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radio interference?

 
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sarg314(at)gmail.com
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PostPosted: Mon Oct 29, 2012 12:11 pm    Post subject: radio interference? Reply with quote
At Ryan field yesterday a fellow I know who has done an incredible job building a perfect Hatz bi-plane asked me for help with a very odd radio problem. He has about 10 flights on it so far.

His brand new ICOM radio produces only very soft, badly garbled reception when his engine (Lyc. O-290) is running.  He can't talk to the tower when he's on-airport without stopping the engine.  The really odd part is that the same is true if he uses a hand-held radio.  (The ICOM has been benched checked by an official shop and looks fine.)

His belly mounted antenna was obviously poor since it had a 90 deg. bend it it about 2 inches after it emerges from the plane.  It has what looks like a good ground plane.  He's switching to a plain, straight whip antenna for next flight.  But I doubt it will work any better given the performance of the hand-held.  He was also going to fly it once with the magnetos shorted at the magneto (so the wires going from the mag to the switch will be disconnected, thinking they may be radiating somehow).

Engine installation looks beautiful, like the rest of the plane.  Grounding looks good, all parts are real aircraft parts, spark plugs, wires, etc.  Any ideas?
--
Tom Sargent
[quote][b]

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nuckolls.bob(at)aeroelect
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PostPosted: Tue Oct 30, 2012 6:07 am    Post subject: radio interference? Reply with quote
At 03:10 PM 10/29/2012, you wrote:
At Ryan field yesterday a fellow I know who has
done an incredible job building a perfect Hatz
bi-plane asked me for help with a very odd radio
problem. He has about 10 flights on it so far.

His brand new ICOM radio produces only very soft,
badly garbled reception when his engine (Lyc.
O-290) is running. He can't talk to the tower
when he's on-airport without stopping the
engine. The really odd part is that the same is
true if he uses a hand-held radio. (The ICOM
has been benched checked by an official shop and looks fine.)

His belly mounted antenna was obviously poor
since it had a 90 deg. bend it it about 2 inches
after it emerges from the plane. It has what
looks like a good ground plane. He's switching
to a plain, straight whip antenna for next
flight. But I doubt it will work any better
given the performance of the hand-held.

I agree. Pretzel shaped antennas may not perform
as well as the idealized configurations . . . but
gross performance is not likely to be the root
cause of a noise/interference issue in the radio.

Is the hand-held on a rubber-duck when these
experiments are tried or is it plugged into
ship's antenna?

 He was also going to fly it once with the
magnetos shorted at the magneto (so the wires
going from the mag to the switch will be
disconnected, thinking they may be radiating somehow).

Good experiment . . . but the flavor of noise
emitted by magneto p-lead circuits is of the
short duration, repetitive 'spark plug' variety.
The phenomenon you describe sounds more like the
effects of a smoother, more contiguous stress.
Noise? Are these radios being tested with a common
mic/head-set? If so, try another mic/head-set.

Bob . . .

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sarg314(at)gmail.com
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PostPosted: Tue Oct 30, 2012 7:09 am    Post subject: radio interference? Reply with quote
Bob:
I'll have to check for sure, but I think the hand held was on its own antenna and not connected into the plane's antenna.  Switching headsets is a good suggestion. I'll tell him about that.

I agree that his description of the noise doesn't sound like ignition noise.  It's like another radio is is squashing the signal somehow.  I can't figure how the ICOM could do that to itself and still check out on the bench, though.  Weird.

Thanks for the suggestions.
--
Tom Sargent
[quote][b]

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sarg314(at)gmail.com
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PostPosted: Mon Nov 05, 2012 9:01 am    Post subject: radio interference? Reply with quote
Bob:
The radio problem has been resolved. Experimenting with the leads going to the Magnetos revealed the issue.  When the wires were originally stripped and had terminals crimped, too much shielding was cut away.  There was about 4 inches of unshielded wire. When that stretch of unshielded wire was reduced to a minimum, the radio worked normally (also the old 90 deg. bend antenna was replaced with a straight one). 

This explanation surprised me because the description of the problem didn't sound like ignition noise.  The only thing I can think of is that the modern (probably digital) squelch circuitry in the ICOM radio was interacting with this interference to make it sound different than a simple, analog squelch circuit would.  I'm guessing these com radios nowadays are mostly digital inside and you end up hearing what the digital signal processor computer thinks is there.  It can confuse us old geezers who are used to simple analog stuff.

Anyway, thanks for your suggestions (he had tried different headsets, BTW).

On Tue, Oct 30, 2012 at 8:06 AM, thomas sargent <sarg314(at)gmail.com (sarg314(at)gmail.com)> wrote:
Quote:
Bob:
I'll have to check for sure, but I think the hand held was on its own antenna and not connected into the plane's antenna.  Switching headsets is a good suggestion. I'll tell him about that.

I agree that his description of the noise doesn't sound like ignition noise.  It's like another radio is is squashing the signal somehow.  I can't figure how the ICOM could do that to itself and still check out on the bench, though.  Weird.

Thanks for the suggestions.
--
Tom Sargent

--
Tom Sargent
[quote][b]

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