Lowering cockpit RF interference

On Saturday, December 15, 2012 2:32:26 PM UTC-8, Martin Gregorie wrote:
On Sat, 15 Dec 2012 11:49:06 -0700, Dan Marotta wrote:

I think I tried a ferrite core on the 12v side of the voltage converter

without success and then remembered that Maplins (the UK equivalent of

RadioShack) had these capacitors at a reasonable price. Better yet it was

easy to fit: I just soldered it across the T&B power switch.

martin@ | Martin Gregorie

gregorie. | Essex, UK

org


A couple of thoughts here . . .

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.

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.

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.

If you have enough slack in wiring leads, and the size of the ferrite allows, you get almost double the iductance for loopiing the wire/s through the ferrite twice, (or even more for 3 times etc). Sometimes adding a capacitor in concert with the ferrite helps even more, especially when noise is coupling via power leads (ferrite or inductor in series and cap across the wires in parallel to form a classic pi filter).

Ferrites are not all the same, they have a frequency band at which they will be most effective. Same thing with the size of capacitors used in filtering apps. (do a search on inductive and capacitive reactance and resonant frequecy). In the U.S., Digi-Key is one good source for ferrites etc.

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