xponder ground plane

[Henk Roelofs <henk@loginet.nl>]

In reviewing the excellent photos of Thomas Scherer on the Europa
Club Web site I see a rather small ground plane for the transponder
antenna. I was told that it must be at least 65cm (26") in diameter.

Was I misinformed? Does anybody know the minimum size for the ground 
plane? Since the copper sheet we are using is quite heavy it seams 
that we can save some weight here.

Thanks in advance,
Henk & Bart #264
(still waiting for the BMW)