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

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

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

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.

(Jay) wrote:

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.

I used to be able to turn OFF my car radio and STILL get a loud
"bbBBBRRRRrrrrzzzz" every time the long range radar swept past my car
about 1/4 mile away on US1 in Key West (you could tell which way the
dish was turning in the radome by how long the sweep took).

Talk about "detection by overload"...

Mark Hickey

"Mark Hickey" wrote in message
...
(Jay) wrote:

In very high fields with amplitude modulation you can
get what's often called "detection by overload".

Some people have reported picking up radio on their
fillings in their teeth.

I used to be able to turn OFF my car radio and STILL get a loud
"bbBBBRRRRrrrrzzzz" every time the long range radar swept past my car

You can get that sitting in the car on top of one of the car parks in
Heathrow.

Paul

On 4 Nov 2003 06:01:30 -0800, (Snowbird)
wrote:

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?

An example of how difficult IM can be to find:

Two years ago we received a report of interference from our repeater
(on 147 MHz) to one of the services in Lansing. (that's about 70
miles). The interference was intermittent.

It turned out that there were three stations involved and none were
within 30 miles of each other.

Out repeater signal and a pager in (I believe it was Chesasining MI)
was mixing in a commercial repeater (fire I believe) and being
retransmitted on the input frequency for a Police repeater in Lansing.
The resultant signal being well off the frequencies used in the other
repeaters was understandably weak, but strong enough to key the Police
repeater and be clearly understandable.

It took several weeks of dedicated hunting by a number of crews in an
area nearly a 100 miles by about 30 miles to finally locate the
offender.

snip

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.

IF the signal bothers one receiver far more than the other then I
would think the problem is more likely on the plane. IF it is
external and close, both receivers on the same frequency should have
the same problem.

To me that says "bad connection" some where.

There are many other possibilities, but with that information I'd tend
to lean heavily toward "on board"

Roger Halstead (K8RI EN73 & ARRL Life Member)
www.rogerhalstead.com
N833R World's oldest Debonair? (S# CD-2)

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

On 4-Nov-2003, (Snowbird) wrote:

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

That would be pretty difficult because of the huge number of different
combinations of out-of-band signals that could cause the intermodulation
interference.



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

Generally, the locations of all commercial broadcast sites in the US are
public domain information available from the FCC. Some or all of this info
is probably available on-line. Unfortunately, I am not particularly
knowledgeable when it comes to tools that might be available for searching
the appropriate FCC data bases.


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

The fact that three different radios that you tested were similarly affected
suggests that the problem is not a specific malfunction. A "solution" would
most logically involve placing a bandpass filter between each of your com
antennas and the corresponding transceiver. I do not have knowledge of such
filters being commercially available, but others might know of such
products.


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.

In that case I certainly think that you should bring the problem to the
attention of the FAA.

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

I'm no expert about FSDO organization, but I would start with the office of
the Facilities Director.

--
-Elliott Drucker

I would start by disconnecting the antennas from ALL of the receivers
or if the antenna does not have a separate connector then removing the
receivers and using the hand held to detect the offending signal. If
you inject a strong enough signal into the antenna input you force the
receiver's RF amp into overload and it generates the intermod products
and sends them back OUT the same antenna to be picked up by the other
antenna's
There is one brand of ELT that is infamous for this type of problem
but I can not remember the model and brand.

On 4 Nov 2003 06:01:30 -0800, (Snowbird)
wrote:

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 in message ...
I would start by disconnecting the antennas from ALL of the receivers
or if the antenna does not have a separate connector then removing the
receivers and using the hand held to detect the offending signal. If
you inject a strong enough signal into the antenna input you force the
receiver's RF amp into overload and it generates the intermod products
and sends them back OUT the same antenna to be picked up by the other
antenna's

OK, here is what I don't understand:

We have this problem with the airplane's master switch turned OFF

Can the receiver's RF amp generate intermod products when the
power is switched OFF?

We have some avionics which don't have separate off switches
(the KMA20 audio panel/mb is one) but the only thing which
operates independently of the master switch is the airplane's
clock.

If the antenna farm itself is generating intermod products,
can it be picked up by an installed antenna (connected to a
receiver which is turned OFF) and re-radiated to our comm
antennae?

There is one brand of ELT that is infamous for this type of problem
but I can not remember the model and brand.

This was suggested to us initially. We disconnected the ELT
(but DH left it sitting in the back seat of the plane unfortunately,
instead of on the ground) and we still had the problem.

So people at the time thought that pretty much absolved the ELT
(it's an old one-Narco 10). I could certainly disconnect and
physically remove the ELT and see if that helps.

It's not that I'm unwilling to disconnect all the antennae in the
plane, but some of them are a terrible PITA to reconnect and I'd
like to understand the theory of what's supposed to be happening
to produce this problem with the power to all of the receivers
turned OFF.

Thanks,
Sydney