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 ohms.

     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.

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