Sydney,

Here are a few ideas concerning your interference problem (worth at least as
much as they cost...

*** You mentioned 127.0 might have the strongest interference. This could be
the result of mixing between the TV channel 2 visual carrier (55.25 MHz) and the
channel 4 aural carrier (71.75 MHz); alternatively, the ch. 2 aural carrier
(59.75 MHz) could be mixing with the ch. 4 visual carrier (67.25 MHz) to produce
the same result.

I looked at all the high-power broadcast services (VHF-TV & FM, as listed in the
FCC databases) within 10 km of the location you gave; trying all the 2nd & 3rd
order intermod possibilities I could think of, the above result seems the most
likely.


*** FCC databases:

FM: http://www.fcc.gov/mb/audio/fmq.html
TV: http://www.fcc.gov/mb/video/tvq.html

You can scroll to the bottom of these and enter a lat-long and radius.


*** According to the TV database, channel 2's transmitter (100 kW) is the
closest to the location you gave. If the intermod is being produced EXTERNAL to
your airplane, IMO this is the most likely source; a ch. 4 signal (from a
different, but nearby, antenna) is received by ch. 2's transmitting antenna,
travels back into the high-power output stage where it mixes with the ch. 2
signal, and the resultant 2nd-order intermod product then radiated from ch. 2's
antenna.


*** You mentioned in another post that on *one* radio, the controller becomes
"very faint" under the intermod, while on the other radio and the handheld the
controller is still heard nearly normally. This may be a clue that the *one*
radio may be the SOURCE; the intermod is perhaps being generated in it's input
circuit (wiping out ATC on that radio) and also re-radiated from it's COM
antenna into the other radios (but not as strong in the others due to the
attenuation [12-18 dB or so] between the COM antennas.)

As others have mentioned, it's possible for the radios to generate intermod even
when powered off (the "crystal radio" principle is applicable here.) The
easiest way to check this would be to simply disconnect the antenna from the
*suspect* radio(s), ideally at the antenna end (IMO, no need to bother
"terminating" the open connectors) and fly out to the Antenna Farm and note the
result.


*** Take your handheld in your car and drive around near the Farm, note the
results. If no intermod, the problem's most likely in the plane; else, the
problem could *still* be in the plane (and the handheld), but *could* also be
the fault of one of the TV station.

If you suspect a TV station (Channel 2 most likely IMO) is at fault, or even if
you don't, I would suggest getting in contact with the station's Engineering
Department and (diplomatically) explaining the situation. They're technically
qualified folks, are well-equipped with test equipment (spectrum analyzers,
ect), and will likely be VERY interested in finding and correcting any such
problems in their transmitting plant. If there's an RF emissions problem they
aren't aware of (that could possibly result in the station being fined), they'd
be most appreciative of it being brought to their attention.


*** Hope this helps. Have fun...

Mark


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"