Earlier, (Jay) wrote:

- required dimension

Larger in radius than the wavelength you are working with.

As Wayne Paul pointed out, for a quarter-wave whip like a typical
transponder antenna, a quarter-wave radius ground plane is really all
you need. A full wavelength would be around 22" in diameter. That's
way overkill for one of those little antennas.

My actual experience seems to validate that a 5.5" dia round or square
ground plane is adequate.

Thanks, and best regards to all

Bob K.