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

Jack Shirley N8DX jcshirle at gmail.com
Sun Aug 20 20:59:35 EDT 2017

I guess it would be too practical to take a VNA and measure Z with
different IDs as you mention?


On Sun, Aug 20, 2017 at 7:06 PM, N1BUG <paul at n1bug.com> wrote:

> I have re-read both RFI-Ham.pdf and KillingReceiveNoise.pdf twice each
> over the last couple of days. Great work which I refer to frequently, but
> neither really answers my question.
> 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.
> 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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