On Sep 12, 9:50*pm, Owen Duffy wrote:
Back to the Carolina Windom, a common explanation holds that there is
common mode current on the feedline between the dipole feedpoint and the
'isolator'. The notion that common mode current exists on one side of the
isolator and not on the other is an interesting one, one better explained
by advertising hyperbole than radiocommunications theory.

As you know, common-mode RF obeys the rules of the reflection model.
From an (ideal) physics standpoint, there is nothing technically wrong
with having zero common mode current between the isolator and the
source while having a common mode current maximum (loop) 1/4WL back
toward the antenna feedpoint from the isolator. (Assume an ideal
isolator with an infinite choking impedance.)

Consider the following example:

Source--------1/2WL coax1--------isolator-------1/2WL coax2----------
antenna feedpoint

The net common-mode current on each side of the (ideal lumped)
isolator must be equal to satisfy Kirchhoff. There is no technical
reason why the net standing-wave common-mode current could not be zero
on each side of the isolator where the isolator is causing a standing-
wave current node (minimum). 1/4WL back from the isolator toward the
source, there is no technical reason why the net standing-wave common-
mode current could not be zero. 1/4WL forward from the isolator toward
the antenna, there is no technical reason why the net standing-wave
common-mode current could not be at a high (maximum-loop) value. Since
it is theoretically possible, one should not dismiss it as
"advertising hyperbole" without having performed the measurements to
prove that particular statement applies to the Carolina Windom because
of poor isolator performance, not because it violates Kirchhoff's
laws.

Incidentally, this is the same conceptual error that some folks have
made when they reported measuring no phase shift in the current
through a large air-core 75m loading coil when installed on a standing-
wave antenna. Hint: Pure standing wave current has zero relative phase
shift so it obviously cannot be used to measure phase shift. EZNEC
confirms that fact.
--
73, Cecil, w5dxp.com