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Re: [RFI] Motherboard RFI

To: "rfi@contesting.com" <rfi@contesting.com>
Subject: Re: [RFI] Motherboard RFI
From: "Jim Brown" <jim@audiosystemsgroup.com>
Date: Sun, 03 Jan 2010 17:00:26 -0800
List-post: <rfi@contesting.com">mailto:rfi@contesting.com>
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.

Yes. http://audiosystemsgroup.com/RFI-Ham.pdf

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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