A New Wide-Band Balun

This paper describes a form of balun which is useful for matching a balanced circuit to an unbalanced circuit of nearly the same impedance over a wide frequency range. The bandwidth increase is obtained by the use of a quarter-wave transmission line section which is placed inside one of the balanced arms, thereby minimizing the over-all physical length. It is shown that a practical balun of this type, designed to match a 50-ohm unbalanced line to a balanced 70-ohm antenna, has a voltage standing-wave ratio of 1.4 or better over a frequency band of about 2.8 to 1. The wide bandwidth, simplicity, and reasonable physical dimensions make the balun particularly useful in connection with the adjustable-length dipole antennas which are generally used for vhf and uhf fieldstrength measurement.