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

N1BUG paul at n1bug.com
Sun Aug 20 19:06:23 EDT 2017

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.

Paul N1BUG

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