On Monday, December 30, 2019 at 6:44:50 AM UTC-8, Dan Marotta wrote:
In the absence of antenna diversity, I'm pretty sure all general
aviation factory built aircraft have the transponder antenna on the
bottom of the aircraft.Â* I would think the manufacturers must know
something.

When transponders first came out, the only receiver/interrogator was ground based. So anything certificated prior to TCAS, would definitely have the antenna on the bottom.
Then TCAS came along, and now ADS-B. These will interrogate/receive from any direction. So to be visible from any direction, one should probably have an antenna on top as well as bottom... And satellite based ADS-B likely demands it.
I'm sure Darryl has all the gory technical details :-) But my guess is that unless told otherwise, and considering cost of certification, etc., putting the antenna "where it's always been" is why it's on the bottom. :-)

5Z