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

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"

"Snowbird" wrote in message
om...
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"

I fly a sailplane with a battery powered Microair 760 and no other avionics.
Occasionally, in certain locations, a country western AM station will break
through at high volume. In other locations, it is a TV station. This
problem is also encountered in other sailplanes with other radios. The
thing here is that there are no other onboard electronics to confuse the
situation. The unwanted signals are usually encountered at less than 1500
feet AGL so I suspect groundwave propagation from the commercial
transmitters.

I think the commercial stations are poorly controlling their sideband
radiation and some of their RF power is spilling onto the aviation band.
Aircraft radios are generally well designed and reject signals that are not
intended for them, but they can't do anything about spurious signals from
sloppy maintenance at a commercial radio stations.

Maybe contacting the FCC would be your next step.

Bill Daniels

I respectfully disagree with this statement. There is precious little that is
engineered as well as a commercial radio station. Even sloppy maintenance
wouldn't get the sidebands this out of whack...and besides, those sidebands
would have to be from the FM band, and we are talking about 20% away from the
carrier. Ain't no way.

I live next door (literally) to a 10kW AM transmitter, and occasionally I tune
the spectrum analyzer to their frequency and put an 80 dB notch filter on the
center of the carrier. With a 120 dBm noise floor, I **still** can't see any
splatter outside their allocated bandwidth.

While aircraft radios are generally good with filters, also, there comes a limit
to any design when the volts coming in the front end are just too much for the
microvolt amplifiers. It is called fundamental front end overload, and we are
all susceptible to it.

Both Microair and King use varactor tuned filters in the front end for
selectivity. While these are good design choices, you must remember that a
varactor is nothing more than a diode, and a varactor is one of the best
multipliers in the world. Whap that front end diode with a couple of volts and
you've got a little comb generator second to none.

I think we'd best look elsewhere for the solution.


Jim




"Bill Daniels"
shared these priceless pearls of wisdom:

-I think the commercial stations are poorly controlling their sideband
-radiation and some of their RF power is spilling onto the aviation band.
-Aircraft radios are generally well designed and reject signals that are not
-intended for them, but they can't do anything about spurious signals from
-sloppy maintenance at a commercial radio stations.

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

Quick question(s) ...

Is TV channel 3 a local station for you? Is their transmitter in this antenna
farm? Do other aircraft report the same interference?

Jim

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

"Jim Weir" wrote in message
...

I think we'd best look elsewhere for the solution.

Sounds like intermodulation to me. Look for a sum/difference with the
frequencies at the antenna farm?

Rich S.

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

In article , Rich S.
wrote:

Sounds like intermodulation to me. Look for a sum/difference with the
frequencies at the antenna farm?

That's my thought. I used to fly over Voice of America's transmitter
farm in southwest Ohio before they shut it down. Their transmissions
always leaked into the aircraft radios when in close proximity.

In rec.aviation.owning Jim Weir wrote:
: Quick question(s) ...

: Is TV channel 3 a local station for you? Is their transmitter in this antenna
: farm? Do other aircraft report the same interference?

: Jim

Jim, the FCC has a nice antenna search by geo coords page:
http://wireless2.fcc.gov/UlsApp/AsrS...tionSearch.jsp

Using Sydney's coords (38 deg 31' 54" N 90 deg 21' 45" W) it shows 2
antennae at that site, both belonging to KTVI Fox channel 2. Within 5 km
of that site there's KSDK TV5 (38 34' 05"N, 90 19' 55"W), and some cell towers
owned by SpectraSite (38 34' 24"N, 90 19' 30"W). Nothing else with any height,
though.

--
Aaron Coolidge (N9376J)

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

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?