In article ,
Snowbird wrote:


OK, we're still having our RF interference problem and
our avionics guy pleads 'stumped'. Meanwhile we're going
nuts whenever we need to get radar vectors for the ILS at
our local Class D or when we depart IFR to the SE.

Here is what we know

1) the problem is intermittant. occurs both at night and during day.
2) when it does occur, the problem occurs in a specific
area -- heading towards a local antenna farm
3) legitimate radio transmissions come through loud and clear
4) the interference isn't just random noise, but sometimes has
voices in it (like a radio or TV show)
5) we have disconnected the ELT from its antenna (but left
it turned off in the back seat of the plane) -- problem persists
6) marker beacons on, marker beacons off, nav radios different freqs,
nav radios off, no effect
7) swapped our KMA 20 audio panel for a loaner KMA 20 no difference
8) we have tried turning off the airplane's entire electrical
system and listening for interference on a handheld radio with its
own "stick" antenna. Problem persists (!!!!)
9) we have tried different frequencies while experiencing the
interference -- not exhaustively. here is a list (- means no
interference + means interference)

124.00 -
124.20 -
124.52 -
125.00 -
126.00 +
126.50 +
126.50 mb on, mb off, nav 111.9, nav 110.8, nav off
126.50 handheld w/ alt off, airplane electrical system off
127.00 +
127.10 -
127.25 -
127.27 +
127.30 -
127.50 +
127.97 -
128.00 -
129.00 +
130.00 -
131.00 -
132.00 +

(126.5 is the local tracon frequency where the interference is
problematic for us, which is why I focused there. 127.0 might
be the strongest interference)

geographical location where interference seems strongest
(there's an antenna there, and when we were directly over
it interference stopped)
38 31 90
90 21 75

Can we figure out the frequency and maybe the station which
is causing the problem from the above info?

Ideas? Other tests? Things to check? Help! If we still
get the problem with the plane's entire electrical system off
and using a radio/antenna which is not connected to the plane,
is there ANYTHING we can do or must we just grit our teeth and
bear this?

Everything you say points to "intermodulation interference" -- where
multiple transmissions are hetrodyning against each other, and creating
a byproduct signal on the channel you're listening on. The bad news is
that locating the 'source' of the problem is *difficult*. In large part
because there isn't _a_ source. There are somewhere between 2 and "many"
'contributors' to the problem -- *all* of which have to be operating for
the problem to occur (that's why "sometimes it's there, sometimes it
isn't" -- sometimes one or more of the 'critical' contributor sources a
are -not- running.)

The fact that it happens with the plane electrical off tends to exonerate
_almost_all_ of the avionics from complicity.

There are, in essence, three possibilities remaining:

One: it is *possible*, albeit unlikely, that the hetrodyning is occurring
in an antenna/cabling/"front end" of one of the radios (even without
power applied), and being _re-radiated_ to be picked up by other radios,
including the hand-held.

The test, to eliminate this possibility, involves disconnecting antenna
cables (one at a time) from whatever they're plugged into, then (a)
shorting the center conductor to the shield, and (b) putting a 'dummy
load' (a 50 ohm resistor) across the radio connection. that radio
should now be totally 'deaf'. *IF* _it_ hears the interference, then
the 'intermod' is occurring at IF, and additional shielding around the
radio may help.

*IF* the problem 'goes away' with one of the antenna's shorted, you've
'localized' the problem, check for a corroded connection, cold-solder
joint, etc. consider replacing the entire assembly.

If, as is likely, nothing 'interesting' happens during the above testing,
It's time for _one_ additional test. using the _same_ hand-held, and
a *different* airplane, fly into the same area, at a time when you have
established that the problem is present. If the interference shows
up on the hand-held, you _have_ eliminated everything in the plane as
causative agent. It *is* 'inherent' in the locale If the radios in this
2nd plane are not picking it up, it is, for lack of a better term, a
"front end overload" problem in _your_ radios. Sensitivity to this
problem varies with the design of the radio, _and_ (although generally to
a minor degree) even from unit to unit within a given model line.

This is possibility #2, "out-of-band signal overload at the radio
front-end". A "High Q-Factor band-pass filter" installed on the
receive side of the radio can greatly reduce this problem. Depending
or radio design, this can range from 'reasonable' to 'outrageously
difficult and expensive' to implement.

The third possibility is that the hetrodyning, and re-radiation is
occurring somewhere _external_ to the airplane. e.g. _on_ some component
of that transmitting tower. In which case about the *only* 'solution'
is to treat it as a "DDT" problem -- since the problem occurs only when
you fly into that specific area, the solution is "Don't Do That!"
Don't fly into that area, and you won't have the problem.

Note: if this -is- the situation, then pretty much _everybody_ flying into
the vicinity should be experiencing the same problem, on a recurring basis.
Do other pilots report similar difficulties?