Well here is a link to a picture of a Ventus with the transponder antenna located as per Schempp Hirth.
Seems to work well only is straight flight. In any banking flight, the antenna is shadowed by the fuselage and/or wings.
Maybe a location on the turtledeck is better ?

https://www.airliners.net/photo/-/Sc...HgcaJ1/QJ2lC9A

Dan