Aircraft is a Lancair with antenna hidden in the horizontal stab. How
'bout directing us to some reading material about how to stop it.

A friend of ours had this exact problem in his SX-300. I helped him
troubleshoot it ("OK, key the mike!" for hours on end...) while he
stood on his head fiddling with antenna wires.

He found that the culprit was the antenna wiring going up into the
tail. Since replacing that wiring would have required opening up the
vertical stab, he found that he could stop it from happening by
shielding it with strategically placed pieces of aluminum foil. He also
minimized it by re-routing the cable, and replacing some of the
connectors that were "leaking".

By doing this he was able to eliminate the porpoising on one of the two
frequencies it was happening on, and he minimized it dramatically on
the other.
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"