> Recall that common mode chokes work by adding a high resistive impedance in
> series with the common mode circuit.
Recall also that the common-mode circuit for coaxial line is on a separate
conductor from those carrying the differential-mode signal, creating a
"three-wire" system in which the common-mode currents are flowing on the
outside of the coax shield, completely independently of the differential-mode
currents flowing on the inside of the shield and the outer surface of the
center conductor. That means remedies for the common-mode signal can be
applied to the "third wire" without affecting the differential-mode signal.
For parallel-conductor transmission line, though, there is no "third wire" -
the common-mode signal flows equally on both wires of the line, along with the
differential-mode signal. Anything that affects the common-mode signal is
going to have an effect on the differential-mode signal, as well.
73, Ward N0AX
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