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

(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

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

Aaron Coolidge wrote in message ...

Sydney, have a look at the fcc link I posted a couple messages ago. It lets
you look up towers' owners by lat/long. Your tower is owned by KTVI chan 2.

Thanks.

: Do other aircraft report the same interference?

: Not that I've heard, but then, I might not have heard.
: Or, like us, they might have assumed it was a problem in
: their airplane.

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.

So it *is* something specific to our plane I guess.
Although it's an intermittant problem for us, too.

: We didn't have this problem before last spring.

Did channel 2 recently add a digital TV transmitter? Like, last spring?

I believe so, yes.

So here's what I'm thinking.

That tower is TV Channel 2 (60-65 MHz I think?)
Channel 5 which is nearby would be 79-84 MHz.

This makes me think that marker beacons, at 75 MHz,
are the logical suspect for causing a problem.

But can the marker beacon antenna, by itself, be somehow
bringing signals into the plane to be received by the
rubber whip antenna of our handheld?

If this is a possibility, how do we safely remove the
marker beacon antenna for testing purposes? Do we need
to put some kind of load on the cable heading for the
marker beacon receiver, since we can't turn the MB off
while the power in the plane is on?

If removing the MB antenna seems to cure the problem,
what do we test or do? Is the antenna itself likely to
be bad and in need of replacement, or is this likely to
be a ground type issue where maybe we should replace the
coax, or at least redo the connections?

If it seems far-fetched that the MB antenna itself is
the culprit, where next do we look?

Thanks,
Sydney

: Sydney, have a look at the fcc link I posted a couple messages ago. It lets
: you look up towers' owners by lat/long. Your tower is owned by KTVI chan 2.

: Thanks.

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!

: So it *is* something specific to our plane I guess.
: Although it's an intermittant problem for us, too.

Aha, I think we're on to something.

: : We didn't have this problem before last spring.
:
: Did channel 2 recently add a digital TV transmitter? Like, last spring?

: I believe so, yes.

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

: So here's what I'm thinking.

: That tower is TV Channel 2 (60-65 MHz I think?)
: Channel 5 which is nearby would be 79-84 MHz.

: This makes me think that marker beacons, at 75 MHz,
: are the logical suspect for causing a problem.

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

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?
This would take their front end circuitry out of the area. Then try the
other one. The COM radios would also be out of the picture. Perhaps
you've already done this?

The reason that I'm harping on radios is that intermodulation
distortion needs a detector or a modulator to occur, such as in the
RF front end of a radio. I don't think that an antenna by itself is
sufficient to cause it.

Also, you might try taking out the nav antenna splitter. I'm not sure
that this should make any change, but if we're using buckshot methods...

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!

: But can the marker beacon antenna, by itself, be somehow
: bringing signals into the plane to be received by the
: rubber whip antenna of our handheld?

If you disconnect the MB antenna from the MB receiver, it is unlikely
that the end of the coax could act as much of a radiating element. I have
made a passive radiator before, but that's 2 antennas connected to each
other.

: 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, disconnecting the little floating
wire will disconnect the MB antenna from the in-plane electronics, though
I'm inclined to dismiss the MB system.

If you wish to electrically remove the antenna from the plane while leaving
it physically in place, you can get a "terminator" cap from most electronics
stores that cater to the ham radio crowd. I'm not sure Radio Shack sells them.
You'd want a 50-ohm terminator, and whatever adapters are needed to connect
it to the end of the antenna coax. If I were doing this, I'd probably
terminate the RF input to the radio, as well.

Please keep us (me) informed, we're trying to help the best we can!
--
Aaron Coolidge (N9376J)

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

In article ,
Snowbird wrote:


Aaron Coolidge wrote in message
...

Sydney, have a look at the fcc link I posted a couple messages ago. It lets
you look up towers' owners by lat/long. Your tower is owned by KTVI chan 2.

Thanks.

: Do other aircraft report the same interference?

: Not that I've heard, but then, I might not have heard.
: Or, like us, they might have assumed it was a problem in
: their airplane.

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.

So it *is* something specific to our plane I guess.

"Not necessarily", although it _is_ likely. Different radios _do_ have
different degrees of susceptability to intermod interference. It _is_
possible that you have 'unlucky' radios.

I repeat: "possible", yes; "probable", no.

Although it's an intermittant problem for us, too.

The _definitive_ test -- to confirm that it *is* something with
your plane -- is to get her, in one of her planes that does *not*
exhibit the problem, to fly in the area, *WITH*YOUR*HAND-HELD*,
at a time when you _are_ able to reproduce the problem on *your* radios.

If the hand-held does _not_ misbehave, then it _is_ confirmed to be
something in your plane.

There is _also_ a REMOTE possibility that, *because*the*hand-held*is*aboard*,
The in-board radios _will_ hear interference. This would be conclusive
proof of 'front end overload' in the hand-held, with it _re-radiating_ the
spurious signal, which is then being picked up by _their_ recievers.


: We didn't have this problem before last spring.

Did channel 2 recently add a digital TV transmitter? Like, last spring?

I believe so, yes.

So here's what I'm thinking.

That tower is TV Channel 2 (60-65 MHz I think?)

54-60 mHz carrier at 55.125

Channel 5 which is nearby would be 79-84 MHz.

76-82 mHz carrier at 77.125

This makes me think that marker beacons, at 75 MHz,
are the logical suspect for causing a problem.

No particular reason to suspect the marker beacons.

'intermodulation interference' occurs when the frequencies of
two (or more) transmitters add/subtract to give a result that
is the same as the "real signal" you're looking for. Frequently,
the 'source' signals are _far_removed_ from the frequency that
is getting fouled. e.g. a Tx at 400.0mhz, and a 2nd one at 526.50mhz,
combining to give a spurios signal at 126.50 mHz. (note: I'm pulling
figures out of thin air here, *no* reason to believe there's anything
at 400.0, or 526.5, in your area.

But can the marker beacon antenna, by itself, be somehow
bringing signals into the plane to be received by the
rubber whip antenna of our handheld?

yes, and no. grin

*Any* place where two _dis-similar_ pieces of metal come into contact
is a "low grade" transistor junction, providing a place where 2 or more
signals can "mix", generating a 'spurious' hetrodyne, or intermodulation
product. This "generated" (low strength) signal can then 'radiate' from
_any_ antenna-like piece of metal that is electrically connected to the
point where the mixing occured.

If this is a possibility, how do we safely remove the
marker beacon antenna for testing purposes? Do we need
to put some kind of load on the cable heading for the
marker beacon receiver, since we can't turn the MB off
while the power in the plane is on?

If removing the MB antenna seems to cure the problem,
what do we test or do? Is the antenna itself likely to
be bad and in need of replacement, or is this likely to
be a ground type issue where maybe we should replace the
coax, or at least redo the connections?

If it seems far-fetched that the MB antenna itself is
the culprit, where next do we look?

There are _still_ two possibilities to deal with -- 1) it _is_
something inside the plane, 2) it is *not* something inside
the plane.

The fact that you've got a hand-held that will 'hear' the problem
is a good start at a 'signal sniffer'.

A "sniffer" needs two capabilities that aren't 'standard' on the
hand-held. One, a way to reduce the incoming signal strength to
a point where you can 'hear' or 'see' (on an s-meter, if present),
comparatively small changes in signal strength. And, two, a
'directional' antenna.


Assuming the hand-held has a -removable- antenna (probably the
so-called "rubber duckie" type), this is all relatively _easy_ to
do.

Step 1 is to build a 'variable attenuator'. The ideal enclosure to
build this in is a small "U-box" (available from radio shack, among
other places -- all metal, 2 pieces, each shaped sort-of like the
letter "U"). several minature DPDT switches, each of which switches
an attenuator stage (of varying degree). the attenuator stages
consist of a series resistor, and a resistor to ground, so as to
provide a constant impedence. Eached switched stage is wired in
series to the next switch, so you can "add" attenuation, just by
switching in additional stages. Without having _any_ idea of
signal levels, I'd suggest 5 stages -- one at 3db, one at 6db,
one at 10db, and 2 at 20db ea. This lets you cut signal strength
in 3db increments (cutting the signal seen by the reciever in half)
all the way to nearly 70db of attenuation (enough to drop out a
fairly high-powered source at close range.

Step 2, a 'directional antenna' is required. It doesn't have to be
an 'efficient' antenna, just 'directional'. In fact, 'efficieny'
in _this_ application is *not* a good thing. A simple one can be
manufactured with nothing more than: (a) a piece of co-ax, (b) a
short board to serve as a 'handle' (something like a 12" piece of
1x2 is near-ideal), and a 'medium' (circa 6" across the top) _metallic_
funnel. Drill a hole through the board, a couple of inches down from
one end. big enough that the spout of the funnel just fits in it.
Take the piece of coax, and strip back the outer cover, and shielding,
about 2/3 of the distance from the top of the funnel to the tip.
strip off the inner dielectric as well, but _leaving_ a section that
is roughly the length of the spout. "Apply" the coax to the funnel,
from the spout end, so that you have the bare center wire sticking up
in the middle of the funnel. Solder the shield to the tip of the spout.
Lastly, fit this 'monstrosity' back into the hole in the wooden 'handle'.

Now, by simply 'waving the stick around', you can point the funnel in
any direction you choose.

Run this 'antenna cable' to the attenuator box, and cable from the
box to the antenna connection on the radio.

Ok, you're equipped for 'transmitter hunting'. *grin*

A little experimenting with a "known" station -- say a 'weather' announce
loop, will show how the beastie works. Starting with all the attenuation
'switched out', hold the stick upright, and rotate it 360 deg. If you
hear the station at all angles, start switching in some attenuation. You'll
get to a point where you only hear the station when the funnel is pointed
"more or less" in it's direction. The station is located roughly in the
middle of that arc where you can hear it.

*NOW* you're ready to see what's to be seen about the source of your
interference.

Fly into the problem area, and tune to the interference. and locate
what direction it's coming from. Now, make another pass through the
area, on a course 90 degree removed from the first attempt.

When you localize the direction _this_ time, one of two things will occur.
either the _absolute_ direction will be the same (e.g "due East"), or
it will be the same _relative_bearing_ (e.g. 45 degrees left of 'straight
ahead'). If it's the first, the problem is *outside* the plane. If
the second, it _is_ in the plane.

If the source is outside the plane, you're conclusively dealing with front
end overload intermodulation interference. There are two possible solutions:
1) high-Q bandpass filtering, to eliminate the out-of-band interference
sources.
2) replace the radios, with ones that aren't as suceptable to interference.

If the source is in the plane, you keep hunting. Do the direction check from
different places in the plane. Remember to check on all axis (roll,pitch,
yaw) too. (maximize on one axis, then hunt 90 degree _relative_ to that one,
and confirm by using 3rd 90) By kicking in additional attenuation, you should
be able to get fairly narrow arcs where you can hear the interference, and
tell "something" about where problem is located. if it shows 'straight ahead'
when checked near the left window, but 45 deg. left of straight ahead
when checked from the right window, this tells you a _lot_ about where
to look. Admittedly, if the problem is 'way back in the tail', you're
not going to get a very precise indication. Unless you get "Mini-Me"
to go back there with the antenna stick, that is. guffaw

In article ,
Aaron Coolidge wrote:


: Sydney, have a look at the fcc link I posted a couple messages ago. It lets
: you look up towers' owners by lat/long. Your tower is owned by KTVI chan 2.

: Thanks.

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!

: So it *is* something specific to our plane I guess.
: Although it's an intermittant problem for us, too.

Aha, I think we're on to something.

: : We didn't have this problem before last spring.
:
: Did channel 2 recently add a digital TV transmitter? Like, last spring?

: I believe so, yes.

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

: So here's what I'm thinking.

: That tower is TV Channel 2 (60-65 MHz I think?)
: Channel 5 which is nearby would be 79-84 MHz.

: This makes me think that marker beacons, at 75 MHz,
: are the logical suspect for causing a problem.

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

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?
This would take their front end circuitry out of the area. Then try the
other one. The COM radios would also be out of the picture. Perhaps
you've already done this?

The reason that I'm harping on radios is that intermodulation
distortion needs a detector or a modulator to occur, such as in the
RF front end of a radio. I don't think that an antenna by itself is
sufficient to cause it.

Also, you might try taking out the nav antenna splitter. I'm not sure
that this should make any change, but if we're using buckshot methods...

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!

: But can the marker beacon antenna, by itself, be somehow
: bringing signals into the plane to be received by the
: rubber whip antenna of our handheld?

If you disconnect the MB antenna from the MB receiver, it is unlikely
that the end of the coax could act as much of a radiating element. I have
made a passive radiator before, but that's 2 antennas connected to each
other.

Doesn't have to be -- _strong_ out-of-band signals can be picked up,
albeit inefficiently, 'mix' at any dissimilar metals contact, genrating
a signal to which the antenna is tuned, and which _will_ then re-radiate.
All in a single-antenna system.

: 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, disconnecting the little floating
wire will disconnect the MB antenna from the in-plane electronics, though
I'm inclined to dismiss the MB system.

If you wish to electrically remove the antenna from the plane while leaving
it physically in place, you can get a "terminator" cap from most electronics
stores that cater to the ham radio crowd. I'm not sure Radio Shack sells them.
You'd want a 50-ohm terminator, and whatever adapters are needed to connect
it to the end of the antenna coax. If I were doing this, I'd probably
terminate the RF input to the radio, as well.

Recommendation is to put a terminator on the receiver input, but
simply *short* the antenna cable center-lead to the shield. Putting
a terminator on the antenna cable encourages re-radiation, *if* the
problem is coming from a 'diode-ing' spot somewhere closer to the
antenna. Shorting tends to suppress any passive re-radiation.

"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

We've got one here but it may be a bit far to come.

Paul
Guildford, UK.

"Aaron Coolidge" wrote in message
...
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!

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.