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

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"

"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

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" 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.

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.

"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

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

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/AsrS...tionSearch.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)

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

: 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)