Cecil Moore wrote in
t:

Owen Duffy wrote:
I know there is a proposition that chokes such as the W2DU balun need
only have choke impedance about 10 times the nominal Zo to be
effective.

I believe that rule of thumb applies only to matched lines,
i.e. Z0 loads. The other rule of thumb that I have heard
is 5x the antenna feedpoint impedance.

Ah Rules of Thumb (ROT) abound.

The first ROT relates to what is happening inside the coax at the choke
location (or is it somewhere else), and the choke is on the outside of
the coax.

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.

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.

Owen

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