Practical_Antenna_Handbook_0071639586

226 P a r t I I I : H i g h - F r e q u e n c y B u i l d i n g - B l o c k A n t e n n a s
L
Tuner
L /4
Coax
VSWR meter
L /4
Transmatch
To XMTR
Random
Figure 8.6 Random-length longwire antenna.
However, off-center feedlines suffer from an inherent lack of balance between the
two sides of the feedline because there is no electrical or physical symmetry at the point
where they attach to the antenna. As a result, it is much harder to keep the feedline from
radiating and becoming an unintended part of the antenna.
Windom
The Windom antenna (Fig. 8.7) has been popular since the 1920s. Although Loren Windom
is credited with the design, there were actually multiple contributors. Coworkers
at the University of Illinois with Windom who should be cocredited were John Byrne,
E. F. Brooke, and W. L. Everett. The designation of Windom as the inventor was probably
due to the publication of the idea (credited to Windom) in the July 1926 issue of QST
magazine. Additional (later) contributions were rendered by G2BI and GM1IAA.
The Windom is a roughly half-wavelength antenna that will also work on even
harmonics of the fundamental frequency. Like the off-center-fed dipole, the basic premise
is that a dipole’s feedpoint resistance varies from about 50 Ω at the center to about
5000 Ω at either end, depending upon the location of the feedpoint. In the Windom
antenna of Fig. 8.7A, the feedpoint is placed about one-third the way from one end,
presumably where the impedance is about 600 Ω.
The Windom antenna works “moderately” well—but with some caveats. It is important
to again recognize that the return path for a single-conductor feedline is the
ground system underneath the antenna and feedline. In distinct contrast to similar horizontal
dipoles that are center fed, the extent and quality of the ground beneath the
antenna is a major factor in the overall radiation efficiency of the Windom. Further, this