[RFI] impedance mismatching in coax

[Jim Brown]

Some thoughts to help understand the physics behind Frank's 
observations. First, ferrites are a semiconductor, and act as a 
dielectric, so causes there to be capacitance between windings, even 
when the windings are widely separated on the ferrite core (which is the 
configuration that provides the least capacitance between windings). 
When the two windings are bifilar, as is often done, the capacitance is 
MUCH greater.

I've done neither measurements nor calculations to study the 
difference.  But since the mu of the ferrite cores we use for 
transformers are much much greater than 1, virtually all the flux from 
one winding on a toroid couples to the other, so I've always viewed the 
practice of bifilar windings to be misguided simply because it does 
greatly increase the interwinding capacitance.

I'd love to hear from anyone who might have done experimental work to 
put numbers to it.

73, Jim K9YC

On Wed,12/28/2016 12:58 PM, donovanf at starpower.net wrote:
> If you measure the shield isolation of a 1:1 transformer you'll
> discover -- contrary to popular belief -- that it provides only a
> modest amount of common mode rejection. K9YC's common
> mode chokes are vastly superior.
>