[amsat-bb] Re: ground plane on
ko6th_greg at hotmail.com
Sun Apr 17 22:49:54 PDT 2011
On 17/04/2011, at 15:53, KF1BUZ <kf1buz at gmail.com> wrote:
> A Copper Jpole, has this been tried?
> Just thinking it might make my getting into the birds better.
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,
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