A Wide-Band Transformer from an Unbalanced to a Balanced Line

The frequency characteristic of a particular type of "balun" (a transformer from an unbalanced to a balanced line) is discussed, and it is shown that by inserting a properly designed pair of quarter-wave sections to match the balun to the balanced line of higher characteristic impedance, the standing-wave ratio can be kept below 1.25 over frequency bands of the order of 4 to 1. Design parameters are given for the case of 128-ohm and 150-ohm balanced lines, for a 50-ohm unbalanced line.