Owen Duffy wrote:
The second ROT relates to the antenna impedance, wherever that is
(perhaps it assumed feedpoint adjacent to the choke), and again relates
to the differential mode Z (or some transformation) whereas the choke
acts on common mode current.

The second ROT is of course for cases where the choke is
installed at the antenna feedpoint. The main function of
such a choke is to choke off *conducted* common-mode current.
The effect of the choke on inducted common-mode current is
usually unknown unless measured.

Take for example the use of a common mode choke at the junction between
open wire line and coax in a G5RV, its behaviour seems to me to be quite
unrelated to the differential mode Z at that point, or at the dipole
centre, or anywhere else.

Again, the G5RV choke is trying to prevent *conducted* common-
mode currents and only has a chance of success on certain
bands where the differential impedance is relatively low at
the junction point. The effect of the choke on inducted common-
mode current is hard to quantify without measurements.
--
73, Cecil http://www.w5dxp.com