Darryl Ramm wrote:
The TT-21 as a Transponder utterly blows away any other transponder
available in the USA for use in gliders.

Just be aware that the TT21 is a class 2 tansponder, i.e. not certified
for use above 15,000 feet, and therefore not suited for gliders. Buy the
TT22 instead which is class 1.