[RFI] RFI suppression cores for 147 MHz

Christopher Brown cbrown at woods.net
Wed Feb 27 16:07:48 EST 2013

Fair-rite 0431164281 is a good fit for that size cable.

Where I was getting common mode inband on 2M I added 3 to each end of
the cable.  These are large enough they will slide around on cable, need
to be restrained, I used velcro "table" as commonly available for cable

When 3 per end not enough I added a second set of 3 ~ 19" from each.

Mouser has them for < $2 each in lots of 10.  I picked up 100 last year,
for about $130 IIRC.

I do similar on all of my VHF and up LMR400 runs with the solid
2631102002.  Slip a batch of them on when making the cable and
group/space approp to the use.

On 2/27/13 8:22 AM, Jim Brown wrote:
> 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
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