Effect of Off-center Feed on Radiation Pattern of Dipole

Author: Edmund A. Laport

The radiation pattern for a half-wave dipole is always spoken of as symmetrical about the normal to the antenna axis. This is true only for strictly symmetrical feed at the center. One that is end-fed has its maximum field strength pushed about 20 degrees away from the normal in the direction away from the end that is fed.

When two collinear dipoles are end-fed from a common balanced feeder, the tilt effects of the two patterns are equal and opposite so that they quite thoroughly neutralize each other and produce a combined pattern that is again normal to the axis of the dipoles.

When several collinear cophased dipoles are end-fed one from the other in series, the attenuation of the feed current tilts the resultant pattern away from the normal, toward the free end. Unsymmetrical feed always produces an unsymmetrical pattern. Figure 3.23 (abstracted from Cleckner, D. C. ) illustrates this effect. It is desirable in all cases to employ symmetrical systems of radiators with symmetrical feed in broadside arrays, to obtain symmetrical radiation patterns.