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

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

(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

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

I think we are chasing our tails here, folks. Snowbird says that she gets the
interference on her handheld. Let's do the binary troubleshooting tree.

Split the problem into two parts. It is either IN the aircraft or OUT of the
aircraft that the problem is located.

Depending on how often this problem occurs (once a week? once a day? once an
hour?...) DRIVE the handheld out near the antenna farm and sit there and listen
on one of the most affected frequencies.

Does it still happen? Then with 99% probability, you've got a problem not of
your own making.

Does it not happen? Then you've got a problem in the aircraft.

Let's settle THAT one and we can go from there.

Or, rather than drive out and sit for hours in the wintertime, do you have any
friends that live near the farm? Would they be willing to sit your handheld in
their window and listen for a few days?

All else at this point is conjecture.

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

On Wed, 05 Nov 2003 11:10:29 -0800, Jim Weir wrote:


I think we are chasing our tails here, folks. Snowbird says that she gets the
interference on her handheld. Let's do the binary troubleshooting tree.

Split the problem into two parts. It is either IN the aircraft or OUT of the
aircraft that the problem is located.

Depending on how often this problem occurs (once a week? once a day? once an
hour?...) DRIVE the handheld out near the antenna farm and sit there and listen
on one of the most affected frequencies.

As a suggestion, try a bigger antenna on the HT if it has the
connector.


Does it still happen? Then with 99% probability, you've got a problem not of
your own making.

HTs are notorious for running the front ends wide open so it could be
the HT. Although out of all the HTs I've owned including commercial
(Motorola), I've only had one that was bad on intermod. Given a
strong enough signal any of them would intermod, or rather cross mod,
but virtually any radio will do that.

OTOH, I've had two out of 7 mobile rigs that had a problem. Course
when all is totaled up I've run into as many commercial installations
in the area that were either intermoding with each other, or
transmitting spurs. (which ain't many after this many years)

My biggest problem is a paging system about two miles from me that has
a problem about once a year.


Does it not happen? Then you've got a problem in the aircraft.

Let's settle THAT one and we can go from there.

Or, rather than drive out and sit for hours in the wintertime, do you have any
friends that live near the farm? Would they be willing to sit your handheld in
their window and listen for a few days?

If possible stick an antenna out the window of a nearby home and then
set one of those voice activated tape recorders next to it. Come back
about the time you expect the batteries to go dead. It makes for a
good check and doesn't require constant attention.


All else at this point is conjecture.

They are bad enough to find even using a systematic approach. :-))

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


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

Roger Halstead wrote in message . ..
On Wed, 05 Nov 2003 11:10:29 -0800, Jim Weir wrote:
I think we are chasing our tails here, folks.

Well, I'm not sure the decision tree is quite this binary from
what folks are saying.

If I drive to the antenna farm and get the interference,
does it prove the problem is outside my plane, or just that
the handheld is also more susceptible to it?

If I drive to the antenna farm every day for a month and don't
get the interference, does it prove the problem is in my plane,
or that the interference is several things combined some of
which aren't line-of-sight to my current ground location?

This is not to say that I don't think it's worth at least
a drive to the area, and a flight in someone else's plane
with my handheld.

But what I'd like to understand is this:

How could my nav radios (or my handheld) be contributing
to this problem when they are *powered off*?

Could someone explain this to me please? I'm not an electronics
wizard (obviously) but I do know a little bit and this just seems
very "twilight zone".

Thanks,
Sydney

: How could my nav radios (or my handheld) be contributing
: to this problem when they are *powered off*?

Did you ever build a "crystal" radio when you were a kid? It's an AM
radio that receives stations and powers an earphone using only the
energy received through the antenna. If there is enough RF energy in
the air (say at an antenna farm), there is enough power being received
by the antenna to make the powered off radio into an analog of the
"crystal" radio. The powered off radio uses the received RF energy to
become an oscillator (transmitter).

: Could someone explain this to me please? I'm not an electronics
: wizard (obviously) but I do know a little bit and this just seems
: very "twilight zone".

Does that help?
--
Aaron Coolidge (N9376J)

(Snowbird)
shared these priceless pearls of wisdom:


-
-Well, I'm not sure the decision tree is quite this binary from
-what folks are saying.

Well, then take other folks advice who have had this problem and solved it. I
gave you the method that works for me.

-
-If I drive to the antenna farm and get the interference,
-does it prove the problem is outside my plane, or just that
-the handheld is also more susceptible to it?

It proves that the problem is outside your aircraft.

-
-If I drive to the antenna farm every day for a month and don't
-get the interference, does it prove the problem is in my plane,
-or that the interference is several things combined some of
-which aren't line-of-sight to my current ground location?

That is possible. I am trying to eliminate one thing at a time. And, that's
why I suggested that you find a friend that lives close to the farm and let them
listen around the house for a week or so. It MAY be when the cops key up their
repeater that it is mixing with channel 4. It MAY be when the local hams key up
their machine that it is mixing with Rock 102. It MAY be any combination, and
if it is intermittent in your aircraft, the odds are good that at SOME point in
the week your handheld will hear it.

-
-This is not to say that I don't think it's worth at least
-a drive to the area, and a flight in someone else's plane
-with my handheld.

Drive first. Fly second.

-
-But what I'd like to understand is this:
-
-How could my nav radios (or my handheld) be contributing
-to this problem when they are *powered off*?

The nav radios (or com radios, or ELT, or...) have an input circuit that is
comprised of a transistor. However it has to get to this "RF Amplifier"
transistor, through whatever filtering, it gets to the transistor. The
transistor is nothing more than a couple of diodes back-to-back. A diode,
powered or not, is an inherent "mixer". A mixer takes two signals and outputs
the sum and difference of these signals, plus (in decreasing strength) the sum
and difference of all integer multiples of those frequencies.

Let me make the math simple. Take two signals, one at 50 MHz. in the 6 meter
ham band and one at 60 MHz. at the tag end of channel 2.

Turn your nav radio off. These two signals will get in to the front end of your
nav receiver and mix in the RF stage. How much signal is getting in is a
function of the steepness of the filter your nav radio designer put into the
receiver. No filter is perfect; there will ALWAYS be some little bit of signal
leaking in, and the more powerful the extraneous signal, the more it will power
its way into the front end.

So now we've got 50 and 60 MHz. in the radio. The RF transistor takes those two
signals and mixes them so that you get 110 MHz. and 10 MHz. (sum and
difference). You ALSO get 160 MHz. (2x50 +60), 170 MHz. (50 + 2x60), 220 MHz.
(2x50 +2x60), 20 MHz. (2x60 - 2x50)... and so on ad infinitum. If any of these
"spurious mixer products" falls within the passband of the nav input filter, it
will be reradiated out the nav antenna directly into your com antenna.

Now go figure out how many AM, FM, TV, public service, amateur, and CB
transmitters there are in your area. Do a sum and difference, plus a harmonic
(integer multiple) sum and difference for ANY COMBINATION of them, and you begin
to get an idea of the magnitude of the problem in finding the culprits.

Did that help?

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

OK, so this afternoon I took my trusty ICom aviation handheld,
suction cupped its duckie to the windshield of DH's car, and
off we went

Going on an Antenna Hunt
Gonna Catch a BIG One
Sorry, too much before-bedtime reading to the toddler *g*

What we learned:
*our handheld ain't exactly a precision filter. I could
hear things like the turn signal and the electric windows
rolling down. power lines -- tcccchhk!
*nevertheless, we could hear aircraft on 126.5, and occasionally
(faint) the tracon
*adjusting so that such transmissions broke squelch but at
least some of the dreck was filtered out, we sure could tell
when we got close to them big antennae. RF interference up
the Ying Yang no ifs ands or buts
*TV Channel 2 antenna was the big winner. Where the other
big antennae just produced noise, I could sit near the
base of the Channel 2 antenna and listen to a program about
college football. Further out, came through in snatches. Right
there by the antenna, came through 3x5 on 126.5 and 5x5 on 127.0
*Didn't have time to fly and conduct the obvious experiment of
leaving Mr Handheld on the ground
*Now here's where it gets wierd: according to the TV schedule,
channel 4 and Channel 5 were broadcasting college football.
Channel 2 was showing a movie. Didn't have time to stay and
listen until we got a station identification. From what I
heard, sounded like Channel 5.

Will try to return to confirm lat-long of antenna (foolishly
didn't take my GPS along, though I think I have them straight)
and to get station ID if I can. Also trying to reach Channel
2 to see if they changed programming -- they're the station
which carries Mizzou football and might conceivably have
altered programming to carry football at that time.

Soooooooo Guri, what does *this* tell us?

What is TV channel 2 digital sound frequency and normal
Channel 2 sound frequency? Just wondering if by chance
the difference between the two might be...127.0?? How
about Channel 2 and Channel 5?

Jim Weir wrote in message . ..
Well, then take other folks advice who have had this problem and solved it. I
gave you the method that works for me.

I take my hat off, then Jim. I could barely scrape up the
hour to drive around, much less weeks or days or even hours
to park patiently. We're going to have to get at this by
pragmatic combinations.

-If I drive to the antenna farm and get the interference,
-does it prove the problem is outside my plane, or just that
-the handheld is also more susceptible to it?

It proves that the problem is outside your aircraft.

But....does it? The handheld is normally *in* my aircraft.
Albeit, it is normally disassembled from the battery pack
and with the ducky antenna disconnected from it.

And..what about the testimony of the DE that she flies in
lotsa planes in that area all the time and never heard that
problem before?

why I suggested that you find a friend that lives close to the farm

No such critter alas. We live about as close to it as anyone we
know.

-How could my nav radios (or my handheld) be contributing
-to this problem when they are *powered off*?
good explanation snipped
If any of these
"spurious mixer products" falls within the passband of the nav input filter,
it will be reradiated out the nav antenna directly into your com antenna.
...
Did that help?

Yes, thank you! That was very clear.

But just to confirm: what I'm hearing is that frequencies can
mix and be reradiated by a turned-off radio, *but they need an
antenna* to get anywhere? ie, if the ELT is in the plane but
disconnected from the antenna, it's not likely the source of
the problem...likewise, if the portable nav/com is disconnected
from its antenna?

Have we likely absolved my plane at this point?

Sydney