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

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

"Paul Sengupta" wrote in message . ..
http://www.leeselect.com/shopping/pricelist.asp?prid=41

Maybe you could borrow one to try on the handheld. It
could work if the strong out of band signal is causing problems
with the receiver front end, but if the intermod/harmonics/
whatever are external and fall in-band then it's not going to
help.

Paul,

Thanks for the link!

Yes, that looks like the correct kind of product, but my concern
is it includes the nav radio frequencies. If Aaron is correct that
the problem might be via the nav radios, we need a narrower freq.
range.

Best,
Sydney

(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

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.

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

Can you hear this interference through the cabin speaker? Or perhaps
your using active noise-cancelling headsets?

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"

: 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

On 6 Nov 2003 05:34:08 -0800, (Snowbird)
wrote:

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.

snip

It's still a binary decision tree and a process of elimination.

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*?

I'm trying to think of a plain language explanation and Jim could
probably do it more eloquently...and most likely with better accuracy.

The power off problem usually uses a different mechanism to produce
the problem than one that is powered up.

Today's radios have a transistor amplifier on the receiver input.
If you put them in a strong enough RF field (close to a powerful
transmitter) there will be enough voltage on the input transistor to
cause it to conduct. The only thing is, it can conduct in only one
direction so it acts like a switch that turns on and off with each
cycle of the signal. Unfortunately the transistor doesn't start to
conduct until the voltage is already on the upward cycle and this
causes the transistor to switch on abruptly. It's sorta like the
rusty fence/poor connection/rubbing a screwdriver on the guy wire sort
of thing, but it's not just noise. It can generate signals although
they usually sound muffled, or garbled.

Actually the same thing can happen in the audio stages of receivers
with power on, but it normally happens with those using long speaker
leads. here the speaker leads act like an antenna and pick up the
signal. The audio transistors act like a switch and rectify the
signal producing a garbled sound on top of the audio to which the user
was trying to listen.

Without getting in too deep, intermod (with power applied) can be
caused by a signal so strong it exceeds the design limits of the
amplifier and it goes into what is called a non linear operation (It
becomes a mixer instead of just an amplifier).

I hope this makes sense. I've found the longer I've been in a
particular field the more difficult it becomes to explain things in
plain language.

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