[RFI] Wired vs Wireless Routers for RFI

Jim Brown jim at audiosystemsgroup.com
Sat Jan 12 11:09:41 EST 2008

On Fri, 11 Jan 2008 10:55:40 -0500, Paul Christensen wrote:

>I just purchased a box of 0.750" 
>Fair-Rite #31 core material and the result is that router birdies are 
>somewhat reduced, but still very annoying.  Half the battle with CAT-5 
>is in getting enough windings on the core without running out of room. 
>Adding cores on the PC end seems to offer little befit, so it would appear 
>that the majority of the RFI source is coming right out of the router.

>Any suggestions from here?  

First, you MUST use enough turns before you begin seeing much benefit from 
these chokes. The particular cores you purchased may be too small for this 
use. I recommend the 2.4-inch o.d. 1.4-inch i.d. x 1/2-inch toroids (renamed 
FT-240 by vendors who mark them up by 5X to hams) and the "big clamp-on" 
listed in my tutorial (1-inch i.d. by a bit more than 1 inch long). These 
parts allow enough turns for good choking action at 10 MHz -30 MHz both with 
coax and with Ethernet cables. 

Second, with the cores you have on hand, try using two or three cores 
combined for a single multi-turn choke. The objective is to lower the 
resonant frequency of the choke to the range of birdies you're trying to 
suppress. See the guidelines in the tutorial for estimating the reduction in 
resonant frequency for 2-4 turns based on the Fair-Rite data for a single 

Third, don't forget what I call "threshold effect." (see the tutorial).

Fourth, once you've killed radiation from the cables, there may still be 
radiation from the router itself. 

Fifth, once you've killed radiation from the router end, you may start 
hearing the lower level trash from the other end. 

Sixth, once you've killed your own stuff, you may hear your neighbors. 



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