Roy Lewallen wrote:
So any core you put over the coax doesn't
see or interact with the common mode current or its fields at all, and
you can completely ignore it when analyzing balun action.

Roy probably meant "differential" above instead of "common".
Cores on coax definitely "interact with the common mode current
or its fields" and that is their purpose.
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73, Cecil http://www.qsl.net/w5dxp