Please all, remember,

The common-mode currents the choke is suppressing are not necessarily
related to the antenna impedance (the load) or the transmit/receive
currents flowing in the antenna, though there can and will be some
coupling between the two on an antenna (if you measure antenna Z
with/without a choke they will usually differ).

To place the choke in the best spot, you need to measure the common
mode current on the feedline and not the load impedance. If you have
common mode currents induced by an unbalanced situation in the antenna
itself, right at the antenna is probably best. Other places might be
good if you are coupling electrical noise to your feedline.
W1HIS's article was most interesting in this respect.

I would assume that because the feedline current is common-mode (the
same in both conductors), it will usually travel at the velocity factor
of a wire in air rather than the feedline velocity factor, though I do
not know this.

73,
Glenn AC7ZN

wrote:
I would assume that because the feedline current is common-mode (the
same in both conductors), it will usually travel at the velocity factor
of a wire in air rather than the feedline velocity factor, though I do
not know this.

Seems like a reasonable assumption given skin effect and the
outside braid of the coax. Common-mode current also, no doubt,
forms standing waves with current minimums and maximums 1/4WL
apart. A choke would be most effective at a standing wave
common mode current maximum point.
--
73, Cecil http://www.qsl.net/w5dxp