Unfortunately, the frequency that the transponder transmits on is
determined by a vacuum tube in a cavity resonator. Anything that
changes the physical dimensions or impedance of the cavity detunes
it, causing a frequency error.

Changing the coax/antenna shouldn't be enough to mistune the
cavity, but if it was off to one side already, then a changed
load impedance could send it off far enough to fail the test.

A good transponder check measures the center frequency of the
transponder pulse while the transponder is in the aircraft...

MikeM


Ray Godwin wrote:

I just had a VFR Cert. done and the avionics shop said that the antenna
of coax was causting the frequency of the transponder to be out of
limits. Of coarse they had to bench adjust the frequency of the
transponder to compensate. Has anyone ever had this problem before?
Thanks,
Ray