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

"Rich S." writes:

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?

I'd ask other owners. Then try to find your regional FAA frequency
coordinator or similar, in other to locate a sympathetic ear. He'd
know who would go by with a 494 spectrum analyzer or such...iffen they
ever have time.

It could well be intermod from tower guy joints or such. It could
also be the front end of the radios. (Don't suppose you have 3 element
cavity tha would fit in the back seat ;-?)

If all else fails, maybe filing a NASA report would get a response.
--
A host is a host from coast to
& no one will talk to a host that's close........[v].(301) 56-LINUX
Unless the host (that isn't close).........................pob 1433
is busy, hung or dead....................................20915-1433

Jim Weir wrote in message . ..
Quick question(s) ...

Is TV channel 3 a local station for you?

Negative. Channel 2 and Channel 4.

Is their transmitter in this antenna farm?

I don't know whose transmitters are there. I got the
lat-long coordinates; any way to find out which transmitters
are there?

Do other aircraft report the same interference?

Not that I've heard, but then, I might not have heard.
Or, like us, they might have assumed it was a problem in
their airplane.

We didn't have this problem before last spring.

Any assistance sorting this out would be greatly appreciated;
we do have a pretty good local avionics guy but he frankly
seems stumped (at least he's honest and good enough not to
simply suggest replacing all the radios in the stack, which
one shop did).

Cheers,
Sydney

Admin is correct as far as he goes.
This problem is also sometimes referred to as the "rusty bolt
problem". If you have a rusty bolt or rusty connection between two
metal parts on the antenna, the aircraft airframe , the transmitting
antenna and anywhere the field is strong enough the corrosion can act
as a diode and generate intermod products which is what you are most
likely hearing.

I would disconnect "ALL" of the antennas on the aircraft and use the
handheld while flying over this location. If this fixes the problem
connect one antenna at a time until the problem comes back. If the
signal is still present with all of the antennas disconnected it is
something on the ground or airframe corrosion. The next step is try
a different airplane with the same hand held. If it is still present
the problem is in or near the antenna farm on the ground.
Good luck
John


On Tue, 04 Nov 2003 02:18:14 GMT, ()
wrote:

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?

: I don't know whose transmitters are there. I got the
: lat-long coordinates; any way to find out which transmitters
: are there?

Sydney, have a look at the fcc link I posted a couple messages ago. It lets
you look up towers' owners by lat/long. Your tower is owned by KTVI chan 2.

: Do other aircraft report the same interference?

: Not that I've heard, but then, I might not have heard.
: Or, like us, they might have assumed it was a problem in
: their airplane.

: We didn't have this problem before last spring.

Did channel 2 recently add a digital TV transmitter? Like, last spring?

--
Aaron Coolidge (N9376J)

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, do you think that TV Channel 2 (54 MHz) could be interfering with the
NAV radios? I do know that I can receive distorted FM on my nav radio
at 108.00 (we have a 107.9 FM station in town here).

--
Aaron Coolidge

Sydney,

What you have here is a pretty classic case of intermodulation interference.
Judging from he results of your investigation (good work on that, by the
way) I would say that the most likely cause is the frequency mixing
("heterodyning") of two very strong out-of-band signals in the "front end"
of whatever radio you are listening to. Yes, it is possible that the mixing
could be taking place in some corrosion of an antenna mounting or some such,
but that is, in my opinion, far less likely.

It's a good bet that at least one, and very likely both, of the very strong
out-of-band signals is in the commercial FM broadcast band. There may be
several such signals being broadcast at high power from the antenna farm.

At this point the question is whether the interference poses a threat to
safety or merely an annoyance. When the interference "breaks squelch while
you are tuned to the TRACON freq are you still able to hear the controller
when he/she transmits?

It may be a good idea for you to notify the FAA about the problem. Further
investigation may involve the use of a spectrum analyzer, not a tool you are
likely to have laying around in the garage. If the Feds determine that the
potential for intermodulation interference is a hazard they can take
corrective measures, such as reassigning the TRACON frequency in the area to
one that has a lower risk of interference from whatever signals are being
transmitted from the antenna farm.

--
-Elliott Drucker

wrote in message .. .

What you have here is a pretty classic case of intermodulation
interference.

Elliot, given what we did, is there a way to pinpoint what
signals might be the source of the problem?

Given the lat-long, is there a way to find out what antennas
are located there?

Is there something which might be a problem in our radios,
which could be fixed?

Yes, it is possible that the mixing
could be taking place in some corrosion of an antenna mounting or some such,
but that is, in my opinion, far less likely.

Well, one of the things which we did when the problem started
was take off all the easily-accessible antennae and clean the
connectors and check their ground. They all looked very good,
very well sealed and tight although the comm antennae didn't
have as strong ground as I'd like at first. The only antennae
we didn't check were the VOR/Glideslope because they're way
up at the top of the horizontal stab. and a b**ch to get at.

So unless it's the VOR antenna, I don't think corrosion in the
antenna mounting is likely, either.

At this point the question is whether the interference poses a threat to
safety or merely an annoyance. When the interference "breaks squelch while
you are tuned to the TRACON freq are you still able to hear the controller
when he/she transmits?

On one installed radio and the handheld, yes. On the other installed
radio, the controller becomes very faint against a background of
continued noise.

It may be a good idea for you to notify the FAA about the problem.

OK, I was thinking about this. Can you suggest which person in the
FSDO I'd ask for?

Thanks,
Sydney

wrote:

Admin is correct as far as he goes.
This problem is also sometimes referred to as the "rusty bolt
problem". If you have a rusty bolt or rusty connection between two
metal parts on the antenna, the aircraft airframe , the transmitting
antenna and anywhere the field is strong enough the corrosion can act
as a diode and generate intermod products which is what you are most
likely hearing.

I would disconnect "ALL" of the antennas on the aircraft and use the
handheld while flying over this location. If this fixes the problem
connect one antenna at a time until the problem comes back. If the
signal is still present with all of the antennas disconnected it is
something on the ground or airframe corrosion. The next step is try
a different airplane with the same hand held. If it is still present
the problem is in or near the antenna farm on the ground.

It may be an anathema in aviation circles to suggest this, but why not
just DRIVE to somewhere near the site with a portable and see if you
get the same effect? By doing this, you've eliminated EVERYTHING
other than the site itself. If the interference is still there on the
same frequencies, you know there's really nothing you can do about it
(at least without legal intervention and/or high explosives).

Or I suppose you could fly over it in a J3 sans electrical system and
get the same results...

Mark Hickey

I sent Sydney an off line reply about this problem. As several have
mentioned, and my message did also, is to call the Spectrum branch of
your local FAA. I did in the Dallas area when I was having interference
on a certain frequency and locale. It turned out to be a pager
transmitter in NE Oklahoma. He also had other interesting case studies
and I had him come to our EAA meeting to give a presentation. He drove
in with his "office". All this electronic gear crammed into a van.

wrote:

Sydney,

What you have here is a pretty classic case of intermodulation interference.
Judging from he results of your investigation (good work on that, by the
way) I would say that the most likely cause is the frequency mixing
("heterodyning") of two very strong out-of-band signals in the "front end"
of whatever radio you are listening to. Yes, it is possible that the mixing
could be taking place in some corrosion of an antenna mounting or some such,
but that is, in my opinion, far less likely.

It's a good bet that at least one, and very likely both, of the very strong
out-of-band signals is in the commercial FM broadcast band. There may be
several such signals being broadcast at high power from the antenna farm.

At this point the question is whether the interference poses a threat to
safety or merely an annoyance. When the interference "breaks squelch while
you are tuned to the TRACON freq are you still able to hear the controller
when he/she transmits?

It may be a good idea for you to notify the FAA about the problem. Further
investigation may involve the use of a spectrum analyzer, not a tool you are
likely to have laying around in the garage. If the Feds determine that the
potential for intermodulation interference is a hazard they can take
corrective measures, such as reassigning the TRACON frequency in the area to
one that has a lower risk of interference from whatever signals are being
transmitted from the antenna farm.

--
-Elliott Drucker

How close are you flying to the antenna farm? The field strength
decays exponentially with radial distance. Put another way, as you
get close, you may be flying into a VERY strong field, outside the
design parameters of the radios your using.

The spectrum analyzer (if you can get one) will answer your inter mod
products question. You might make friends with your local HAM group.
Some of these guys may enjoy the hunt, an airplane ride, and have the
high dollar equipment to do it. But I'm betting that the cause
something simpler than that. The fact that you mentioned that they
were AM broadcasts reminds me of the times I used to get AM radio on
my telephone. In very high fields with amplitude modulation you can
get what's often called "detection by overload". Any non-linear
circuit element in your radio- front end through audio amp have all
kinds of semiconductors which can "detect" and demodulate the AM
broadcast if the signal is powerful enough. In the case of my
telephone, the bridge rectifier that protected the polarity of the
phone was the detector.

If this turns out indeed to be the case, and you need to fly your
plane into fields that are that strong, you might be able to get some
relief with improved grounding, sealing radio case gaps with copper
foil, a filter network right before the antenna enters the radio.