There is a unique antenna that works fine as a transponder antenna - with no
blade or stub to break off or impede airflow. It is called an annular slot
antenna. Many antenna engineering textbooks contain information on the,
but they are not common. The entire antenna is flush with the aircraft skin
and they are vertically polarized. They would work with fiberglass
aircraft.

I did find a photograph and description of a commercial one at
www.mircrowaveeng.com Look for the L-Band Annular Slot Data Sheet (8 of
10). I suspect this particular unit may be beyond the budget of the average
sailplane owner, but you can see how what they look like.

It would be a good candidate for a slippery sailplane that does not want
stuff hanging out.

Colin Lamb