Lowering cockpit RF interference

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.

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.


"Martin Gregorie" wrote in message
...
On Sun, 16 Dec 2012 17:12:24 -0700, Dan Marotta wrote:

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

Even at those low voltages and currents?

But what does solving the problem by fitting a simple capacitor across
the supply say about the relative amounts of RV vs. electrical noise?
Bear in mind that there's around 50cm of unshielded cable between the
capacitor on the switch and the T&B - cable that was supplied with it.


--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |

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?


--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |

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?

martin@ | Martin Gregorie
gregorie. | Essex, UK
org |


Nope! Wasn't me...

On Monday, December 17, 2012 11:42:00 AM UTC-5, Dan Marotta wrote:

Either or both of these of the proper values and connected in the proper way will greatly reduce or eliminate RF interference.


Thanks Dan for for your input. Since exercise is good for the brain, inquiry minds want to know how to determine proper capacitor and RF choke values and how to connect correctly.

Frankly, I've been out of school far too long to do a decent job of
designing a filter. I've included a link on filter design below if you want
to slap a few components together. Or you could simply go to your local
electronics supplier and tell them that you've got a buzzing in your radio
caused by something switching DC and you want to insert a low pass filter or
an RF choke in the DC line.

This will help you design a lowpass filter:

http://www.radio-electronics.com/inf...ter-design.php

However, simply wrapping your +12v line through a ferrit ring will serve the
purpose.
wrote in message
...
On Monday, December 17, 2012 11:42:00 AM UTC-5, Dan Marotta wrote:

Either or both of these of the proper values and connected in the proper
way will greatly reduce or eliminate RF interference.


Thanks Dan for for your input. Since exercise is good for the brain,
inquiry minds want to know how to determine proper capacitor and RF choke
values and how to connect correctly.