[amsat-bb] Re: Ferrite RFI filters and Traps? (yes!)

Robert Bruninga bruninga at usna.edu
Thu Apr 3 06:14:52 PDT 2014

>> I cut the RFI Ferrite blobs off of all trash Keyboards and stuff.
>> These are great Ferrite devices.
>> But what are they best for?

> Check out this article from Chuck Councilman W1HIS:
> http://www.yccc.org/Articles/W1HIS/CommonModeChokesW1HIS2006Apr06.pdf

Wow, a long one, but on page 24 of 42 I learned a great technique.  He
says, simply put a turn or two on any ferrite core and hook it to an
antenna analyzer.  Then look at the impedance over frequency.  At DC (or
your lowest frequency) the impedance should be very low (and 90 degrees)
Meaning the R is very low and the +jX is very high.  Keep going up in
frequency to where the R and jX terms are equal.  That is where the Q is
now 1 and the ferrite is starting to get quite lousy.

He also said that as you double frequency, the R part of the impedance
should also double.  When you get to the frequencies where a doubling of
frequency no longer doubles R, then you are starting to get into
frequencies where this core is less effective.

That is exactly the kind of rule-of-thumb advice I was looking for!  Now I
know what to do with all these cores!

Bob, WB4aPR

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