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I have the same problem with my
Bob Archer XPDR antenna, which in my ES is mounted on the inside of the large
inspection cover on the left side of the empennage, just under the HStab.
Typically, when I'm 20 or 30 miles out and heading directly toward an APC radar,
they'll call me and complain they're not receiving my transponder. I've
learned to ask them to just hold on for a few miles, as it usually shows up when
I get closer. They usually grump and admit they do have my primary
echo. A couple of times I've had to do a 90-degree turn for ID -- they'll
see it, then sometimes not see it again when I turn back toward the radar
location. So far no problems with transmission to either side or to the
rear.
I did talk to Bob Archer on the
telephone about it. He seemed mildly surprised I was getting any forward
transmission at all, given the amount of junk forward of the antenna location
(spars, gear plates, flap motors, pilots, panel, engine, etc.). The
size and shape of Bob's antenna being what they are, I can't think of another
location for it that would fix the problem. Probably the only way around
it is to put an external antenna on the belly, and build a ground plane inside
with copper foil tape, a la Jim Weir. So far it hasn't been enough of a
problem, often enough, to justify any action. If I flew a lot of legs into
congested areas, however, I guess I'd have to do it.
Jim Cameron
Lancair Super ES, N143ES
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