Transponder antenna ground plane - Help!

On Monday, March 24, 2014 8:37:57 PM UTC-7, Bill T wrote:
Don't let them cut your fiberglass...

That's all well and good if it is indeed fiberglass. I can't remember if the original poster said what their fuselage is made of. But if it's carbon fiber, an internal antenna will not work because of the opacity of carbon to radio waves.

With carbon fuselages, you have to either put the antenna somewhere that isn't carbon (sometimes the fin and/or rudder are made with radio-transparent fiberglass or aramid fibers), or you must install an external antenna.

With my gliders, the fuselage and fin is made with carbon, but we put the com antenna in the aramid fiber rudder. We haven't installed any transponder antennas yet, but when we do we'll probably just install an external blade antenna and call it good.

Thanks, Bob K.

Since you mentioned the Schleicher TN and the Schleicher TN's list the RAMI AV-22 as a suitable antenna I assume you're installing the transponder in a Schleicher. RAVI's installation instructions on their website give no guidance as to sizing the ground plane as they only refer to externally mounting the antenna on a metal airframe, but as you found their engineer is quoted on the Cumulus website as recommending 30cm minimum diameter... I would ask them and refer them to the aircraft manufacturer's TN spec of 13cm and see if they think it's alright.

Just for good measure the AC43-13 says that a transponder ground plane should be 12 inches diameter minimum.

Isn't t it nice to get contradictory information from the authorities? Confusion is fun!

I'm not sure whose instructions are considered the final authority under the FAA. Perhaps it's the aircraft manufacturer's as they give your A&P and avionics shop the only instructions specifically about your glider to use to authorize the installation. It seems unlikely that Schleicher's instructions would result in unacceptable performance though given how many gliders flying in EASA airspace must have had transponders installed in accordance with them by now.

The L2 antenna does seem convenient but it sounds like the original poster requires a TSO'd antenna. The L2 isn't so far as I know.