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

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

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

: 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)

(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

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

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?

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 a
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

Snowbird wrote:

Mark Mallory wrote in message ...
Isn't troubleshooting fun? ;^)

Just Ducky (pun intended).

So what do you suggest to distinguish?

My ideas a
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...

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

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