Symmetry Versus Balance in Balancing Networks for Dipolar Antennas

Imperfect balancing networks (baluns) have been identified as a source of error in emission and site attenuation measurements. For this reason, performance tests have been developed to characterize the symmetry of baluns. We draw a distinction between symmetry and balance as they relate to baluns and describe both quantitatively in terms of three-port network parameters. It is shown that a symmetric balun alone does not necessarily eliminate common-mode (CM) current on the feed transmission line. CM current on the feed transmission line is minimized by use of a current balun. However, for a given implementation, such as a transmission-line transformer, some types of baluns are more difficult to implement than others and an imperfect, asymmetric current balun may perform worse than another type of balun (e.g., voltage), which is more symmetric. The case of a vertically polarized biconical antenna situated over a ground plane and driven from a coaxial feed line via a balun is examined for three symmetric baluns: a voltage balun, a current balun, and a $180^{\circ }$ power divider. It is shown that although all three baluns are symmetric, only the current balun maintains the dipolar pattern under the asymmetrical influence of the ground plane.