[RFI] Ethernet RFI

Ford Peterson ford at cmgate.com
Sat Jan 22 20:41:37 EST 2005


> 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)?
> Alan NV8A


One of my pet peeves is when computer technicians wire cat5 cables incorrectly.  There is a right way and a wrong way to do it.  The right way is the most difficult.  Guess which way most people do it?  

The 8 conductors in that cable are actually 4 pairs of twisted cables.  Each pair is a "hot" and a ground return.  When you hook up the cables to keep the colors all the same on each pin at each end, you will have conductivity between the correct pins, and the network will work, but you completely lose any shielding benefits of the twisted pairs.  You should not be hearing anything from that cable.  

I do not have the specification for those cable ends handy.  But as I recall, the ground returns are the 4 pins on one end of the connector.  Before rewiring your house with EMT, I would make sure the connectors are installed correctly.  

I have one run here that an 'installation expert' installed for me.  I queried him explicitly on the correct wiring pattern.  A lively discussion ensued.  Realizing that I was arguing with an idiot with 7th grade shop class electrical wiring experience, and that I was paying him $2 a minute to argue with me, I let him wire it his way.  Not surprisingly, that particular 75' run of Cat5 never runs over 10mb in a 10/100 hub.  The hub shifts into low gear to deal with the noise.  

I'm sure short runs work just fine wired however you want.  Wired the 'easy' way, you get 2 pairs carrying all the data and two pairs acting as the ground return.  I'm sure there is some shielding afforded by lashing them into the same run of wire, but they are not acting correctly.  It takes 4 thumbs to wire them correctly.  Find the spec and wire them right.

ford at cmgate.com

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