Lowering cockpit RF interference

Been a long time since EE school, but, IIRC, "switching on and off" creates
a LOT of RF noise.


"Martin Gregorie" wrote in message
...
On Sun, 16 Dec 2012 11:02:57 -0800, bumper wrote:


When you say you soldered it "across" the power switch, that might lead
one to assume you soldered it to the two terminals of your switch, i.e.
in parallel with the switch contacts). Normally a filter capacitor would
be connected across the power leads, i.e. from + to -. While a filter
inductor (such as a ferrite) is connected in series.

The switch is a double pole push-on push-off type, so one pole each for
ground and 12v I was able to use it as a two position tag strip, which
made wiring easier. The capacitor is across the +12 and ground connectors
on the T&B side of the switch.

When trouble shooting RFI, it helps to be able to quantify results. If
it's interferance on comm freqencies, a hand held radio can be useful -
squelch off, and distance away from suspected RFI source as appropriate
one can use the handheld as a field strength meter for radiated
interferance.

It was purely electrical noise that cut in when the T&B was nearly up to
speed. I don't think it was RF because:

(a)ferrites didn't have any effect

(b) if it was coming from the DC-DC 12-28v solid state adapter I'd have
expected to hear it loudest when the T&B started because thats when you
see the biggest current drain from an electric motor.

(c) an old, mechanical T&B shouldn't be an RF source unless its sparking
like hell. This is a Mil-spec R C Allen type MD-4A, so possibly ex-USAF,
that has been modified to suit a glider's rate of turn rather than a
power plane. I'm told that these devices have a centrifugal switch that
sets the RPM and has the side effect of switching in and out at several
tens of cycles a second when its up to speed. That is what the racket
sounded like too.

This is not so much use when dealing with closely spaced instruments on
the panel where one is causing problems with another (LNAV did that to
Comm on one of previous glider). There, just slapping in clamp-ferrites
did the job.

This was very easily localized: T&B off: no problem. T&B turned on: nasty
noise on the radio once it had spun up. No effect from turning any other
stuff on or off.

...other good stuff snipped.


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martin@ | Martin Gregorie
gregorie. | Essex, UK
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