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

Bear in mind that the radios involved may have different front
ends. It might just be a coincidence that your handheld and
panel radios are affected and nothing to do with the airframe.

How about 127.00-10.7 (IF mixer) = 116.30
116.30/2 = 58.15?
(i.e. mixing of the second harmonic of 58.15MHz)

Ok, unlikely, just playing with numbers!

Paul
G1YJY

"Snowbird" wrote in message
om...
Update: I talked to a local DE who is also doing piles of
instrument instruction. She says she's flying in beaucoup
planes in that area, without the same problem.

Aaron Coolidge wrote in message ...
Cool, I thought that you might have kill-filtered me for some reason, because
no one other than me seemed to be getting my messages!

No, not at all, though I'm reading news through groups.google.com
which is flaky on propegation

BTW someone said they emailed me. I didn't receive it. This
email address no longer works. If anyone wants to email me
try hoeltzli at swbell dot net. Though this discussion seems
on-topic and of possible general interest so I'd just as soon keep it
on the net.

On further reflection, this may be a red herring. Digital TV is in the
220+ MHz region.

Can this kind of intereference be caused by subtracting frequencies?

Channel 2 is 54 to 60 MHz, the 2nd harmonics of are 108 to 120 MHz.

That's too low for the frequencies where we're getting interference
(124.2 is clean, 126.5 is trash)

This leads me to suspect one of the *NAV* radios. Can you physically
remove them from your plane, one at a time, and leave them in your car?

Not a problem. Can this really be caused by a radio which is *powered
off*? Because I did try switching the navs to different frequencies
and also turning them off, along with all the electrical power in the
plane.

Since I changed jobs I don't have my trusty HP 8591E spectrum analyzer
anymore, if I did I'd consider a trip to St Louis!

Oh, bummer! That would have been nice.

: If this is a possibility, how do we safely remove the
: marker beacon antenna for testing purposes? Do we need

If you've got the bent metal rod kind

Pretty sure it's a blade, though I admit the MB antenna is "out of
sight out of mind" to me.

If you wish to electrically remove the antenna from the plane while leaving
it physically in place

I'm perfectly happy to take it off and slap some 50 mph duct tape over
the resulting hole, providing it's not going to hurt the MB receiver
to be attached to dangling coax.

If I were doing this, I'd probably
terminate the RF input to the radio, as well.

IIRC this would be a PITA -- the coax goes to the antenna with a
standard BNC connector, but is soldered on to the radio tray.

Please keep us (me) informed, we're trying to help the best we can!

I appreciate this. It's a vexing problem and one which apparently
lies outside our radio guy's experience

Thanks,
Sydney

"Paul Sengupta" wrote:

"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.

And to think there are people scared about the effects of cellular
towers! Heh.

Mark Hickey

wrote in message
link.net...
54-60 mHz carrier at 55.125

(55.125 * 2) + (2 * 10.7) = 131.65
(55.125 * 2) + 10.7 = 120.95

76-82 mHz carrier at 77.125

(77.125 * 2) - (2 * 10.7) = 132.85

225.525 (digital? ham repeater?) - 77.125 - (2 * 10.7) = 127.00.

Just playing...

If it's intermittent then are there any ham repeaters or other
comms repeaters in the area?

And, two, a 'directional' antenna.

If an AM radio station was causing the problems and you
wanted to know which mast was causing it, you could tune
in the ADF and see where the needle points! :-)

If the source is outside the plane, you're conclusively dealing with front
end overload intermodulation interference.

Unless you're getting in-band interference from outside the plane,
such as harmonics, intermod from rusty bolts or other transmitters
as others have mentioned, etc.

If that's the case:
1) high-Q bandpass filtering, to eliminate the out-of-band interference
sources.
wouldn't work. I'd say it was worth a try first though, relatively
easy to stick in-line with an antenna and it would get rid of most
of the out of band signal straight off. Not to leave the filter there
but just as a trial to see if the interference is in-band or out-of-band.

Paul

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, "Paul Sengupta"
wrote:

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.

The problem with devices like the one you suggest (see above link) is that
they are not designed to carry the high current that will be present when
the com antenna to which it is attached is used for transmission.
(Remember, our com radios transmit as well as receive.) If the filter
doesn't burn up under that current load it will at least significantly
attenuate the transmit signal. The specs for this device show an in-band
attenuation of 4 dB. That's a lot!

It would be possible to rig up a transmit bypass using circulators or coax
relays, but it would be messy.

--
-Elliott Drucker

Yes, I was only thinking using it as a troubleshooting tool
to see if the interference was in-band or out-of-band by the
time it hit the receiver front end. My dim memory is telling me
that there are setups using by-pass relays for transmit due to
the adding of a receive pre-amp, though I can't remember where
I saw it...I may be thinking of a different field entirely, I know
they're used in amateur radio...

Paul
G1YJY

wrote in message
...
The problem with devices like the one you suggest (see above link) is that
they are not designed to carry the high current that will be present when
the com antenna to which it is attached is used for transmission.
It would be possible to rig up a transmit bypass using circulators or coax
relays, but it would be messy.

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

: This leads me to suspect one of the *NAV* radios. Can you physically
: remove them from your plane, one at a time, and leave them in your car?

: Not a problem. Can this really be caused by a radio which is *powered
: off*? Because I did try switching the navs to different frequencies
: and also turning them off, along with all the electrical power in the
: plane.

Yes, it can be caused by a radio switched off. With those ACK elt units that
cause problems, they are off when their problems occur!

: I'm perfectly happy to take it off and slap some 50 mph duct tape over
: the resulting hole, providing it's not going to hurt the MB receiver
: to be attached to dangling coax.

It won't.

: If I were doing this, I'd probably
: terminate the RF input to the radio, as well.

: IIRC this would be a PITA -- the coax goes to the antenna with a
: standard BNC connector, but is soldered on to the radio tray.
:

It would be OK to terminate the coax where the antenna was connected.
As someone else pointed out, you can short out the antenna at its
BNC connector to electrically remove it from the plane. You could
probably make a BNC shorting plug from parts found at Radio Shack.

--
Aaron Coolidge (N9376J)

(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.