Roy Lewallen wrote in
:

The guy wire requirements would be about the same as for a "current
balun" (common mode choke) -- somewhere around 500 - 1000 ohms is
typically necessary. At that impedance level, it makes no difference
whether the impedance is reactive or resistive from the standpoint of
effectiveness in choking current or in terms of dB loss. But there can
still be enough power dissipated to overheat the cores if they're
resistive and the power level is very high. Then you're stuck with
using ferrites which are more reactive and less resistive (e.g.,
Fair-Rite 60 series), but they also give you a lot less impedance per
core so you need more cores yet. That makes the ferrite solution even
less attractive.

Roy,

I have attempted to model the famous Carolina Windom. It takes some
guessing since it contains proprietary (ie secret) components, namely the
"balun" and the "isolator". My flat top is at 10m height.

Since they argue that the vertical feedline is an effective radiator, I
make the assumption that the balun is a "voltage" balun, and that it is
transparent to common mode current, so I have modelled the feedline as a
wire attached to (as it happens) the inner end of the short leg of the
flat top.

The "isolator" is argued to prevent the current flowing on the line above
itself from flowing on the line below itself. It is a naive notion, since
there is mutual coupling... but lets guess that it is a bunch of
supression sleeves on coax.

At 7.2MHz, the isolator is subject to appreciable current, and it does
not effectively force a current minimum until it is well over 1000+j1000.
(BTW, about half the power is dissipated in a 1000+j1000 isolator.) 2000
+j2000 is becoming reasonably effective.

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 think that design constraint is effective in limiting the extent to
which such a balun unbalances the load by its own shunt impedance dropped
accross one load leg for a load~=Zo, but I think the criteria has nothing
to do with the choke's influence in forcing a current minimum in the
region of itself. It is the difference between a bench test criteria for
insertion VSWR, and what is needed when plugged into an NEC model where
the intention is to force a current minimum.

Thoughts?

Owen