I'm not an antenna expert, but I have installed a few that work.
A couple VERY general rules I use for antenna ground planes.
A transmitting antenna needs a ground plane. Com, xpnder, elt, etc-yes. GPS-no.
The ground plane radial should be the same length as the antenna . Since this is impossible in our aircraft, as large as possible is a fair compromise. 2 ft square works ok for the CI 122 com antenna.
The elt antenna works ok with a smaller ground plane. In the IV's an aluminum plate spanning the empennage under the fiberglass portion works well for a small antenna farm, including the elt and gps.

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Jon Hadlich
AI Systems
(541) 815-7381