[RFI] Ethernet RFI

Jim Brown jim at audiosystemsgroup.com
Sat Jan 22 20:31:15 EST 2005

On Sat, 22 Jan 2005 20:05:27 -0500, Alan NV8A (ex. AB2OS) wrote:

>My wife's computer is located about 20' below and almost immediately 
>under the fed end (via an Icom AH-4 autotuner) of my wire antenna. The 
>computer is connected via CAT5e cable to a Linksys 8-port switch.
>When this cable was connected, I had S7 signals at various points on 20m 
>(and chokes -- three turns through a toroid -- at both ends made little 
>difference), so I decided to try shielding the cable by running it 
>through EMT.
>I was surprised to find that grounding the EMT to the #4 Cu wire that 
>comes in from outside INcreases the level of these unwanted signals by 2 
>S-units. Any ideas why, and what I could do to get rid of this RFI 
>(until I get the tower up, at which time the feed point will be much 
>farther away from any of the computer equipment)?

See this link to a new applications note on my website about how to use ferrite 
chokes to kill stuff like this. This one is geared toward keeping RF out of audio, 
but the physics is exactly the same. I am writing a version of this geared toward 
ham radio for QST. 

The short answers to your problem are that:

1) The stuff on the cable is common-mode, so when you run the cable through the 
conduit, it capacity couples via the conduit to ground and the conduit radiates 
thanks to the current. 

The fix to this common-mode stuff is a serious choke on the cable at each end (both 
ends transmit alternately). See my app note for details of the chokes. 

2) The box itself may also radiate due to lousy (or non-existent) shielding. You 
won't fix that with chokes.  Instead, you need to give it the "bucket treatment." 
That is, fill a bucket with water, put the Linksys in twice, take it out once. The 
Linksys gear are known to be RFI dogs, but other brands are too. 

3) Don't be surprised if you kill your stuff completely and still hear stuff on 
nearly the same frequencies -- I hear several of my neighbors' systems.  

4) Go wireless as much as possible, treat the cable between your internet and the 
wireless xmtr as short as possible, and choke it too. 

Here's the link: 


Jim Brown  K9YC

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