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

David Robbins k1ttt at verizon.net
Mon Aug 21 08:10:27 EDT 2017

I scanned a few papers this morning that talk about things like the ferrite
window fill factor and variations of inductance with wire size... then
pulled out my vna and a ferrite to try a quick experiment.  My conclusion is
that if the ferrite is much larger in diameter than the wire going through
it the inductance will be decreased.  My quick explanation of this is that a
current through a wire generates a magnetic field around the wire that is
strongest near the wire and gets weaker the farther away you go.  To get the
most flux in the ferrite in order for it to increase the inductance the
ferrite must be close to the surface of the wire.  my simple test with the
vna used a 22ga clip lead going once through a 2" diameter ferrite, when the
wire was in the middle of the core the |Z| of the single turn coil was like
38 ohms, when it was touching the inside edge it increased to 42 ohms, so it
appears to be a real and probably not insignificant effect.

David Robbins K1TTT
e-mail: mailto:k1ttt at arrl.net
web: http://wiki.k1ttt.net
AR-Cluster node: 145.69MHz or telnet://k1ttt.net:7373

-----Original Message-----
From: RFI [mailto:rfi-bounces at contesting.com] On Behalf Of Jack Shirley N8DX
Sent: Monday, August 21, 2017 01:00
Cc: rfi at contesting.com List
Subject: Re: [RFI] Snap-on RFI suppression core size vs cable size

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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