Hi Eric,
I initially wrote:
> >A J-pole is not as ground independent as one might think or
> >read. It is an end-fed 1/2 wl antenna , fed by a 1/4 wl
> >stub. Because the end impedance of a 1/2 wl is not infinite, but
> >rather some finite impedance between a few hundred and a few
> >thousand ohms (depending on conductor diameter), the terminal
> >current of the balanced line driving the actual radiator are not
> >balanced. This causes the balanced feedline section of the
> >J-pole to have common mode current adding destructive radiation
> >to the pattern that raises the wave angle and lowers gain.
> We did not observe any telltale indications of an excessive
> feedline radiation problem with the one we used on 160 a few
> years back. And we were looking for this effect during the
> tuneup phase of the project.
Respectfully Eric, it depends on how you "looked". The antenna doesn't work
"bad", it just doesn't work the way most people or models predict.
> We did isolate the coax at the feedpoint (turned out to be only a
> few inches out from the short for 50 ohm feed) with a bunch of
> ferrite beads. But we never had any bead temperatures above
> ambient even when running significant power into the system.
First the terms. The top 1/2 sticking straight up is the radiator and
intended to radiate. The parallel section below it is the (matching) stub
and is assumed to not radiate in the regular J.
Was the stub part of the pole isolated or grounded to another structure?
If the stub section is isolated from other conductors, including the mast
and feedline, common mode excitation is minimized by the stub section and
antenna forming an unbalanced to balanced connection. In that case the
antenna will work about like a 1/2 wl vertical, especially if the radiator
is thin compared to effective diameter of the two wire stub used to feed
the radiator.
If the stub section is grounded or connected to a low impedance mast or
tower (or anything else metallic), maximum common mode excitation of the
stub takes place no matter what you do with the coax. The low Z at the base
of the stub cause the top of the stub to be a high Z, and the overall
structure is excited by the junction of the unbalanced radiator to the
balanced stub. This is particularly true when the radiator is thick,
because one terminal of the two wire stub drives an open and while the
other drives the thick radiator which can have an end impedance in the
hundreds of ohms (if thick).
At that junction, the power divides in proportion to impedances, with most
of the power appearing in the HIGHER impedance load. Grounding the stub at
the bottom or making the stub thin compared to the radiator can make the
stub radiate as much or more than the actual 1/2 wl element, and current is
180 degrees out-of-phase with the intended radiator. That places a pattern
null at zero degrees elevation.
If a model does not include real-world effects like grounding or
less-than-perfect feedline choking, it was not an accurate model. This very
thing happen in an article about end-fed zepps, where someone modeled the
end-fed zepp as a perfect antenna with a perfect ground independent power
source exactly where the antenna and feeder were optimized. In the real
world, especially if someone attempts to feed an end-fed zepp (which is
really a bent J-pole) with a voltage-type balanced tuner or transformer,
the feeder (or matching stub in the J-pole) will radiate quite
significantly.
You can use that feeder radiation to advantage in the J-pole by
intentionally feeding the bottom of the stub with a balanced voltage
source, intentionally making each voltage at the terminal offset from
center with reference to a ground plane. By using the correct offset, you
can force the antenna to be excited with in-phase currents and behave like
a 1/4 wl in-phase with a 1/2 wl and get a few dB of stacking gain.
> Also, we had no "RF in the shack" types of problems and the coax
> feedline dress or length had zero effect on the observed feed
> impedance. And that was true both with and without the beads on
> the coax.
Feedline radiation is another topic. RF in the shack is an esoteric term.
You might have had "RF in the Shack" and not even known it. Or the feedline
might have coupled off all the RF through mutual coupling to surroundings
before it got there!
> Bottom line is that it worked well for us and allowed us to use
You had a good reason to use one. I did also, I wanted to use a 190 ft
tower on 80 meters with a low wave angle. The only difference is I
optimized mine by forcing the antenna to radiate like a skirt collinear. No
one would be able to tell five dB of difference without a reference A-B
test, we all know that. For example, people are often enamored with their
B&W dipoles even though they are ten dB down from a regular dipole on many
bands. Myself, I like every last dB.
Once again, try to find Danny (K6MHE's) CQ article on J-poles. These
electrical problems are the reason why commercial systems never adopted the
j-pole, despite its mechanical advantages.
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