Off-Center Feed - The "Windom" Antenna
By Bill Buchholz - K8SYH
The Windom antenna was widely used
in the 1930s and is named after the amateur that wrote a comprehensive article
about it (which I do not have in my library). It consists of a half-wave
dipole at its lowest frequency band, with a single-wire feeder connected
off-center, dividing the antenna into pieces of .36L and .64L, where L (ft)
= 468/f (MHz).
This will operate satisfactorily on the fundamental and
even harmonics with an impedance of about 600 ohms to ground. It depends
on an efficient ground system. This may be attached directly to a vacuum
tube rig with a pi network tank circuit. This antenna requires the use of
a tuner, as no modern transmitter likes such a hi-Z load A possible problem
with this system is that at certain feed length-frequency combinations there
may be lots of RF "floating" around the shack.
A more recent off-center-fed antenna uses 300-ohm line
as the feedline instead of the single wire. This is miscalled a "Windom".
It is a half wave antenna (at its lowest band) divided into pieces of .326L
and .674L, where L (ft) = 468/f (MHz). It is claimed that the feedpoint impedance
is about 300 ohms, but there is no theoretical support for this.
This arrangement is particularly susceptible to
parallel line currents and the balanced line probably acts like a single
wire feed. Since the usual configuration uses a balun to join the balanced
line to coax, the parallel currents are effectively choked off. Using a 4:1
balun would convert 300 ohms to 75 ohms, if the feedpoint were actually 300
The above information is taken
from the 1970 edition of the ARRL Antenna Handbook. These antennas are not
found in some of the more recent editions.
A note from K8SYH - The principal
virtue of the second version of this antenna is that it allows a multiband
antenna of sorts with at least part of the feedline being coax. This removes
the physical difficulties of using balanced line right into the shack where
the tuner is located (next to the window for those of us lucky enough to
do this). There is no problem with RF in the shack, and in most cases the
increased losses caused by an elevated SWR in the coax can be tolerated.
It is a popular antenna and has allowed many stations to get on multiple
HF bands with a relatively inexpensive and trouble-free antenna system.