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

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?

Plane's equipment:
Sigtronics SCI-4 intercom
KMA20 audio panel/mb
King KI-170B nav/com
TKM 170B nav/com
Apollo 2001 IFR GPS
King KN-75 glideslope receiver
King KT-76 Transponder
no ADF or DME

THANKS!
Sydney
Grumman AA5B "Tigger"

It sounds like IMD in the receiver frontend. As Jim pointed out it is
probably being caused by the input amp stage. The difficult thing is the
cure. I don't know if a high power band pass filter is available so that
the interfering frequencys don't get into the reciever front end. If you
can pinpoint the source frequency of the interferance a simple coax
tuned stub filter will notch it out but that would not be broad enough
for a TV transmitter, but usually only for an AM transmitter, or other
narrow band interferance.
Good Luck
Jim Stockton

On Mon, 03 Nov 2003 17:39:03 -0600, Jim Stockton
wrote:

It sounds like IMD in the receiver frontend.

It could also be passive intermodulation (PIM). If several high-power
transmitters are at the same location (as in an antenna farm), then
these signals can mix and reradiate. All that is necessary is
dissimilar or corroded metals in the radio towers.

After a Google search, I came up with this reference:

http://www.summitekinstruments.com/p...qcont.html#top

Notice the mention of "rusty bolt noise".

You say that your hand-held radio receives the same interference. Do
other pilots have the same issue? If so, this is a good indication of
PIM. Of course, it could also show that other receivers are similarly
overloaded with some strong out-of-band signals. But, once you have
some more evidence that it is PIM (i.e., others with the same issue),
then perhaps the FCC or the FAA might take an interest in doing an RF
site survey to isolate the cause of it.


Rob-
------------------------------------------------------------------
Robert Cherney e-mail: rcherney(at)comcast(dot)net