Symmetry vs. 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 3-port network parameters. It is shown that a symmetric balun alone does not necessarily eliminate common-mode (CM) current on the feed transmission line. Common-mode current on the feed transmission line is minimized by use of a current balun. However, for a given implementation such as 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.