On Sep 3, 7:18*am, Cecil Moore wrote:
wrote:
For a balanced transmission line, the characteristic impedance is not
expressed in differential mode terms, it IS common mode so I do
not know why you ask about differential mode characteristic impedance.

Well, there's your problem. In a properly functioning
transmission line, whether balanced or unbalanced, the
currents in the two conductors are differential, not
common-mode. The two currents are equal in magnitude
and 180 degrees out of phase and their fields tend
to cancel which minimizes radiation.

Common-mode currents only occur when the two currents
are not perfectly differential, i.e. not equal amplitudes
and/or not 180 degrees out of phase.
--
73, Cecil *http://www.w5dxp.com

Nope. The CM choke works precisely because the common mode currents
are mirror images, 180 degrees out of phase. If they weren't, the CM
choke could not act to cancel them and you would need differential
mode chokes to get rid of them. On your balanced feedline, the voltage
at any point is delta between the amplitude of line 1 and 2.