Transponder Antenna Location

[robcaldwell]

I originally mounted my xpdr and ADSB antennas down the belly along the stringers. After realizing I had almost no ground plane in that configuration, I did some research. I learned that shorter coax runs are better, and forward of comm antennas is better than aft. A few references were made to the placement of xpdr and ADSB mounted on the bottom of the cowl, as shown here. ADSB will go on the other side. Any thoughts? (Com antenna will at the back of the tunnel with the whip pointing rearward.)

IMG_1522.JPG

[Mark Goldberg]

I have put the transponder antenna more or less like you did in the picture. But I have put my comm antennas on top of the wing usually as you see on certified aircraft. Mark

[spinningwrench]

My Citabria has the transponder on the bottom of the boot cowl. American Champion puts the COM antenna on the top of the fuselage half way between the wings and tail. They install a large aluminum ground plane underneath the antenna and in recent years have put even a larger ground plane in. It all works very well.

[rodsmith]

According to the Aeroelectric Connection antenna chapter, the transponder antenna only requires a 6" diameter ground plane.

[Archer39J]

I don't recall if it's in the AEC or if I've seen it mentioned on the forum, but performance drops off quickly if your radius isn't at least 3". So it's recommend you make it larger or just put it on a surface that's larger, like the boot cowl panel. I'd keep it 3" or more from any edge.

[swpilot3]

I put mine exactly in the same place. ADSB on one side and Transponder on the other. Works great. I always pick up ADSB and ATC has never complained about my transponder. I do quite bit of radar flight following and class B transitions here in the PHX area.