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

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

On 5-Nov-2003, (Snowbird) wrote:

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?

Very, very unlikely. Those who propose this theory assume that an
interfering intermod is being generated within the antenna system or preamp
of one radio and then being re-radiated out the antenna to another com
antenna, where it interferes with reception on the second radio. What this
theory fails to take into account is the large attenuation that the intermod
would be subject to in the propagation between antennas. The far more likely
scenario is that the antenna farm is transmitting two (or more) very strong
out-of band signals that cause intermodulation interference in your com
receivers. My guess is that this problem exists for others as well,
depending upon the band selectivity of the front ends of their radios.

There is a relatively simple way you can test this theory. What you need is
a 6 dB RF coaxial attenuator. These little buggers cost around $30 each,
but maybe you can borrow one from your avionics shop. Put the attenuator
between the antenna and receiver on your handheld. (You will, of course,
have selected an attenuator with he proper RF connectors on it. Otherwise,
you will need suitable adaptors.) Then fly to the location where the
problem exists. I'll bet that with the attenuator you will no longer get
the interference on the handheld, but you will probably be able to receive
the TRACON signal. Assuming I'm right, here is what's going on: The 6 dB
attenuator attenuates ALL signals going through it by 6 dB, including the
strong out of band signals that are causing the intermods. However, the
amplitude of the intermod that these signals cause is thereby attenuated by
more like 18 dB, probably enough to make them too weak to cause problems.
The TRACON signal is also attenuated by 6 dB, but it is probably strong
enough to begin with so that you will still be able to receive it OK. One
caution: do NOT transmit on the handheld when the attenuator is in place.
Depending upon the TX output power of the handheld and the power rating of
the attenuator, it (the attenuator) could be damaged. In any case,
transmissions would be attenuated by 6 dB, which might make them to weak to
be received by the ground station.

--
-Elliott Drucker

On Wed, 05 Nov 2003 18:02:44 GMT, wrote:


On 5-Nov-2003, (Snowbird) wrote:

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?

Very, very unlikely. Those who propose this theory assume that an

It can happen easily.

For instance a ham had aa problem with Television Interference. His
station was clean...a team did some snooping and found the culprit was
an attic mounted prearmp hooked to an antenna. There was no power to
the preamp and no cable to the preamp other than the TV antenna.

The offender was causing problems over nearly a city block.

interfering intermod is being generated within the antenna system or preamp
of one radio and then being re-radiated out the antenna to another com
antenna, where it interferes with reception on the second radio. What this
theory fails to take into account is the large attenuation that the intermod

When we had the 144 MHZ repeater here, I could walk out and wipe a
screwdriver blade up and down a guy wire (still new and shiny wire)
while the transmitter was active. It created enough hash to drowned
out a 50 watt mobile less than 4 miles away.

It only takes a few micro volts of signal to cause a problem.

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

would be subject to in the propagation between antennas. The far more likely
scenario is that the antenna farm is transmitting two (or more) very strong
out-of band signals that cause intermodulation interference in your com
receivers. My guess is that this problem exists for others as well,
depending upon the band selectivity of the front ends of their radios.

There is a relatively simple way you can test this theory. What you need is
a 6 dB RF coaxial attenuator. These little buggers cost around $30 each,
but maybe you can borrow one from your avionics shop. Put the attenuator
between the antenna and receiver on your handheld. (You will, of course,
have selected an attenuator with he proper RF connectors on it. Otherwise,
you will need suitable adaptors.) Then fly to the location where the
problem exists. I'll bet that with the attenuator you will no longer get
the interference on the handheld, but you will probably be able to receive
the TRACON signal. Assuming I'm right, here is what's going on: The 6 dB
attenuator attenuates ALL signals going through it by 6 dB, including the
strong out of band signals that are causing the intermods. However, the
amplitude of the intermod that these signals cause is thereby attenuated by
more like 18 dB, probably enough to make them too weak to cause problems.
The TRACON signal is also attenuated by 6 dB, but it is probably strong
enough to begin with so that you will still be able to receive it OK. One
caution: do NOT transmit on the handheld when the attenuator is in place.
Depending upon the TX output power of the handheld and the power rating of
the attenuator, it (the attenuator) could be damaged. In any case,
transmissions would be attenuated by 6 dB, which might make them to weak to
be received by the ground station.

(Snowbird) wrote:

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


This Google link may be of interest,

http://tinyurl.com/trcf


P.S. About six years ago, I ferried a single engine airplane from
Miami to Atlanta. For reasons I won't go into here, I didn't have
full confidence in the engine. Just north of Lake Okeechobee, a
religious broadcast station bled through the airplane's com receiver
and suggested that I "repent now!". True story.

(Snowbird) writes:

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?

Yes. All it takes is a non-linear device, i.e. a diode.
Iffen the tranceiver used diode-switching in the front
end, and there's enough RF to make the diode conduct..

The trouble is, a diode can also be one slightly corroded aluminum
joint.

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?

It's now sounding like the non-linear junction is on your airframe
somewhere.


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.

Well, first fly the handheld in another airframe. That establishes
iffen it's part of the solution or the precipatate.

If all's clean in that case, start looking at all grounds on yours.
Disconnect the easy antennas first.... and see if anything changes.
Keep good notes!

If all else fails...http://www.spacemodel.com/pic355.html

--
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

("David Lesher" wrote)
snip
Well, first fly the handheld in another airframe. That establishes
iffen it's part of the solution or the precipatate.

If all's clean in that case, start looking at all grounds on yours.
Disconnect the easy antennas first.... and see if anything changes.
Keep good notes!

If all else fails...http://www.spacemodel.com/pic355.html


I've been fascinated with this thread. I'm following the discussion without
really understanding what it is I'm following ...but I'm able to follow it -
sort of.

I was going to offer my advice (a while ago) based on my boat trailer
experience - when in doubt, it's the ground. When you think you've cleaned
up the ground, replace the ground wire. When you've done that, make yet
another (fresh) ground connection, etc.

We took a friend's little 4x8 (utility) trailer out to SoDak this fall, when
we went pheasant hun'n. We were having all kinds of lighting problems with
that darn trailer before we left. He was very big into diagnosing the
problem. I ran 5 ft of fresh ground wire from the minivan tail lamp socket
to the trailer plug - making two (new) fresh ground connections along the
way. Lights worked fine. Double-grounded the trailer side of the plug with
new wire too - just in case. (20 minutes)

He wanted to know what it is I did, technically speaking. My answer - I'm
not exactly sure, but it always seems to do the trick.

--
Montblack