This is the one time it's easy to put antennas in the fin and route properly sized coax to them (may be worthwhile engaging a microwave engineer).

Schleicher puts the Com antenna in the back of the fin. Think where the transponder and Flarm antennas can go - minimum 30 cm apart.

Air Avionics has announced their Air Traffic unit which can transmit Flarm on both antennas (very handy with carbon fiber fuselage - on the glareshield, there's a blind spot below; in the fin the tailplane makes another blind spot), but so far there's no documentation or word on North American licensing.

Access to the coax connectors might be nice.