[RFI] RFI suppression cores for 147 MHz

[Jim Brown]

On 2/27/2013 8:42 AM, N1BUG wrote:
> I could use a little guidance. I have a case involving radiation from 
> ethernet cables at 147 MHz. The common clamp-on cores seem to offer 
> about 250-300 ohms ESR. It looks like I probably need a lot of them to 
> clean this up, and there are several cables involved. Is there any 
> core which would give more suppression with 2 or 3 turns of the cable 
> through it? Thanks for any info or suggestions.

Hi Paul,

I had that issue in Chicago, and never found a solution.

Suppression works by inserting the resistance of the choke at resonance 
into the common mode circuit. The natural resonance of materials like 
#31 and #43 is around 150 MHz. To use them on the HF bands, we move that 
resonance down by winding turns, which also multiplies the impedance by 
the square of the number of turns. But when the resonance is already in 
the right place, our only solution is more cores. The good news is that 
#43 cores small enough to fit over Ethernet cables cost a lot less than 
the big ones we need for HF. #43 material is less expensive than #31, 
and is slightly better than #31 at VHF.

One thing you might try is using shielded CAT5/6/7 cable.  One thing I 
tried that did not help at all was to use Belden's premium Mediatwist 
cable. It's excellent cable, and it did improve error rates on my 
network, but not RFI.

Another approach, of course, is to use WiFi for as much as practical of 
the network.

73, Jim K9YC