With all due respect to Sinisa, I would trust the L. B. Cebik modeling; as
he uses all the available software and does extensive confidence and
convergence testing of the model. If you model correctly in this way, you
certainly can trust his analysis of the end feed antenna which can be
considered the worst case of asymmetric feeding. I was extending it to the
Windom as a sub case of feeding. One is a half wave antenna, albeit off
center fed. The other is a half wave end fed.
L. B. even has professional antenna modeling software that is not commonly
available to hams, to check the antennas every possible way. There are,
though, some antennas that are very hard to model with today's modeling
programs. These are usually the small volume antennas, not the full size
radiators. However, wire antennas are
extensively analyzed by the present modern programs.
A good discussion and test results for various configurations of the Windom
type antenna, (more properly called the Off Center Fed, or OCF), is
contained in the last antenna book CQ Publications published by the late
Bill Orr, W6SAI. He shows a number of methods that work for some
applications. This, nor any other single antenna is a universal solution to
a ham's desire to operate all HF bands. All, are compromises.
Practical observation by many U.S. hams in the 1950's and 60's was that
their twin lead fed "Windom" type antennas loaded well without a balun or
special device at the feedpoint. This was in part because of the long twin
lead feeder and the use of the universal at the time, Pi Net output
transmitter. Many did not worry with SWR in those days, and in fact did
not possess a way to measure it. Of course a tuner would be used for
multiband or multifrequency operations. In those days you had the Z match
tuners and the Johnson Match Boxes both intended as parallel line tuners.
Much simpler than trying the arduous current balun design route might be to
just feed the "Windom" with parallel line and accept some modest vertical
feeder radiation and use the tuner to move around the band. The problem I
have with baluns mounted near the feedpoint is that weight issues for open
span antennas are a real problem in the windy conditions we have twice a
year. Most Windom types are best used as horizontals rather than hung from
a conductive tower support at their feedpoint. However, one could add a
third non conducting support pole at the feedpoint to take the weight of
added hardware.
I always counsel the new antenna user to use a conventional dipole which can
easily be hung as horizontal, or inverted Vee and is not complicated to
feed. The dipole can even be folded at the ends to be "Zee" in the
horizontal plane, or have vertical hanging tails, or even put up as a part
vertical, part horizontal, or sloping. There are so many ways to fit the
half wave dipole into a space and have a convenient feedpoint, I am somewhat
mystified what appeal the OCF has to the user. It does not possess any
property of radiation not also found with conventional half wave antennas.
Nor does it have gain in its fundamental band application. The only reason
I hear most use them is that it places the feedpoint exactly above an
existing window to the shack. Well, wall penetration insulated sleeves can
be used to route a feeder; or a dipole feeder can be angled somewhat. You
can use standoffs of parallel feeders to route them at right angles to the
dipole, and then direct them along the roof edge on standoffs to wherever
your feeder entry might be.
There is no harm in trying a simple parallel line fed OCF and converting to
simple dipole by one of the means above if problems with OCF feeder currents
are found.
-Stuart Rohre
K5KVH
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