Transponder Antenna Location

We have been researching transponder installation options for our
ASW-27B (carbon fuselage) and have seen or heard about antenna
installations on the bottom of the fuselage in front and behind the
gear ... on top of the deck ... and with internal or external ground
planes ... and without ground planes. All appear to work, although I
have not seen any hard empirical data or testing results that shows
one type of installation is marginal ... or another superior.

Is a ground plane required on a carbon fuselage ... I understand some
German manufacturers don't install GPs during the factory install.

If a GP is required, should it be outside the fuselage ... or will it
also work as well if it is inside?

Will the transponder system work satisfactorily with the antenna on
the top of the fuselage? ... understanding that the radiation pattern
would not be ideal for the ground based interrogation systems.

Gracias'

KK