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

[Snowbird]

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"

[Bill Daniels]

"Snowbird" > wrote in message
om...
> 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"

I fly a sailplane with a battery powered Microair 760 and no other avionics.
Occasionally, in certain locations, a country western AM station will break
through at high volume. In other locations, it is a TV station. This
problem is also encountered in other sailplanes with other radios. The
thing here is that there are no other onboard electronics to confuse the
situation. The unwanted signals are usually encountered at less than 1500
feet AGL so I suspect groundwave propagation from the commercial
transmitters.

I think the commercial stations are poorly controlling their sideband
radiation and some of their RF power is spilling onto the aviation band.
Aircraft radios are generally well designed and reject signals that are not
intended for them, but they can't do anything about spurious signals from
sloppy maintenance at a commercial radio stations.

Maybe contacting the FCC would be your next step.

Bill Daniels

[Jim Weir]

I respectfully disagree with this statement. There is precious little that is
engineered as well as a commercial radio station. Even sloppy maintenance
wouldn't get the sidebands this out of whack...and besides, those sidebands
would have to be from the FM band, and we are talking about 20% away from the
carrier. Ain't no way.

I live next door (literally) to a 10kW AM transmitter, and occasionally I tune
the spectrum analyzer to their frequency and put an 80 dB notch filter on the
center of the carrier. With a 120 dBm noise floor, I **still** can't see any
splatter outside their allocated bandwidth.

While aircraft radios are generally good with filters, also, there comes a limit
to any design when the volts coming in the front end are just too much for the
microvolt amplifiers. It is called fundamental front end overload, and we are
all susceptible to it.

Both Microair and King use varactor tuned filters in the front end for
selectivity. While these are good design choices, you must remember that a
varactor is nothing more than a diode, and a varactor is one of the best
multipliers in the world. Whap that front end diode with a couple of volts and
you've got a little comb generator second to none.

I think we'd best look elsewhere for the solution.


Jim




"Bill Daniels" >
shared these priceless pearls of wisdom:

->I think the commercial stations are poorly controlling their sideband
->radiation and some of their RF power is spilling onto the aviation band.
->Aircraft radios are generally well designed and reject signals that are not
->intended for them, but they can't do anything about spurious signals from
->sloppy maintenance at a commercial radio stations.

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

[Jim Weir]

Quick question(s) ...

Is TV channel 3 a local station for you? Is their transmitter in this antenna
farm? Do other aircraft report the same interference?

Jim

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

[Rich S.]

"Jim Weir" > wrote in message
...
>
> I think we'd best look elsewhere for the solution.

Sounds like intermodulation to me. Look for a sum/difference with the
frequencies at the antenna farm?

Rich S.

[Jim Stockton]

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"

It sounds like IMD in the receiver frontend. As Jim pointed out it is
probably being caused by the input amp stage. The difficult thing is the
cure. I don't know if a high power band pass filter is available so that
the interfering frequencys don't get into the reciever front end. If you
can pinpoint the source frequency of the interferance a simple coax
tuned stub filter will notch it out but that would not be broad enough
for a TV transmitter, but usually only for an AM transmitter, or other
narrow band interferance.
Good Luck
Jim Stockton

[EDR]

In article >, Rich S.
> wrote:

> Sounds like intermodulation to me. Look for a sum/difference with the
> frequencies at the antenna farm?

That's my thought. I used to fly over Voice of America's transmitter
farm in southwest Ohio before they shut it down. Their transmissions
always leaked into the aircraft radios when in close proximity.

[Aaron Coolidge]

In rec.aviation.owning Jim Weir > wrote:
: Quick question(s) ...

: Is TV channel 3 a local station for you? Is their transmitter in this antenna
: farm? Do other aircraft report the same interference?

: Jim

Jim, the FCC has a nice antenna search by geo coords page:
http://wireless2.fcc.gov/UlsApp/AsrSearch/asrRegistrationSearch.jsp

Using Sydney's coords (38 deg 31' 54" N 90 deg 21' 45" W) it shows 2
antennae at that site, both belonging to KTVI Fox channel 2. Within 5 km
of that site there's KSDK TV5 (38 34' 05"N, 90 19' 55"W), and some cell towers
owned by SpectraSite (38 34' 24"N, 90 19' 30"W). Nothing else with any height,
though.

--
Aaron Coolidge (N9376J)

[Rob Cherney]

On Mon, 03 Nov 2003 17:39:03 -0600, Jim Stockton >
wrote:

>It sounds like IMD in the receiver frontend.

It could also be passive intermodulation (PIM). If several high-power
transmitters are at the same location (as in an antenna farm), then
these signals can mix and reradiate. All that is necessary is
dissimilar or corroded metals in the radio towers.

After a Google search, I came up with this reference:

http://www.summitekinstruments.com/passive/faqcont.html#top

Notice the mention of "rusty bolt noise".

You say that your hand-held radio receives the same interference. Do
other pilots have the same issue? If so, this is a good indication of
PIM. Of course, it could also show that other receivers are similarly
overloaded with some strong out-of-band signals. But, once you have
some more evidence that it is PIM (i.e., others with the same issue),
then perhaps the FCC or the FAA might take an interest in doing an RF
site survey to isolate the cause of it.


Rob-
------------------------------------------------------------------
Robert Cherney e-mail: rcherney(at)comcast(dot)net

In article >,
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?

Everything you say points to "intermodulation interference" -- where
multiple transmissions are hetrodyning against each other, and creating
a byproduct signal on the channel you're listening on. The bad news is
that locating the 'source' of the problem is *difficult*. In large part
because there isn't _a_ source. There are somewhere between 2 and "many"
'contributors' to the problem -- *all* of which have to be operating for
the problem to occur (that's why "sometimes it's there, sometimes it
isn't" -- sometimes one or more of the 'critical' contributor sources a
are -not- running.)

The fact that it happens with the plane electrical off tends to exonerate
_almost_all_ of the avionics from complicity.

There are, in essence, three possibilities remaining:

One: it is *possible*, albeit unlikely, that the hetrodyning is occurring
in an antenna/cabling/"front end" of one of the radios (even without
power applied), and being _re-radiated_ to be picked up by other radios,
including the hand-held.

The test, to eliminate this possibility, involves disconnecting antenna
cables (one at a time) from whatever they're plugged into, then (a)
shorting the center conductor to the shield, and (b) putting a 'dummy
load' (a 50 ohm resistor) across the radio connection. that radio
should now be totally 'deaf'. *IF* _it_ hears the interference, then
the 'intermod' is occurring at IF, and additional shielding around the
radio may help.

*IF* the problem 'goes away' with one of the antenna's shorted, you've
'localized' the problem, check for a corroded connection, cold-solder
joint, etc. consider replacing the entire assembly.

If, as is likely, nothing 'interesting' happens during the above testing,
It's time for _one_ additional test. using the _same_ hand-held, and
a *different* airplane, fly into the same area, at a time when you have
established that the problem is present. If the interference shows
up on the hand-held, you _have_ eliminated everything in the plane as
causative agent. It *is* 'inherent' in the locale If the radios in this
2nd plane are not picking it up, it is, for lack of a better term, a
"front end overload" problem in _your_ radios. Sensitivity to this
problem varies with the design of the radio, _and_ (although generally to
a minor degree) even from unit to unit within a given model line.

This is possibility #2, "out-of-band signal overload at the radio
front-end". A "High Q-Factor band-pass filter" installed on the
receive side of the radio can greatly reduce this problem. Depending
or radio design, this can range from 'reasonable' to 'outrageously
difficult and expensive' to implement.

The third possibility is that the hetrodyning, and re-radiation is
occurring somewhere _external_ to the airplane. e.g. _on_ some component
of that transmitting tower. In which case about the *only* 'solution'
is to treat it as a "DDT" problem -- since the problem occurs only when
you fly into that specific area, the solution is "<D>on't <D>o <T>hat!"
Don't fly into that area, and you won't have the problem. :)

Note: if this -is- the situation, then pretty much _everybody_ flying into
the vicinity should be experiencing the same problem, on a recurring basis.
Do other pilots report similar difficulties?

[David Lesher]

"Rich S." > writes:

>> I think we'd best look elsewhere for the solution.

>Sounds like intermodulation to me. Look for a sum/difference with the
>frequencies at the antenna farm?

I'd ask other owners. Then try to find your regional FAA frequency
coordinator or similar, in other to locate a sympathetic ear. He'd
know who would go by with a 494 spectrum analyzer or such...iffen they
ever have time.

It could well be intermod from tower guy joints or such. It could
also be the front end of the radios. (Don't suppose you have 3 element
cavity tha would fit in the back seat ;-?)

If all else fails, maybe filing a NASA report would get a response.
--
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

[Snowbird]

Jim Weir > wrote in message >...
> Quick question(s) ...

> Is TV channel 3 a local station for you?

Negative. Channel 2 and Channel 4.

> Is their transmitter in this antenna farm?

I don't know whose transmitters are there. I got the
lat-long coordinates; any way to find out which transmitters
are there?

> 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.

We didn't have this problem before last spring.

Any assistance sorting this out would be greatly appreciated;
we do have a pretty good local avionics guy but he frankly
seems stumped (at least he's honest and good enough not to
simply suggest replacing all the radios in the stack, which
one shop did).

Cheers,
Sydney

[[email protected]]

Admin is correct as far as he goes.
This problem is also sometimes referred to as the "rusty bolt
problem". If you have a rusty bolt or rusty connection between two
metal parts on the antenna, the aircraft airframe , the transmitting
antenna and anywhere the field is strong enough the corrosion can act
as a diode and generate intermod products which is what you are most
likely hearing.

I would disconnect "ALL" of the antennas on the aircraft and use the
handheld while flying over this location. If this fixes the problem
connect one antenna at a time until the problem comes back. If the
signal is still present with all of the antennas disconnected it is
something on the ground or airframe corrosion. The next step is try
a different airplane with the same hand held. If it is still present
the problem is in or near the antenna farm on the ground.
Good luck
John


On Tue, 04 Nov 2003 02:18:14 GMT, ()
wrote:

>In article >,
>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?
>
>Everything you say points to "intermodulation interference" -- where
>multiple transmissions are hetrodyning against each other, and creating
>a byproduct signal on the channel you're listening on. The bad news is
>that locating the 'source' of the problem is *difficult*. In large part
>because there isn't _a_ source. There are somewhere between 2 and "many"
>'contributors' to the problem -- *all* of which have to be operating for
>the problem to occur (that's why "sometimes it's there, sometimes it
>isn't" -- sometimes one or more of the 'critical' contributor sources a
>are -not- running.)
>
>The fact that it happens with the plane electrical off tends to exonerate
>_almost_all_ of the avionics from complicity.
>
>There are, in essence, three possibilities remaining:
>
>One: it is *possible*, albeit unlikely, that the hetrodyning is occurring
>in an antenna/cabling/"front end" of one of the radios (even without
>power applied), and being _re-radiated_ to be picked up by other radios,
>including the hand-held.
>
>The test, to eliminate this possibility, involves disconnecting antenna
>cables (one at a time) from whatever they're plugged into, then (a)
>shorting the center conductor to the shield, and (b) putting a 'dummy
>load' (a 50 ohm resistor) across the radio connection. that radio
>should now be totally 'deaf'. *IF* _it_ hears the interference, then
>the 'intermod' is occurring at IF, and additional shielding around the
>radio may help.
>
>*IF* the problem 'goes away' with one of the antenna's shorted, you've
>'localized' the problem, check for a corroded connection, cold-solder
>joint, etc. consider replacing the entire assembly.
>
>If, as is likely, nothing 'interesting' happens during the above testing,
>It's time for _one_ additional test. using the _same_ hand-held, and
>a *different* airplane, fly into the same area, at a time when you have
>established that the problem is present. If the interference shows
>up on the hand-held, you _have_ eliminated everything in the plane as
>causative agent. It *is* 'inherent' in the locale If the radios in this
>2nd plane are not picking it up, it is, for lack of a better term, a
>"front end overload" problem in _your_ radios. Sensitivity to this
>problem varies with the design of the radio, _and_ (although generally to
>a minor degree) even from unit to unit within a given model line.
>
>This is possibility #2, "out-of-band signal overload at the radio
>front-end". A "High Q-Factor band-pass filter" installed on the
>receive side of the radio can greatly reduce this problem. Depending
>or radio design, this can range from 'reasonable' to 'outrageously
>difficult and expensive' to implement.
>
>The third possibility is that the hetrodyning, and re-radiation is
>occurring somewhere _external_ to the airplane. e.g. _on_ some component
>of that transmitting tower. In which case about the *only* 'solution'
>is to treat it as a "DDT" problem -- since the problem occurs only when
>you fly into that specific area, the solution is "<D>on't <D>o <T>hat!"
>Don't fly into that area, and you won't have the problem. :)
>
>Note: if this -is- the situation, then pretty much _everybody_ flying into
>the vicinity should be experiencing the same problem, on a recurring basis.
>Do other pilots report similar difficulties?
>

[Aaron Coolidge]

: I don't know whose transmitters are there. I got the
: lat-long coordinates; any way to find out which transmitters
: are there?

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.

:> 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.

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

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

--
Aaron Coolidge (N9376J)

[Aaron Coolidge]

In rec.aviation.owning Jim Weir > wrote:
: Quick question(s) ...

: Is TV channel 3 a local station for you? Is their transmitter in this antenna
: farm? Do other aircraft report the same interference?

: Jim

Jim, do you think that TV Channel 2 (54 MHz) could be interfering with the
NAV radios? I do know that I can receive distorted FM on my nav radio
at 108.00 (we have a 107.9 FM station in town here).

--
Aaron Coolidge

[[email protected]]

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

[Snowbird]

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

[Mark Hickey]

wrote:

>Admin is correct as far as he goes.
>This problem is also sometimes referred to as the "rusty bolt
>problem". If you have a rusty bolt or rusty connection between two
>metal parts on the antenna, the aircraft airframe , the transmitting
>antenna and anywhere the field is strong enough the corrosion can act
>as a diode and generate intermod products which is what you are most
>likely hearing.
>
>I would disconnect "ALL" of the antennas on the aircraft and use the
>handheld while flying over this location. If this fixes the problem
>connect one antenna at a time until the problem comes back. If the
>signal is still present with all of the antennas disconnected it is
>something on the ground or airframe corrosion. The next step is try
>a different airplane with the same hand held. If it is still present
>the problem is in or near the antenna farm on the ground.

It may be an anathema in aviation circles to suggest this, but why not
just DRIVE to somewhere near the site with a portable and see if you
get the same effect? By doing this, you've eliminated EVERYTHING
other than the site itself. If the interference is still there on the
same frequencies, you know there's really nothing you can do about it
(at least without legal intervention and/or high explosives).

Or I suppose you could fly over it in a J3 sans electrical system and
get the same results...

Mark Hickey

[Ross Richardson]

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

[Jay]

How close are you flying to the antenna farm? The field strength
decays exponentially with radial distance. Put another way, as you
get close, you may be flying into a VERY strong field, outside the
design parameters of the radios your using.

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.

If this turns out indeed to be the case, and you need to fly your
plane into fields that are that strong, you might be able to get some
relief with improved grounding, sealing radio case gaps with copper
foil, a filter network right before the antenna enters the radio.

[Roger Halstead]

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?

An example of how difficult IM can be to find:

Two years ago we received a report of interference from our repeater
(on 147 MHz) to one of the services in Lansing. (that's about 70
miles). The interference was intermittent.

It turned out that there were three stations involved and none were
within 30 miles of each other.

Out repeater signal and a pager in (I believe it was Chesasining MI)
was mixing in a commercial repeater (fire I believe) and being
retransmitted on the input frequency for a Police repeater in Lansing.
The resultant signal being well off the frequencies used in the other
repeaters was understandably weak, but strong enough to key the Police
repeater and be clearly understandable.

It took several weeks of dedicated hunting by a number of crews in an
area nearly a 100 miles by about 30 miles to finally locate the
offender.

<snip>

>> 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.

IF the signal bothers one receiver far more than the other then I
would think the problem is more likely on the plane. IF it is
external and close, both receivers on the same frequency should have
the same problem.

To me that says "bad connection" some where.

There are many other possibilities, but with that information I'd tend
to lean heavily toward "on board"

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

[Mark Hickey]

(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

[[email protected]]

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

[Snowbird]

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

[Aaron Coolidge]

:> 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)

[[email protected]]

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.

[Paul Sengupta]

"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

[Paul Sengupta]

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!

[Paul Sengupta]

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.

[Snowbird]

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

[Mark Hickey]

"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

[Paul Sengupta]

> 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

[Snowbird]

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

[[email protected]]

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

[Paul Sengupta]

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.

[[email protected]]

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

[Aaron Coolidge]

:> 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)

[anon]

(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. :)

[Jim Weir]

You can stop playing with 10.7 as a source of the problem. I cannot recall an
aircraft navcom using 10.7 as the IF frequency.

Jim


"Paul Sengupta" >
shared these priceless pearls of wisdom:

->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.
->

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

[Jim Weir]

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

[Snowbird]

"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

[David Lesher]

(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

[Roger Halstead]

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.

[Roger Halstead]

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

[Snowbird]

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

[rip]

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"

[Aaron Coolidge]

: 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)

[Jim Weir]

(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

[Roger Halstead]

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

[Snowbird]

rip > wrote in message >...
> Can you hear this interference through the cabin speaker?

Don't know. Haven't tried that one -- frankly I can hardly
hear anything through the cabin speaker

> Or perhaps your using active noise-cancelling headsets?

'rip', we are -- but we have done the obvious experiment of
seeing if it's still there when the ANRs are turned OFF and
we're listening on a passive headset hooked to the handheld
radio. We have had problems with RF noise from a bad ANR
in the past -- however, it was just noise, not the audio
part of some TV or radio program.

However, the switched OFF ANRs are still sitting in the plane
if anyone thinks that makes a difference

I'm still trying to understand how a powered OFF radio with
the power OFF in the plane might be contributing to this
problem and hoping someone will explain this to me.

Cheers,
Sydney

[[email protected]]

On 6-Nov-2003, (Snowbird) wrote:

> I'm still trying to understand how a powered OFF radio with
> the power OFF in the plane might be contributing to this
> problem and hoping someone will explain this to me.



Sydney,

It IS possible, but it is also extremely unlikely to be the cause of your
problem.

Those that propose this theory are assuming that the RF signals being picked
up by your com antennas are strong enough BY THEMSELVES to provide power to
the receiver of a radio that is not otherwise powered. This seemingly
strange phenomenon is most often observed when a car (with a typical AM/FM
car radio that is turned off) is driven right up to an antenna farm.

In your case, I assume that you are flying either far enough horizontally
away from or high enough above the antennas for safety. Let's say a minimum
of 1/2 mile horizontally. Now lets assume that the strongest broadcast
signal is 100,000 watts "effective radiated power" and a frequency somewhere
around 100 MHz. At a distance of 1/2 statute mile the power from that
signal that would be picked up by an aircraft com antenna would be about 10
milliwatts. This is a spectacularly strong signal for purposes of
reception, and certainly more than enough to make it impossible for the
attached receiver to pick up any other signal, but nowhere near strong
enough to cause the receiver, if not otherwise powered, to generate and
re-radiate an intermodulation product.

So, if the signal WERE strong enough to provide enough power to cause an
otherwise unpowered receiver to generate and re-radiate an interfering
signal, the same powerful signal would pretty much wipe out operation of any
other nearby receiver that IS powered on.

By all accounts, what you have is plain old garden variety intermodulation
interference. The intermodulation products doing the interfering are being
generated in the same receiver that is being interfered with. In an earlier
post I offered a relatively simple way to prove this.

--
-Elliott Drucker

[Mark A. Matthews]

In article >,
(Snowbird) wrote:
> I'm still trying to understand how a powered OFF radio with
> the power OFF in the plane might be contributing to this
> problem and hoping someone will explain this to me.

Does it help to think about how crystal radios work without any power at
all? Except for that of the transmitted signal.

Similarly, the energy for creating the interfering signal could be
coming from two or more transmitted signals that are mixing in the
front-end stage of an on-board radio receiver. Powered-off doesn't
matter; the energy is supplied by the transmitted signals and the mixing
action provided by one of the semiconductors in the first RF stage of
the receiver. Resulting interfering signal is then re-radiated back out
the same antenna the original signals are coming in on. You then hear
the interfering signal in another radio, such as a handheld.

Similarly, some corrision at an antenna terminal can also provide a
mixing action to a pair of strong signals that is then heard on one or
more radios. Or the aforementioned "rusty bolt syndrome" in that a
corroded bolt in a radio tower leg provides the mixing action for two
strong signals heard in your radios as you get close to it.

I would take the handheld and a car (or another airplane) and explore
the area where the interference occurs to eliminate the airplane from
the equation. Once you've convinced yourself it is or is not in the
airplane you have a direction to follow to find resolution. Until then,
you just don't know which way to look.

--
-Mark

[Jim Weir]

->In your case, I assume that you are flying either far enough horizontally
->away from or high enough above the antennas for safety. Let's say a minimum
->of 1/2 mile horizontally. Now lets assume that the strongest broadcast
->signal is 100,000 watts "effective radiated power" and a frequency somewhere
->around 100 MHz.

In a metropolitan area 100kW ERP would rank you in the lower third of broadcast
signals, but for grins and giggles, let's make that presumption.



At a distance of 1/2 statute mile the power from that
->signal that would be picked up by an aircraft com antenna would be about 10
->milliwatts.

More like 5 milliwatts, but let's not debate how many milliwatts can dance on
the head of a dipole.


This is a spectacularly strong signal for purposes of
->reception, and certainly more than enough to make it impossible for the
->attached receiver to pick up any other signal, but nowhere near strong
->enough to cause the receiver, if not otherwise powered, to generate and
->re-radiate an intermodulation product.

Oopsie. 10 milliwatts is about 0.7 volts RMS, or about a volt peak. If for
whatever reason that front end were wide open to the interfering signal, a volt
is sure as little green apples capable of turning on the B-E diode of the RF
amplifier. Now any lower power signal is perfectly capable of being mixed with
our newfound "LO" and being reradiated.


->
->So, if the signal WERE strong enough to provide enough power to cause an
->otherwise unpowered receiver to generate and re-radiate an interfering
->signal, the same powerful signal would pretty much wipe out operation of any
->other nearby receiver that IS powered on.

Ah, no. NOt if the other receiver had enough filtering in the front end to get
rid of it.

->
->By all accounts, what you have is plain old garden variety intermodulation
->interference. The intermodulation products doing the interfering are being
->generated in the same receiver that is being interfered with. In an earlier
->post I offered a relatively simple way to prove this.

I'm not debating that it is intermod. I don't know. I'm not there. But front
end reradiation is a phenomenon that should be investigated also. And, for that
matter, that is exactly why two brands of ELT have had factory recalls...the C-B
junction in the output transistor was a wonderful intermod generator.

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]

Ah, ok, thanks for that! What do they use? Is it standard?

Paul

"Jim Weir" > wrote in message
...
> You can stop playing with 10.7 as a source of the problem. I cannot
recall an
> aircraft navcom using 10.7 as the IF frequency.

[Paul Sengupta]

The rate at which the attenuation increases away from the centre
frequency is affected by the "order" of the filter (first order, second
order, etc). It would be difficult to filter out nav frequencies from the
com band. Even if the nav radios are the cause, it's unlikely to be a
signal actually in the nav band causing the problem...the thinking is that
it would produce interference in the com band.

The filter from the link would only be used really to see if the cause
of the interference was due to a TV band (50-80MHz?) signalling
causing problems in the receiver, or whether it was something
external causing problems within the com band.

Question (Jim?)...I would expect the image channel rejection to be
pretty good in aircraft radios, is this the case? How about with
handhelds? (and what IF is used?)

Paul

"Snowbird" > wrote in message
om...
> 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.

[David Lesher]

(Snowbird) writes:


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


Do you remember a "crystal set" radio? It had a chunk of Galena and
a cat-whisker -- they made a diode.So do 2 pieces of metal with a little
corrosion between them.

All it takes is a diode and a bunch of RF, and you get not just the
original RF signals, but mixtures of the signals....

Not there's no battery in a crystal set.

Now, given the inefficiencies in the process, it usually takes a
good antenna...or LOTS of RF and an ungood antenna..and you clearly
have the 2nd case.

--
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

[Mark Mallory]

Sydney,

Here are a few ideas concerning your interference problem (worth at least as
much as they cost... :)

*** You mentioned 127.0 might have the strongest interference. This could be
the result of mixing between the TV channel 2 visual carrier (55.25 MHz) and the
channel 4 aural carrier (71.75 MHz); alternatively, the ch. 2 aural carrier
(59.75 MHz) could be mixing with the ch. 4 visual carrier (67.25 MHz) to produce
the same result.

I looked at all the high-power broadcast services (VHF-TV & FM, as listed in the
FCC databases) within 10 km of the location you gave; trying all the 2nd & 3rd
order intermod possibilities I could think of, the above result seems the most
likely.


*** FCC databases:

FM: http://www.fcc.gov/mb/audio/fmq.html
TV: http://www.fcc.gov/mb/video/tvq.html

You can scroll to the bottom of these and enter a lat-long and radius.


*** According to the TV database, channel 2's transmitter (100 kW) is the
closest to the location you gave. If the intermod is being produced EXTERNAL to
your airplane, IMO this is the most likely source; a ch. 4 signal (from a
different, but nearby, antenna) is received by ch. 2's transmitting antenna,
travels back into the high-power output stage where it mixes with the ch. 2
signal, and the resultant 2nd-order intermod product then radiated from ch. 2's
antenna.


*** You mentioned in another post that on *one* radio, the controller becomes
"very faint" under the intermod, while on the other radio and the handheld the
controller is still heard nearly normally. This may be a clue that the *one*
radio may be the SOURCE; the intermod is perhaps being generated in it's input
circuit (wiping out ATC on that radio) and also re-radiated from it's COM
antenna into the other radios (but not as strong in the others due to the
attenuation [12-18 dB or so] between the COM antennas.)

As others have mentioned, it's possible for the radios to generate intermod even
when powered off (the "crystal radio" principle is applicable here.) The
easiest way to check this would be to simply disconnect the antenna from the
*suspect* radio(s), ideally at the antenna end (IMO, no need to bother
"terminating" the open connectors) and fly out to the Antenna Farm and note the
result.


*** Take your handheld in your car and drive around near the Farm, note the
results. If no intermod, the problem's most likely in the plane; else, the
problem could *still* be in the plane (and the handheld), but *could* also be
the fault of one of the TV station.

If you suspect a TV station (Channel 2 most likely IMO) is at fault, or even if
you don't, I would suggest getting in contact with the station's Engineering
Department and (diplomatically) explaining the situation. They're technically
qualified folks, are well-equipped with test equipment (spectrum analyzers,
ect), and will likely be VERY interested in finding and correcting any such
problems in their transmitting plant. If there's an RF emissions problem they
aren't aware of (that could possibly result in the station being fined), they'd
be most appreciative of it being brought to their attention.


*** Hope this helps. Have fun...

Mark


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"
>

[Jim Weir]

Nope. They are all over the map. I chose a random book from the library...it
happens to be the King KX-170B. The high COM IF is 9.0M and the low COM IF is
861.25k. The high NAV IF is 15.1875M and the low NAV IF is 1.1857M. I could
pick half a dozen books from the shelf and no two would be the same.

Jim


"Paul Sengupta" >
shared these priceless pearls of wisdom:

->Ah, ok, thanks for that! What do they use? Is it standard?
->
->Paul
->
->"Jim Weir" > wrote in message
...
->> You can stop playing with 10.7 as a source of the problem. I cannot
->recall an
->> aircraft navcom using 10.7 as the IF frequency.
->

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]

Blimey, thanks. Any idea how they decide to use these figures? Just out of
curiosity.

Paul

"Jim Weir" > wrote in message
...
> Nope. They are all over the map. I chose a random book from the
library...it
> happens to be the King KX-170B. The high COM IF is 9.0M and the low COM
IF is
> 861.25k. The high NAV IF is 15.1875M and the low NAV IF is 1.1857M. I
could
> pick half a dozen books from the shelf and no two would be the same.

[Jim Weir]

Back in the old days we did it by trial and error and glommed onto what worked
best. When the digital computer came along, we got a full blown printout of the
inter/cross/spuri mod products across a frequency band and picked a
frequency(ies) for the IF that produced minimum spurious products.

Note the word MINIMUM. There has never been a receiver produced by the
superheterodyne process that is totally free of spurious, including the
magnificent Collins S-line or 51J series of receivers.

Jim



"Paul Sengupta" >
shared these priceless pearls of wisdom:

->Blimey, thanks. Any idea how they decide to use these figures? Just out of
->curiosity.
->
->Paul
->
->"Jim Weir" > wrote in message
...
->> Nope. They are all over the map. I chose a random book from the
->library...it
->> happens to be the King KX-170B. The high COM IF is 9.0M and the low COM
->IF is
->> 861.25k. The high NAV IF is 15.1875M and the low NAV IF is 1.1857M. I
->could
->> pick half a dozen books from the shelf and no two would be the same.
->

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]

On Fri, 07 Nov 2003 12:39:37 -0800, Jim Weir > wrote:

>Back in the old days we did it by trial and error and glommed onto what worked
>best. When the digital computer came along, we got a full blown printout of the
>inter/cross/spuri mod products across a frequency band and picked a
>frequency(ies) for the IF that produced minimum spurious products.
>
>Note the word MINIMUM. There has never been a receiver produced by the
>superheterodyne process that is totally free of spurious, including the
>magnificent Collins S-line or 51J series of receivers.

And they were tube type equipment which I think did far better at
rejecting intermod than transistors. OTOH todays FETs are pretty good.

Of course a miser is a mixer is a mixer ... which was designed to mix
the signals. It takes sojme careful design to prevent unwanted signals
from getting into (and out of) the mixer.

As you say, none of them are perfect.

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

[Gene Seibel]

I operate out of St Louis Regional and have noise on 126.00 on a KX170B.
--
Gene Seibel
Hangar 131 - http://www.pad39a.com/gene/plane.html
Because I fly, I envy no one.


> > 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.
>
> We didn't have this problem before last spring.
>
> Any assistance sorting this out would be greatly appreciated;
> we do have a pretty good local avionics guy but he frankly
> seems stumped (at least he's honest and good enough not to
> simply suggest replacing all the radios in the stack, which
> one shop did).
>
> Cheers,
> Sydney

[Montblack]

("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

[Montblack]

("Paul Sengupta" wrote)
> Some people have reported picking up radio on their
> fillings in their teeth.

http://www.tvtome.com/tvtome/servlet/EpisodeGuideSummary/showid-599/season-2

Second season (In Color)

46. Hi-Fi Gilligan

Gilligan's mouth becomes a radio after he is accidentally hit on the head.
When their regular radio is broken, Gilligan becomes their sole source of
information on the approaching typhoon.

b: 25-Nov-1965 w: Mary C. McCall, Jr. d: Jack Arnold


--
Montblack

[Snowbird]

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

[Mark Mallory]

Snowbird wrote:

> *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.


This makes sense! ...a possible intermod scenario (see my other post) is the
ch.4 aural carrier (71.75) mixing with the ch.2 visual carrier (55.25) resulting
in a product at 127.0 MHz. If the mixing were occuring in ch.2's visual xmtr,
IT would be the source of the 127.0 signal (even though the audio you hear is
coming from ch.4's xmtr, some distance away.)


>>->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 could mean:

The antenna farm is clean; the handheld and your plane *both* have a problem.

or,

The antenna farm has a problem; the handheld *may* be clean and your plane *may*
be clean.

Since you don't have enough information to distinguish between the above cases,
you can't rule your plane IN or OUT. On the other hand, if you HADN'T gotten
the interference on the ground with the handheld, you could probably rule the
plane IN (but still not conclusively if the problem is intermittent.)

Isn't troubleshooting fun? ;^)

> 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?


You mentioned the problem was considerably worse on *one* com than the other...
if the problem IS in the plane, that's my guess as to the source (see my other
post). Can you disconnect the antenna from *that* com, and fly with just the
other one?

[Snowbird]

Mark Mallory > wrote in message >...

> >>->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 could mean:
> The antenna farm is clean; the handheld and your plane *both* have a problem.

Or, the handheld *is* the problem, since it's normally in the plane?
(but normally disconnected from its antenna)

> The antenna farm has a problem; the handheld *may* be clean and
> your plane *may* be clean.

> Since you don't have enough information to distinguish between the above >cases,
> you can't rule your plane IN or OUT. On the other hand, if you HADN'T gotten
> the interference on the ground with the handheld, you could probably rule the
> plane IN (but still not conclusively if the problem is intermittent.)

> Isn't troubleshooting fun? ;^)

Just Ducky (pun intended).

So what do you suggest to distinguish?

My ideas are:
1) return to antenna farm w/ handheld and try to stay long enough
to get a station ID, also confirm lat long coordinates
if interference received:
2) fly w/out handheld in the plane
if interference received:
3) remove #2 nav com and ELT from plane
fly again
if no interference:
4) put one back

I called the FSDO last week and got the name of their frequency
guy, talked to him. If the problem reproduces today and I have
the street address and lat-long of the antenna, I'll call him back
with an update since "go right here and see what you find" is
an easier proposition than "there's a problem somewhere in this
vicinity"

I'm also thinking it might be worth a call to the TV station,
to see if they have someone who cares because maybe they have
better equipment and could check this out. (or maybe I can get
the FAA guy to call...)

> You mentioned the problem was considerably worse on *one* com than the > other...
> if the problem IS in the plane, that's my guess as to the source (see my other
> post). Can you disconnect the antenna from *that* com, and fly with just the
> other one?

No problem, but it would be easier (as well as owner-allowed maintenance)
to just remove the radio -- if that would suffice? Or are you thinking
it's the antenna/cable?

IF it's the radio -- unfortunately it's a TKM. Are there bench
checks our local avionics guy should be able to run to see if it's
up to spec, before we ship it out to them?

Thanks!
Sydney

[Gene Seibel]

Channel two visual is 55.25 mHz and aural is 59.75 mHz. Their digital
channel is transmitted on channel 43 with a pilot frequency of 644.31
mHz. Digital audio and video are encoded into one data stream.
--
Gene Seibel
http://www.pad39a.com/gene/plane.html
http://www.pad39a.com/gene/broadcast.html



>
> 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

[Mark Mallory]

Snowbird wrote:

> Mark Mallory > wrote in message >...
>>Isn't troubleshooting fun? ;^)
>
> Just Ducky (pun intended).
>
> So what do you suggest to distinguish?
>
> My ideas are:
> 1) return to antenna farm w/ handheld and try to stay long enough
> to get a station ID, also confirm lat long coordinates
> if interference received:

> 2) fly w/out handheld in the plane
> if interference received:
> 3) remove #2 nav com and ELT from plane
> fly again
> if no interference:
> 4) put one back


Sounds like a good plan.

I would try disconnecting *all* the antennas on the plane except for the *good*
com's (and the transponder's, which you need under the clsB and which IMO is
unlikely to be the problem.) Note the results on the good com, then hook them
back up one by one...


> I'm also thinking it might be worth a call to the TV station,
> to see if they have someone who cares because maybe they have
> better equipment and could check this out. (or maybe I can get
> the FAA guy to call...)


Yes, I mentioned this in my first post... get in contact with their Engineering
Department and explain the problem. They'd be *more* than interested, they're
technically qualified guys, and they have the necesssary equipment.

As a ham radio guy and former Broadcast Engineer myself (1970s), I know I'd be
chomping at the bit to track it down!


>>You mentioned the problem was considerably worse on *one* com than the > other...
>>if the problem IS in the plane, that's my guess as to the source (see my other
>>post). Can you disconnect the antenna from *that* com, and fly with just the
>>other one?
>>
>
> No problem, but it would be easier (as well as owner-allowed maintenance)
> to just remove the radio -- if that would suffice? Or are you thinking
> it's the antenna/cable?


Unless the problem is a corroded connector or such, probably not. I like the
idea of disconnecting the BNC at the antenna because it eliminates as much as
posible.




> IF it's the radio -- unfortunately it's a TKM. Are there bench
> checks our local avionics guy should be able to run to see if it's
> up to spec, before we ship it out to them?


Possibly... if they're clever and have the equipment, they might be able to rig
up two signal generators with a 3db hybrid combiner to simulate the two suspect
interfering signals.

Another idea that doesn't require any test equipment is as follows: go flying
and check the frequency 119.5 (also 119.475 & 119.525) on all your radios, and
see if you receive distorted TV audio. This is the second harmonic of ch. 2's
aural carrier, and if something on the plane is generating second-order intermod
(at 127.0), the same mechanism should generate the second harmonic as well.
(Think of this as the sound carrier mixing with *itself*) It's likely that ch.2
has gone to pains to ensure their radiated 2nd harmonic is quite clean... so you
DON'T hear it, it *might* mean that the problem is outside the airplane (or the
interference is caused by some different mechanism.)

If you drive back to the farm, you can check these frequencies with your
handheld as well.


> Thanks!
> Sydney


Good luck! Keep us posted...

[Snowbird]

Mark Mallory > wrote in message >...

> I would try disconnecting *all* the antennas on the plane except
> for the *good* com's (and the transponder's)

Well, but.. which is the good com? They both get the interference..

If you get the impression I'm not too thrilled about disconnecting
all the antennae in the plane, you got that right. Removing
avionics and leaving them on the ground is easy, for the most part
(and legal for me to do AFAIK).

Disconnecting some of our antennae is a cast-iron b**** and
reconnecting them is worse. Notably, the VOR/glideslope antennae.
Which brings me to ask...what does d/c'ing antennae get me that
removing their avionics doesn't?

I'm really hoping to hear from a couple of other folks on this
one, notably Jim Weir since I took his advice, drove to the antenna
farm, and found interference w/ my handheld.

What do you suggest next, Jim?

> Yes, I mentioned this in my first post... get in contact with their
> Engineering Department and explain the problem. They'd be *more*
> than interested, they're technically qualified guys, and they have
> the necesssary equipment.

Well, I did. I can't say they were "more than interested" in fact,
they were initially interested in telling me my equipment was at
fault because "we just had our equipment checked last Tuesday and
we're absolutely clean".

But I'm hopeful I eventually persuaded the guy I talked to
that if I could drive up to the foot of other antenna and just get
some strong RF noise on the aviation band, but drive up to the
foot of his tower stick the ducky out the window and listen to
a broadcast 5x5, maybe this bore investigation. At least he said
he'd call me back in a couple days and gave me his direct number.

We'll see...

Cheers,
Sydney

[Jim Weir]

Pull the darned radios. If the interference goes away, you at least know it was
ONE of the radios that did it. If you have dual coms, pull one, go fly, then
pull the other one, put the first one in, and go fly.

Jim




(Snowbird)
shared these priceless pearls of wisdom:

->I'm really hoping to hear from a couple of other folks on this
->one, notably Jim Weir since I took his advice, drove to the antenna
->farm, and found interference w/ my handheld.
->
->What do you suggest next, Jim?
Jim Weir (A&P/IA, CFI, & other good alphabet soup)
VP Eng RST Pres. Cyberchapter EAA Tech. Counselor
http://www.rst-engr.com

[Stu Gotts]

On Mon, 10 Nov 2003 16:35:28 -0800, Jim Weir > wrote:

>Pull the darned radios. If the interference goes away, you at least know it was
>ONE of the radios that did it. If you have dual coms, pull one, go fly, then
>pull the other one, put the first one in, and go fly.
>
>Jim
>
And if they're the same radios, switch trays, too.
>
>
>
(Snowbird)
>shared these priceless pearls of wisdom:
>
>->I'm really hoping to hear from a couple of other folks on this
>->one, notably Jim Weir since I took his advice, drove to the antenna
>->farm, and found interference w/ my handheld.
>->
>->What do you suggest next, Jim?
>Jim Weir (A&P/IA, CFI, & other good alphabet soup)
>VP Eng RST Pres. Cyberchapter EAA Tech. Counselor
>http://www.rst-engr.com

[Peter Dohm]

Roger Halstead wrote:
>
> On Fri, 07 Nov 2003 12:39:37 -0800, Jim Weir > wrote:
>
> >Back in the old days we did it by trial and error and glommed onto what worked
> >best. When the digital computer came along, we got a full blown printout of the
> >inter/cross/spuri mod products across a frequency band and picked a
> >frequency(ies) for the IF that produced minimum spurious products.
> >
> >Note the word MINIMUM. There has never been a receiver produced by the
> >superheterodyne process that is totally free of spurious, including the
> >magnificent Collins S-line or 51J series of receivers.
>
> And they were tube type equipment which I think did far better at
> rejecting intermod than transistors. OTOH todays FETs are pretty good.
>
> Of course a miser is a mixer is a mixer ... which was designed to mix
> the signals. It takes sojme careful design to prevent unwanted signals
> from getting into (and out of) the mixer.
>
> As you say, none of them are perfect.
>
> Roger Halstead (K8RI EN73 & ARRL Life Member)
> www.rogerhalstead.com
> N833R World's oldest Debonair? (S# CD-2)
>
>

I am sorry that I missed the beginning of the thread.

The King KX170 series is an excellent, if old, radio; and should be able to
regect the FM and VHF-TV band signals from as near as a couple of miles.

Nonetheless, your problem sounds like "front end overload" which is made
worse if the receiver has become slightly detuned over time. Therefore,
even though I usually assert that at least 80% of radio problems are really
antennas and other airframe wiring, I really thing that you will end up
sending the KX170 to the shop in order to solve this problem.

Peter

[[email protected]]

On 10-Nov-2003, (Snowbird) wrote:

> Well, but.. which is the good com? They both get the interference..
>
> If you get the impression I'm not too thrilled about disconnecting
> all the antennae in the plane, you got that right. Removing
> avionics and leaving them on the ground is easy, for the most part
> (and legal for me to do AFAIK).
>
> Disconnecting some of our antennae is a cast-iron b**** and
> reconnecting them is worse. Notably, the VOR/glideslope antennae.
> Which brings me to ask...what does d/c'ing antennae get me that
> removing their avionics doesn't?
>
> I'm really hoping to hear from a couple of other folks on this
> one, notably Jim Weir since I took his advice, drove to the antenna
> farm, and found interference w/ my handheld.
>
> What do you suggest next, Jim?


Sydney,

What your drive to the antenna farm (with the handheld) "proved" is that the
problem is still most likely garden variety intermodulation interference.

Sure, go ahead and pull your radios one at a time as Jim Weir suggests. My
guess is that you will still have the problem on the remaining radio(s).
This is because each radio is generating its own intermodulation products.
But PLEASE don't start yanking antennas. As you note, they are a b***h to
deal with, and it is EXTREMELY unlikely that they are the source of the
problem based upon the symptoms you describe.

Oh, by the way, something that I don't think has been covered in this thread
(and I admit I haven't read every post): Broadcast signals are often much
stronger in some directions (from the broadcast antenna) than others. So,
if you are doing repeated flying tests they should be conducted in the same
location, not just the same distance from the antenna farm.



> Well, I did. I can't say they were "more than interested" in fact,
> they were initially interested in telling me my equipment was at
> fault because "we just had our equipment checked last Tuesday and
> we're absolutely clean".
>
> But I'm hopeful I eventually persuaded the guy I talked to
> that if I could drive up to the foot of other antenna and just get
> some strong RF noise on the aviation band, but drive up to the
> foot of his tower stick the ducky out the window and listen to
> a broadcast 5x5, maybe this bore investigation. At least he said
> he'd call me back in a couple days and gave me his direct number.

I would be VERY surprised if the interfering signals are being generated at
the antenna farm. Of course, as they say, "nothing is impossible."
--
-Elliott Drucker

[Paul Sengupta]

How are we doing on this now by the way?

Paul

> wrote in message
...
> What your drive to the antenna farm (with the handheld) "proved" is that
the
> problem is still most likely garden variety intermodulation interference.

[Snowbird]

"Paul Sengupta" > wrote in message >...
> How are we doing on this now by the way?

Thanks for asking!

Well, DH set out on an "interference hunt" primed to
selectively remove this and that from the plane -- and
couldn't find any interference to remove. Haven't
run into any since.

So troubleshooting is on hold until the trouble shows
up again -- the reason intermittant problems are such
a bear.

Sydney

[Paul Sengupta]

Ah. Maybe they've cleaned up the transmitter? Too much to
hope I guess...

Paul

"Snowbird" > wrote in message
om...
> Well, DH set out on an "interference hunt" primed to
> selectively remove this and that from the plane -- and
> couldn't find any interference to remove. Haven't
> run into any since.