KT-76A frequency

[Ray Godwin]

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

[dave]

Ray,

I have seen this multiple times. I've seen very large swings in freq and
when the problem was fixed, the xponder was back where it needed to be.

The cause of the frequency shift should be fixed, not adjusting the
radio to compensate for it.
It could be a bad connector or a corroded antenna mount.
If you fix it right, then you should be trouble free for the next
inspection.

Dave




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
>

[mikem]

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
>

[John]

I would be looking for rust or water in the RF connectors and the
antenna ground connections of the transponder antenna. The tube that
is used for the older transponders is affected by the load impedance
of the coax and antenna. If the impedance changes then the TX
frequency changes. Mine is still on frequency even after 27 years
without adjustment.
John

On Thu, 11 Mar 2004 00:49:41 GMT, 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