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Re: [TowerTalk] Nasty RF Problem. Advice Please

To: "" <>
Subject: Re: [TowerTalk] Nasty RF Problem. Advice Please
From: "Jim Brown" <>
Date: Sun, 10 Sep 2006 09:17:22 -0700
List-post: <>
On Sun, 10 Sep 2006 10:26:02 -0500, CARY GOOCH wrote:

>The other has an integrated antenna, 2 mile shot and taking so much
>interference from a 3KW FM Stereo antenna 40 feet higher up the tower
>that I can not even log into it (power over Ethernet, shielded cable
>with drain wire.

You're on the wrong list for this kind of question. Check out the RFI 
list, also on

BUT: Since I'm an RFI guy and have wrestled with this kind of problem, 
here are several thoughts. First, that cable shielding does nothing good 
and may CAUSE problems if it is not bonded to the right point, which 
must be the shielding enclosure of the equipment at each end. With most 
Ethernet equipment, it can be next to impossible to find the shielding 
enclosure, especially via a path that is short at the frequencies of 
interfering signals. See the tutorials on my website about "the pin 1 
problem," and think of it in the context of your equipment. 

Second, most Ethernet gear I've run into couples RF common mode on the 
Ethernet cables -- that is, it both sends and receives RF lengthwise on 
the Ethernet cable acting as a long wire antenna. The solution to that 
mode of coupling is a suitable ferrite choke. There are HUGE differences 
in ferrite chokes, and you may need multiple ferrite parts to get 
sufficient choking impedance. See the applications note on 
"publications" section of my website. 

Third, remember that Ethernet is 2-way, so both ends of the cable 
transmit and both receive. This means that both ends are susceptible, so 
both ends may need chokes if the cable is longer than about 1/10 

Fourth, remember that trash can also couple via power supply leads. In 
your case, that would be the power supply leads at the equipment that 
communicates with your POE equipment. 

Fifth, RF can couple via inadequate shielding of equipment -- that is, 
the internal wiring acts as antennas. Equipment may APPEAR to be 
shielded, but there may be gaps in the bonds between pieces of the 
enclosure (for example, paint can get in the way, and there may be slots 
big enough to let the RF in).

Jim Brown K9YC


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