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