On Sat, 2 Jan 2010 13:26:19 -0600, Wes Attaway \(N5WA\) wrote:
>Take a look at Jim Brown's RFI tutorial to get an idea of the proper mix and
>size for the doughnut, plus other noise suppression information.
When thinking about HOW this happens (and how to fix it), it helps to realize
that the equipment design error that we audio folks call "the pin 1 problem"
puts noise on cables to that equipment as a common mode signal. They then
radiate that noise, just like any other antenna would when RF current flows.
Ferrites kill RFI by choking that current, but it's their high resistive
impedance at resonance that does the job, NOT inductance. A ferrite on a
single turn of wire has a resonance in the 150 MHz range. To make the ferrite
useful at HF, you've got to move that resonance down AND increase the
impedance. That's easy to do -- just as with any other coil, you add turns
and the capacitance between turns, and the resonance moves down. Measured
data for some commonly used ferrite parts is in the tutorial.
Jim Brown K9YC
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