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@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.
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