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

rip wrote in message .com...
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

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

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

-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

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.

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.

(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

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"

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

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.