"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"