19 inches from the tip of the J-pole's
long section, for example. The current minima would then be at the ends,
most notably the high voltage end feed point.

Cecil's 40m Zepp diagram indicates that the common mode currents are
comparatively small, against the differential currents that do the work.
Some comfort, there.

I think we mostly have it nailed. Please, let any additional nails NOT
involve my getting crucified. Again.

"Sal"

"W5DXP" wrote in message
...
On Saturday, February 15, 2014 6:59:25 PM UTC-6, Sal wrote:
* But what distance is "not far"? A quarter-wave comes to mind, so a
measurement is called for. Hm-m-m ... is the propagation velocity on the
outside of the coax the same as the inside?

Common-mode chokes work best at the high current points so "not far from the
feedpoint" of a 50 ohm antenna. Two chokes, one at the antenna and one 1/4WL
down the feedline, work well. The velocity factor of a common-mode signal on
the outside braid of the coax is fairly close to 1.0 only slowed down by the
outside insulation layer.
--
73, Cecil, w5dxp.com

Thankew!

"Sal"

In article ,
Jeff Liebermann wrote:


I hate to admit it, but I think I've built them backwards over the
years. Maybe that's why J-poles have such a lousy reputation and why
I think they suck?

Jeff-

Please pardon my ignorance. I understood that a J-Pole is a half wave
antenna connected to a quarter wave stub. The transmission line is
connected to a low impedance point (50 Ohms?) on the stub.

In the literature, it is usually fed as an unbalanced antenna, but it is
not. Therefore, it should not matter which side has the center
conductor or shield - they are both wrong!

Fred
K4DII

"Fred McKenzie" wrote in message
...
In article ,
Jeff Liebermann wrote:


I hate to admit it, but I think I've built them backwards over the
years. Maybe that's why J-poles have such a lousy reputation and why
I think they suck?

Jeff-

Please pardon my ignorance. I understood that a J-Pole is a half wave
antenna connected to a quarter wave stub. The transmission line is
connected to a low impedance point (50 Ohms?) on the stub.

In the literature, it is usually fed as an unbalanced antenna, but it is
not. Therefore, it should not matter which side has the center
conductor or shield - they are both wrong!

Fred
K4DII

I acknowledge the split opinion on the feed. I've often thought it
shouldn't matter, since the idea is to excite the stub (which is, itself, a
half-wave if you count both sides). I follow the crowd, frankly.

How would you change the feed method? I've had generally good performance
from my J-poles but I'll gladly improve what I do, if you have some ideas.

73,
"Sal"
(KD6VKW)

In article ,
Sal salmonella@food poisoning.org wrote:

How would you change the feed method? I've had generally good performance
from my J-poles but I'll gladly improve what I do, if you have some ideas.

One of the older ARRL guides or antenna books shows a balanced method
of feeding a J-pole. A standard half-wave coaxial balun is used. The
two balanced outputs of the balun are tapped onto the two sides of the
J-pole matching section, some distance above the usual "50-ohms-or-
thereabouts" attachment point.

My understanding is that the impedances "seen" on the two sides of the
matching section won't be identical; the short side ends at an
open-circuit point and the other side "ends" at the beginning of the
half-wave section, where the impedance is high but not quite an open
circuit. Hence, you won't achieve complete balance this way - there
will probably be some current flow on the outside of the halfwave coax
balun section.

"David Platt" wrote in message
...
In article ,
Sal salmonella@food poisoning.org wrote:

How would you change the feed method? I've had generally good performance
from my J-poles but I'll gladly improve what I do, if you have some ideas.

One of the older ARRL guides or antenna books shows a balanced method
of feeding a J-pole. A standard half-wave coaxial balun is used. The
two balanced outputs of the balun are tapped onto the two sides of the
J-pole matching section, some distance above the usual "50-ohms-or-
thereabouts" attachment point.

My understanding is that the impedances "seen" on the two sides of the
matching section won't be identical; the short side ends at an
open-circuit point and the other side "ends" at the beginning of the
half-wave section, where the impedance is high but not quite an open
circuit. Hence, you won't achieve complete balance this way - there
will probably be some current flow on the outside of the halfwave coax
balun section.

I don't know which one would be the best but I have seen 3 methods of
feeding the J-pole. If you insulate the bottom then you hook the feedline
to the bottom with the center of the coax to the long side. If you do not
insulate the bottom you tap up the matching segment so that you get a 50 ohm
(if that is the coax used) match with the center of the coax connected to
the long leg. Then there is the balun made out of coax that is hooked up to
the matching segment so that a low swr is obtained.



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Don't forget the Arrow Open Stub J-pole.

http://www.arrowantennas.com/osj/j-pole.html

In article ,
W5DXP wrote:

Don't forget the Arrow Open Stub J-pole.

http://www.arrowantennas.com/osj/j-pole.html

There is (or was) a nice writeup of this variety on Cebik's web
site. He refers to it as a "variant J-pole". The feed arrangement is
different (it's open at the feedpoint, rather than being fed a few
inches above a short), and the arm lengths are different than with a
"classic" J-pole. The current distributions are different, of course,
but the radiation pattern is only very slightly different than the
"classic" variety.

I have one of these dual-band types in my "go-kit" (it's actually a
knock-off, built locally based on the plans that Arrow used to have on
their web site) and it's served me well. I usually mount it at the
top of a two-or-three-segment aluminum mast, sitting in an old hefty
movie-camera tripod base.

Much too large and heavy for a vehicle, of course, but I imagine you
could build a somewhat-similar open-stub J-pole using much lighter
materials (e.g. fiberglass whips with wire fastened inside or
outside).

On Tuesday, February 18, 2014 6:26:40 PM UTC-6, David Platt wrote:
The feed arrangement is
different (it's open at the feedpoint, rather than being fed a few
inches above a short), and the arm lengths are different than with a
"classic" J-pole.

If one will ignore the long 2m element and draw a schematic of it just for the 70cm band, one will realize that it is actually just a standard Zepp antenna.