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Re: [RFI] Computer cases and RFI

To: "RFI List" <rfi@contesting.com>
Subject: Re: [RFI] Computer cases and RFI
From: "Jim Brown" <jim@audiosystemsgroup.com>
Date: Mon, 02 Jan 2006 17:22:05 -0600
List-post: <mailto:rfi@contesting.com>
On Mon, 2 Jan 2006 17:36:24 -0500, Martin, AA6E wrote:

>When I have troubles, it's always with the external
>cabling.  (Without chokes, external wiring can be an efficient
>radiator - comparable in length to a wavelength.)

Oh -- it's the fault of the cable that the computer puts the RF 
current on the cable?  :)  

Let's be clear. ALL of that trash is coming from the electronics 
-- either the computer, the monitor, the network hubs/switches, 
etc. These electronic devices may radiate the trash directly if 
they are unshielded, and they can shove it out on the cables if 
they don't have sufficient common mode filtering (or if they have 
pin 1 problems on those cables). And the trash radiated directly 
by the wiring/circuit board of an unshielded box can vary widely 
depending on how well (or how poorly) the designer minimized the 
size of the current loops carrying RF currents, how well (or how 
poorly) that trash was filtered and suppressed internally. Henry 
Ott, Ralph Morrison, Howard Johnson, and Clayton Paul all cover 
these issues quite nicely in their books and EMC lectures. 

Those mechanisms are additive. When we stick ferrites on the 
cables, we are minimizing (or attempting to minimize) the common 
mode RF current that the electronics is trying to put on those 
cables. The trash we hear will be directly related to the 
magnitude of that current. But even if we manage to reduce that 
current by 30 dB with really effective ferrite chokes, we will 
still hear whatever the various electronic devices are radiating.

Jim Brown K9YC

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