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

[Joe]

Can you now do the same test, but see how the value changes with each 
turn in the core?

IE: loop it through 1 time measure 2x etc.

Joe WB9SBD

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On 8/21/2017 7:10 AM, David Robbins wrote:
> 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
> To: N1BUG
> 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?
>
> N8DX
>
> 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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