On 9/13/2010 2:36 PM, Owen Duffy wrote:

My modelling experience is that other things like connection to
ground, and open ends to conductors have more influence on the
location of a standing wave pattern than typical common mode chokes.

I have created a simple model of a Carolina Windom at 7MHz, assuming
that the device at the dipole feedpoint is a 4:1 voltage balun with
negligible common mode impedance, the isolater is 1000+j0 (your
nomination), and a feedline configuration that demonstrates that the
isolator has not caused a minimum in the common mode standing wave
pattern at that point.

A pic of the current distribution is at
http://www.vk1od.net/lost/Clip043.png .
...

Common mode chokes can be made pretty easily to have an impedance of
more than 1k ohm. Both modeling and measurement show this is usually
adequate in typical installations to drop common mode current to very
near zero at the choke location. But you can easily have substantial
current a quarter wavelength on either side of it.

Didn't work for this case, the current minimum is about half wave
between the isolator (left hand blue square) and the dipole, and the
common current entering the shack (right hand blue square) is quite
large.

Apologies, there was an error in the model... I hadn't installed the
source properly. I have replaced the pic at
http://www.vk1od.net/lost/Clip043.png . The situation is a little
different, but the isolator does not force a current minimim at its
location, and the common mode current flowing at the shack is large.

Owen

A "4:1 voltage balun with negligible common mode impedance" isn't a
common mode choke, and can't be expected to reduce the common mode
current at its location. If that's what the Carolina Windom uses, I
wouldn't be at all surprised to find in practice what you see in the model.

In no way is a voltage balun a common mode choke or "isolator". When a
load is asymmetrical with respect to ground, a voltage balun actually
*forces* a common mode current to exist at its insertion point. Forcing
equal voltages into unequal impedances results in unequal currents in
the two conductors. The difference between the two is the common mode
current. That's why I've tried for a very long time (at least since the
publication of my balun article in 1985) to educate people that voltage
baluns are not the things to use in antenna systems.

My comments were strictly regarding the properties and uses of common
mode chokes (current baluns), not voltage baluns.

Roy Lewallen, W7EL