Hi Dan,
On 17/04/2011, at 15:53, KF1BUZ <kf1...@gmail.com> wrote:
> A Copper Jpole, has this been tried?
> Just thinking it might make my getting into the birds better.
>
> Thanks
> Dan
> KF1BUZ
>
If I interpret this right (subject + message), you are asking if adding a
Ground Plane to a J-pole antenna will improve its performance in a satellite
ground station application. I believe the answer is no, though someone would
need to do the modeling to understand for sure.
A J-pole is an end-fed dipole, with the "J" portion being a 1/4 wave long at
the frequency of the antenna. Recalling some RF theory stuff, a 1/4 wave
"matching section" has a low impedance on one end, and a high impedance at the
other. One end goes to the coax feed line (low impedance), and the other is
attached to the end (high impedance) part of the dipole. You will find that
the single pipe section of the J-pole antenna is about a 1/2 wave long at the
antenna's design frequency, and since it's connected directly to the end of the
matching section, it makes for an end-fed dipole. Some designs use a 5/8 wave
dipole section for a little extra gain towards the horizon.
So a J-pole antenna is actually a pretty effective satellite antenna, similar
to a simple ground plane antenna but mechanically more robust. I've used both
kinds. My very first satellite contact ion 1993 was using one for the uplink
into RS-10, and that contact was followed by many many more. That antenna is
still in service nearly 18 years later. (If you hear or use the KO6TH APRS
iGate, you're using it!) I've got two SO-239-type Ground Plane antenna
carcases in the garage; they didn't last. As a satellite antenna, both Ground
Plane and J-pole antennas do have a null directly overhead, but very few
satellite passes go directly overhead, and when they do, they spend very little
time there. So don't sweat it.
A J-pole with a 1/2 wave section on top will also work as a dual band 2M / 70CM
antenna, with the upper band on the antenna's 3rd harmonic. Using the antenna
that way, I'm told, it has a radiation pattern that is lifted somewhat from the
horizon, so it should be a good match for satellite work.
But, back to your question... The J-pole antenna is a totally balanced system
in itself, and doesn't suffer for not having a ground plane below it. What
putting a ground plane some distance below the end of an end-fed dipole will do
to the radiation pattern, however, is a modeling task for someone at a higher
mental pay-grade than me. My guess is that it will depend very significantly
exactly where the ground plane is mounted. You could alter both the radiation
pattern and the feed point impedance with that addition, and maybe make things
worse.
Hope this helps a little,
Greg KO6TH
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