Today I built my transponder antenna. I had already collected some data . . . read Jim Weir’s stuff, and then ran across some transponder antenna info from Dynon. After a little bit more research, I decided to follow Dynon’s recommendation to go with a square 120mm x 120mm (4.72″ x 4.72″) metal plate for my transponder antenna. This allows for a wider range of frequencies vs a round plate. Also, this size plate is optimized for transponder frequencies. I used aluminum of course for weight, 0.025″ thick.
I cut out the plate & then drilled a 1/2″ hole in the middle of the plate, chamfered the hole & then installed the TED transponder antenna.