RF interference issue again (esp. for E Drucker and Jim Weir and other RF wizards)

On 5-Nov-2003, (Snowbird) wrote:

OK, here is what I don't understand:

We have this problem with the airplane's master switch turned OFF

Can the receiver's RF amp generate intermod products when the
power is switched OFF?

Very, very unlikely. Those who propose this theory assume that an
interfering intermod is being generated within the antenna system or preamp
of one radio and then being re-radiated out the antenna to another com
antenna, where it interferes with reception on the second radio. What this
theory fails to take into account is the large attenuation that the intermod
would be subject to in the propagation between antennas. The far more likely
scenario is that the antenna farm is transmitting two (or more) very strong
out-of band signals that cause intermodulation interference in your com
receivers. My guess is that this problem exists for others as well,
depending upon the band selectivity of the front ends of their radios.

There is a relatively simple way you can test this theory. What you need is
a 6 dB RF coaxial attenuator. These little buggers cost around $30 each,
but maybe you can borrow one from your avionics shop. Put the attenuator
between the antenna and receiver on your handheld. (You will, of course,
have selected an attenuator with he proper RF connectors on it. Otherwise,
you will need suitable adaptors.) Then fly to the location where the
problem exists. I'll bet that with the attenuator you will no longer get
the interference on the handheld, but you will probably be able to receive
the TRACON signal. Assuming I'm right, here is what's going on: The 6 dB
attenuator attenuates ALL signals going through it by 6 dB, including the
strong out of band signals that are causing the intermods. However, the
amplitude of the intermod that these signals cause is thereby attenuated by
more like 18 dB, probably enough to make them too weak to cause problems.
The TRACON signal is also attenuated by 6 dB, but it is probably strong
enough to begin with so that you will still be able to receive it OK. One
caution: do NOT transmit on the handheld when the attenuator is in place.
Depending upon the TX output power of the handheld and the power rating of
the attenuator, it (the attenuator) could be damaged. In any case,
transmissions would be attenuated by 6 dB, which might make them to weak to
be received by the ground station.

--
-Elliott Drucker