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

N1BUG paul at n1bug.com
Mon Aug 21 10:37:40 EDT 2017

Thank you Dave!

This seems to confirm my gut feeling. I suspect if the ferrite were 
almost solid with just a small hole through the center (just larger 
than the wire) Z (or more importantly equivalent series resistance) 
would increase dramatically. This suggests that at VHF using a 
ferrite with an ID significantly larger than the wire passing 
through it *might* not be optimum. It may (or may not) be that using 
a smaller ferrite which more closely fits the wire is better even 
though from the data sheet its Z and Rs may be less than the larger 
part. It may (or may not) be that 2-4x ratio of wire size to ferrite 
ID is not enough to have a significant effect. My point is that 
there are some unknowns here. I have spent a great deal of time 
looking for understandable data on this and have come up with none.

I think it would be very helpful to the VHF community if someone 
with the proper equipment could look into this and publish some 
curves like those for multi-turn HF chokes in K9YC's excellent 
RFI-Ham.pdf paper. Z and Rs at VHF vs. wire diameter for a few 
common sizes of ferrite, for example. If I had a VNA and the 
expertise to use it correctly I would take on this project, but I don't.

I may be able to provide an experiment. Given the unknowns I am 
unwilling to risk buying only a large quantity of just one (large) 
size ferrite. I will buy a large quantity of two sizes that will be 
a close fit for many of the cables I need to deal with. For the time 
being I will buy a relatively small quantity of a smaller size which 
would be a close fit for my smallest cables, as these are the ones 
where I question the best choice. Assuming I find a small cable upon 
which installation of ferrites reduces my receive noise, I will 
compare noise reduction with three differnet size ferrites on that 
cable: one that is a close fit, one that is a loose fit, and one 
that is even more loose. This may at least tell me whether I am 
better off buying the smaller ferrites in quantity or going with 
more of the larger size.

Paul N1BUG

On 08/21/2017 08:10 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.

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