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

Gedas w8bya at mchsi.com
Mon Aug 21 10:25:28 EDT 2017

Hi Joe. About a year ago I did just what you guys are talking about. I 
had purchased a whole bunch of assorted split bead cores of various 
sizes and shapes off Amazon. No specs with them but the price was so 
right that I picked up 40-50 of each flavor I could find.

At the time I did not have my HP-8712ES VNA so I used my AIM 4170 to 
experiment a little bit and just have some fun to see what my new cores 
looked like RF wise. I took a given core and tested it with 1-T, then 
2-T, then 3-T etc....then for fun put two of those cores in series (each 
with 1-T) to see for myself what effect the various configurations had 
on the impedance (real & imaginary) over frequency. Then tried 3 cores 
in series etc, etc.

It was interesting to see how superior a single core with multiple turns 
of wire worked vs. a single turn of wire through 2, 3, or even 4 of the 
same cores all placed in series with each other.

I may be able to find the plots that I saved and if you are interested I 
would be happy to share them with you. But I want to point out that Jim 
(K9YC) has already done all this hard work (and a whole lot more) and 
made those results along with an excellent narration available for all 
of us via his published papers. I am sure his test methodology and 
accuracy is far superior to what I did....again I was just messing 
around wanting to get a quick idea of what happened under different 
scenarios with my generic cores. 73

Gedas, W8BYA

Gallery at http://w8bya.com
Light travels faster than sound....
This is why some people appear bright until you hear them speak.

On 8/21/2017 9:34 AM, Joe wrote:
> 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
> Sig
> The Original Rolling Ball Clock
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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
>>> _______________________________________________
>>> RFI mailing list
>>> RFI at contesting.com
>>> http://lists.contesting.com/mailman/listinfo/rfi
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