I have placed a Blade antenna on the top of the turtle deck as far
back as I can reach. This is about even with the trailing edge of
the wing on my Nimbus 3D. I consulted a neighbor who works in an
Avionics division of a local aerospace company. His comment was the
placement wouldn’t matter much as there is always some angle to the
interrogating station.

I have placed a third battery on board solely to power the
transponder. There is a Battery mount forward and to the right of
the box that the landing gear retracts into. This is accessed by
removing the panel behind the rear seat back.

Hope this helps.

Paul Cordell
Scottsdale Az

PS - Eric Greenwell has produced a document regarding transponders.
He's used a photo of my antenna mount. Maybe he can provide a link to
the document.