The minimum size I have seen quoted for the transponder ground plane is 5.5
inch diameter. The height of the antenna is 2.65 inches above the ground
plane. Both of these dimensions are from "The Aero Electric Connection" .
The 26 inch dimension you quoted is the minimum size for irregularly shaped
ground planes.
With a round ground plane the antenna should be located at the center, of
course . A simple way to insulate the antenna from the ground plane is to use
a BNC connector in the center of the ground plane and solder the antenna to
the center conductor. The thickness of the ground plane is not critical.
Aluminum works about as well as copper. I made one from 0.010 inch aluminum
which can be cut to shape with scissors. Weight of this ground plane is 12
grams.
Not flying yet so can't say how it will actually work.
Regards, John A044
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