Some Aspects of the Balun Problem

W hy all the mystery surrounding baluns? Here’s some straight talk to dispel the rumors! The balun — to use, or not to use — is one of today’s hottest topics in Amateur Radio. Because certain aspects of the connection between a coaxial feed line and a balanced antenna have been ignored, misunderstanding still exists concerning the function of baluns. Many commercial baluns embody some form of impedance transformer, promoting our tendency to misconstrue them as little more than a matching device, while their primary function is to provide proper current paths between balanced and unbalanced configurations. To help clarify the misunderstanding, this chapter explains some of the undesirable effects that occur when a balun is not used, and some that occur when baluns employing coupling transformers are used. In many cases, these effects cause significant errors in measurements of antenna terminal impedance and SWR. This chapter also describes a simple and inexpensive method of loading the outside of a coaxial feed line with ferrite, which effectively produces a well-balanced, wideband choke balun. Because this configuration eliminates the coupling transformer (with inherent impedance-transfer ratio errors), the accuracy obtainable in antenna impedance and SWR measurements is greatly improved. In addition, antenna-matching networks may be used with this choke balun, because no mismatch limits are imposed by the balun.