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Re: [RFI] Linksys Router Birdies

To: "rfi@contesting.com" <rfi@contesting.com>
Subject: Re: [RFI] Linksys Router Birdies
From: "Jim Brown" <jim@audiosystemsgroup.com>
Date: Thu, 11 Mar 2010 21:04:05 -0800
List-post: <rfi@contesting.com">mailto:rfi@contesting.com>
On Thu, 11 Mar 2010 23:46:37 -0500, Jon D. Garner wrote:

>I only use Belden Cat5e sheilded. and who I recommend.

While Belden makes the best Ethernet cable by far, I have not found it to 
make any improvement in RFI. As others have noted, it's all common mode on 
the cables, and BOTH ENDS OF EVERY CABLE IS A NOISE SOURCE! Radiation is 
NOT the result of any defect in the cable, but rather inferior output 
transformers that put common mode onto that cable. 

In general -- consider the cable a long wire antenna, and put a toroidal 
choke (or a multiturn clamp-on) within 1/10 wavelength of both ends. If the 
cable is shorter than 1/4 wavelength at the highest frequency were you have 
a problem, a single choke in the middle is probably enough.  Chokes wound 
with #31 are good for a 6:1 frequency range if you set the number of turns 
to center them in that range. If you need to cover more frequency, use a 
second smaller choke at each end tuned to the higher frequency. 

EVERY cable is a potential antenna, including a cable to a cable or DSL 
modem, and the power supply for each of those boxes, so any or all of these 
cables may need a choke. What generally happens is that they all 
contribute, so if you have multiple cables, you may not hear much 
improvement until you've choked most of them. 

See http://audiosystemsgroup.com/RFI-Ham.pdf for detailed winding advice 
and part numbers for Fair-Rite parts that work best. 

[There is, however, a good reason for using Belden's cables -- faster 
Ethernet due to lower error rates. I use Belden Mediatwist, which is their 
best, but even their lower grade cables are better because the pairs are 
molded to hold their spacing more uniform.] 


Jim Brown K9YC

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