[RFI] Ferrite Beads

Jim Smith jimsmith at shaw.ca
Wed Oct 20 19:58:40 EDT 2004

I've never split a toroid coil in half so maybe what I'm going to say 
isn't an issue.

I think that if you end up with any air gap at all between the toroid 
faces that the inductance will drop dramatically due to the large 
reluctance of the air gap in series with the flux in the toroid.  
(Reluctance is to magnetic flux as resistance is to electric current.  
Kirchoff's Laws can be applied to magnetic circuits as well as electric 

I have heard, but haven't tried it, that if you touch the highly 
polished face of a split core you will deposit oil from your skin on it 
which will cause a small air gap with consequent reduction in performance.

I have a split core which uses the typical snap-together plastic holder 
to hold it together.  I connected my Autek RF-1 to a 1 turn loop through 
it which displayed (a now forgotten) impedance value.  I then squeezed 
the core hard with my fingers to increase the pressure on the mating 
surfaces.  The impedance went up something like 20%.

The moral of the story is that you have to watch that air gap.  When you 
split your toroids, if a bunch of the material crumbles away you will 
have an air gap.

Still, if it works for you, who cares about air gaps?

I think I would use nylon ty-wraps cinched as tight as you can get them 
to hold the pieces together.  I don't think tape will work at all - too 

Hope this helps (or, at least, doesn't get in the way).

73 de Jim Smith   VE7FO

Ford Peterson wrote:

>  I would prefer to use a toroid and need to select an appropriate mix.  I will split them by hand and slip them over the wires, using tape to secure them.

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