A 12 x 11 belly pan is far too small for a COM ground plane. Find a tube
cluster weld, with four or more tubes joined together and clamp (or weld) a
small plate near the center of the cluster, just big enough to hold the base
of the COM antenna. The tubes will act as the ground plane.

As to the transponder antenna, it is a relatively noncritical design. The
cheap little "rod with a bb on the end" has worked for tens of thousands of
installations and I see no reason it won't work for you. Mount it to the
center of the belly pan and you should be OK.

Jim


"Steve" wrote in message
om...
I bought a Cassutt (Primarily tube, fiberglass and fabric). It has a
light-weight BNC battery, starter and alternator, and a old Grand
Rapids engine monitoring system but no radios. I wish to install a
panel mounted MicroAir Radio and Transponder. I think I can figure out
the wiring of the radios but the only aluminum on the airplane is a
12"X11" aluminum belly pan.

Can I use this as a ground plane for both the VHF and transponder
antennas? If so, where should I position them with respect to each
other and the edge? With regard to the antennas, a transponder antenna
came with the MicroAire Transponder, and I plan to purchase a VHF
antenna from aircraft spruce. Aircraft spruce however, has many
different designs, brands and of course prices. Which one would give
me the best performance for my application? I have about 21" from the
belly pan to the ground in the three point position.



Any help you could give will be greatly appreciated