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

On 6-Nov-2003, (Snowbird) wrote:

I'm still trying to understand how a powered OFF radio with
the power OFF in the plane might be contributing to this
problem and hoping someone will explain this to me.



Sydney,

It IS possible, but it is also extremely unlikely to be the cause of your
problem.

Those that propose this theory are assuming that the RF signals being picked
up by your com antennas are strong enough BY THEMSELVES to provide power to
the receiver of a radio that is not otherwise powered. This seemingly
strange phenomenon is most often observed when a car (with a typical AM/FM
car radio that is turned off) is driven right up to an antenna farm.

In your case, I assume that you are flying either far enough horizontally
away from or high enough above the antennas for safety. Let's say a minimum
of 1/2 mile horizontally. Now lets assume that the strongest broadcast
signal is 100,000 watts "effective radiated power" and a frequency somewhere
around 100 MHz. At a distance of 1/2 statute mile the power from that
signal that would be picked up by an aircraft com antenna would be about 10
milliwatts. This is a spectacularly strong signal for purposes of
reception, and certainly more than enough to make it impossible for the
attached receiver to pick up any other signal, but nowhere near strong
enough to cause the receiver, if not otherwise powered, to generate and
re-radiate an intermodulation product.

So, if the signal WERE strong enough to provide enough power to cause an
otherwise unpowered receiver to generate and re-radiate an interfering
signal, the same powerful signal would pretty much wipe out operation of any
other nearby receiver that IS powered on.

By all accounts, what you have is plain old garden variety intermodulation
interference. The intermodulation products doing the interfering are being
generated in the same receiver that is being interfered with. In an earlier
post I offered a relatively simple way to prove this.

--
-Elliott Drucker