Hi Eric,
This is gone so deep into your specific antenna, my points to the people
considering an HF J-pole are probably missed. My points are..
1.) It is easy to "mess up" a J-pole, because it is a *very* poor or
problematic antenna design (just as end-fed Zepps are).
2.) The unbalanced radiator, fed by a balanced stub, will ALWAYS set up
some common mode excitation of the stub. That common mode current ALWAYS
causes loss of FS at zero degrees elevation.
3.) How much of a problem that creates is a matter of impedance ratios at
the top of the stub and the bottom of the 1/2 wl radiator.
The J-pole (and Zepp) violates one rule of good design. Never make a
balanced to unbalanced transition in a system without proper consideration
of that transition. The J-pole makes TWO transitions from balanced to
unbalanced. One at the junction of the stub and radiator, the other at the
junction of the coax and stub. Often people ignore both areas, and the
result can be MUCH less than optimum performance. Some might not know it,
and might not care, but I suspect many antenna builders want every free dB
they can get!
To maximize performance of a J-pole:
1.) Do NOT ground the bottom of the stub or connect it to any large
metallic structure or feedline (without a choke balun).
2.) Do NOT use a thick radiator, especially when compared to the stub
thickness.
It's always better to install an antenna knowing what to watch out for, if
you want to get maximum performance!
One way to un-optimize a J-pole would be to build the 1/2 wl element out of
thick tubing, and the stub from a small open wire line. Another would be to
ground the bottom end of the stub (considered to be an "electrical neutral
point") to a tower or even a feedline-without-a-balun.
73 Tom
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