----- Original Message -----
From: "Brian Tarmar" <brian.tarmar@post.serco.com>
Subject: Transponder Aerial Location
"Having reached the stage of planning cable routes which separate as much as
possible power from feeders, can I mount the transponder aerial/antenna in
an outer underseat stowage? Either totally within the airframe or
protruding."
Brian,
I fitted a Garmin GTX 320 about a year ago, which comes with an antenna
and installation instructions which list the following considerations:
A. The antenna should be well removed from any major protrusions, the
engine, propeller and antenna masts. It should be as far removed as
practical from landing gear, access doors or other openings which could
alter its radiation pattern.
B. The antenna should be mounted on the underside of the aircraft in a
vertical position when the aircraft is in level flight.
C. Avoid mounting the antenna within three feet of the ADF sense antenna
or any other communication antenna and six feet from the DME antenna.
D. To prevent RF interference, the antenna must be physically mounted a
minimum distance of three feet from the GTX 320.
In respect of this last consideration, Graham Clarke told me that as
long as the antenna cable was at least three feet long this consideration
would be satisfied.
I originally mounted the antenna on the centre line just below the flap
actuating cross tube, which worked well until I took off from a muddy field
and found that it doesn't work at all when caked with wet mud (I have a
monowheel). I then moved it to the inboard underseat bay, which is
relatively horizontal in the flying attitude, and found that it worked
equally well and that as a bonus there was no longer any danger of damaging
the antenna when taking the aircraft on or off the trailer. I have recently
fitted mode Charlie and have had it confirmed that this is working
correctly.
My ground plane is a circular piece of aluminium sheet about 3" in
radius. I was told that the ground plane only has to have the same radius as
the length of the antenna.
Kind regards,
Roger.
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