On Mon, 17 Dec 2012 09:42:00 -0700, Dan Marotta wrote:

You're making my brain hurt thinking about these things! Switching DC
on and off amounts to a square wave which, as we all know, is made up of
an infinite series of sine waves, i.e., DC to light, and somewhere in
there is the RF spectrum.

From my days as an Air Force radio tecnhician, before becoming an
edumacated AF ossifer and pilot, my experience with receivers indicated
that just a couple of micro volts at the antenna input would break
squelch. Now that was from a 5 watt transmitter many, many miles away
(line of sight); and your source is merely inches away. A capacitor of
the correct value acts as a short circuit to RF at the tuned frequency
whereas an inductor (RF choke) acts like a high resistance (at the tuned
frequency). Either or both of these of the proper values and connected
in the proper way will greatly reduce or eliminate RF interference.

Thanks for the clear explanation. Much appreciated.

In my case, with the turn indicator causing the interference, I rummaged
through an electronics parts bin (actually a jelly jar) and found a
ready-made RF choke which consisted of a torroid with two wraps of wire
giving 4 loose ends. I connected one wrap (about 4 or 5 turns) in
series with +12v and the other in series with the ground to the
instrument. Problem solved.

Good one, and another good explanation. I've not previously understood
exactly what an RF choke did.

BTW, about 10 years ago I remember meeting a fellow glider pilot, name of
Dan, at the Sierra Cup free flight bash, but didn't catch his last name.
Was that you by any chance?


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martin@ | Martin Gregorie
gregorie. | Essex, UK
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