>I have a Narco Transponder on my Europa, connected to its aerial on a
>ground plane about 6 foot away. Although my VHF radio (Garmin GNC 250) is
>fed from a separate bus, with widely separated aerial cables, I get
>intermittent interference when the Narco responds to a radar - just a short
>buzz as the identification lamp lights on the Transponder.
>Any ideas on what I can do to suppress the interference?
A very common problem . . . especially with "plastic"
airplanes. Your transponder puts out a stream of
very narrow, high power pulses that carry the digitally
encoded data for your squawk, altitude, etc. Common
propogation modes for the transponder to interfere
with other radios are coupling between antenna feedlines
(not case here because you've separated them), direct
radiation of transponder energy into the antenna of
the victim radio and radiation of transponder energy
into wiring associated with victim radio.
Try turning down the volume on your VHF comm and see if
the transponder noise is still there (of course you
have to do this while the transponder is being iterrogated
by a ground radar). If the noise goes away, then it's
most likely getting into the comm receiver's antenna.
Moving either or both of the antennas to increase
separation may do the trick.
If the noise does NOT go away, then it's getting into
the wiring. You can try ferrite filter "beads" on wire
bundle going into back of comm receiver, also shielding
may help. If push comes to shove, a filter assembly
consisting of inductors and capacitors on each pin of
the wiring to the comm receiver may be necessary.
Metal airplanes dont have the latter problem very
often due to the isolating effect of the aircraft's
skin . . .
Wish there was a "magic bullet" but what you're experiencing
can be one of the hardest problems to fix.
Bob . . .
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