[RFI] Snap-on RFI suppression core size vs cable size

Jim Brown jim at audiosystemsgroup.com
Sun Aug 20 22:34:01 EDT 2017

On 8/20/2017 4:06 PM, N1BUG wrote:
> Suppose I am trying to use common mode chokes to eliminate noise on a 
> conductor at 144 MHz. Suppose the conductor diameter is 1/8 inch. This 
> is VHF so we are not talking about multiple turns - just a wire 
> passing ONCE through a ferrite core.

One turn will work fine. Although I haven't done any measurements to 
confirm it, I wouldn't worry about the loose fit.  Check out this data 
sheet  and note the plot of Z for 1-3 turns at the bottom of the page. 
For a 6M choke, I'd use two turns on a core, placing multiple 2-turn 
chokes in series along the cable.  That's NOT two turns through multiple 
cores in series, but two turns through core #1, then a short length (an 
inch or less) of coax and another two-turn choke, and another.


73, Jim

> A Fair-Rite 0431178181 (.161" ID) would be the best fit for this 
> cable. According to the data sheet its Rs is about 180 at 144 MHz.
> The Rs of a Fair-Rite 0431164281 (.250" ID) is about 330 at 144 MHz 
> but it fits the wire in question very loosely with a larger air gap. 
> How much, if at all, will this "loose fit" reduce the effective Rs?
> Is the answer "it depends"? I've been trying to find an answer to this 
> for years but I keep coming up empty. Maybe it's buried in the 
> engineering talk which is over my head.
> 73,
> Paul N1BUG
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