I installed the Bob Archer Transponder Antenna in the rear of the fuselage just ahead of bulkhead B below the leading edge of the vertical stabilizer. This is a small antenna mounted on a phenolic isolation board but it is very rigid, so I needed to fabricate some standoffs to attach it to the aircraft.
Due to the extreme curvature of the rear fuselage/empennage, I had to custom fit the bracket system while contorting myself into the back of the airplane. Not fun! Ultimately, I got the complete assembly mounted within 2 degrees of centerline and well within tolerance for a quality transponder reading for ATC.
I will cable up the transponder antenna once I begin work on the panel.