(Snowbird)
shared these priceless pearls of wisdom:


-
-Well, I'm not sure the decision tree is quite this binary from
-what folks are saying.

Well, then take other folks advice who have had this problem and solved it. I
gave you the method that works for me.

-
-If I drive to the antenna farm and get the interference,
-does it prove the problem is outside my plane, or just that
-the handheld is also more susceptible to it?

It proves that the problem is outside your aircraft.

-
-If I drive to the antenna farm every day for a month and don't
-get the interference, does it prove the problem is in my plane,
-or that the interference is several things combined some of
-which aren't line-of-sight to my current ground location?

That is possible. I am trying to eliminate one thing at a time. And, that's
why I suggested that you find a friend that lives close to the farm and let them
listen around the house for a week or so. It MAY be when the cops key up their
repeater that it is mixing with channel 4. It MAY be when the local hams key up
their machine that it is mixing with Rock 102. It MAY be any combination, and
if it is intermittent in your aircraft, the odds are good that at SOME point in
the week your handheld will hear it.

-
-This is not to say that I don't think it's worth at least
-a drive to the area, and a flight in someone else's plane
-with my handheld.

Drive first. Fly second.

-
-But what I'd like to understand is this:
-
-How could my nav radios (or my handheld) be contributing
-to this problem when they are *powered off*?

The nav radios (or com radios, or ELT, or...) have an input circuit that is
comprised of a transistor. However it has to get to this "RF Amplifier"
transistor, through whatever filtering, it gets to the transistor. The
transistor is nothing more than a couple of diodes back-to-back. A diode,
powered or not, is an inherent "mixer". A mixer takes two signals and outputs
the sum and difference of these signals, plus (in decreasing strength) the sum
and difference of all integer multiples of those frequencies.

Let me make the math simple. Take two signals, one at 50 MHz. in the 6 meter
ham band and one at 60 MHz. at the tag end of channel 2.

Turn your nav radio off. These two signals will get in to the front end of your
nav receiver and mix in the RF stage. How much signal is getting in is a
function of the steepness of the filter your nav radio designer put into the
receiver. No filter is perfect; there will ALWAYS be some little bit of signal
leaking in, and the more powerful the extraneous signal, the more it will power
its way into the front end.

So now we've got 50 and 60 MHz. in the radio. The RF transistor takes those two
signals and mixes them so that you get 110 MHz. and 10 MHz. (sum and
difference). You ALSO get 160 MHz. (2x50 +60), 170 MHz. (50 + 2x60), 220 MHz.
(2x50 +2x60), 20 MHz. (2x60 - 2x50)... and so on ad infinitum. If any of these
"spurious mixer products" falls within the passband of the nav input filter, it
will be reradiated out the nav antenna directly into your com antenna.

Now go figure out how many AM, FM, TV, public service, amateur, and CB
transmitters there are in your area. Do a sum and difference, plus a harmonic
(integer multiple) sum and difference for ANY COMBINATION of them, and you begin
to get an idea of the magnitude of the problem in finding the culprits.

Did that help?

Jim
Jim Weir (A&P/IA, CFI, & other good alphabet soup)
VP Eng RST Pres. Cyberchapter EAA Tech. Counselor
http://www.rst-engr.com