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

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...

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

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

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

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

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.

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