baluner

A balun, short for balance-unbalance, is a device that connects a balanced electrical circuit to an unbalanced one, often providing impedance transformation in the process. Its primary purpose is to allow balanced antennas or circuits to be fed from unbalanced transmission lines such as coaxial cable, while minimizing RF currents on the outside of the feed line that can distort radiation patterns or increase losses.

There are several common types of baluns. Transformer baluns use windings on a core to convert between balanced and unbalanced sides, with common impedance ratios including 1:1 and 4:1. The 4:1 type can transform a 50-ohm unbalanced feed to a roughly 200-ohm balanced load, depending on the application. Sleeve or coaxial baluns employ a section of coax with a conductive sleeve around the outer conductor to create a balanced feed without an explicit transformer. Current baluns, or choke baluns, suppress common-mode currents on the feed line by presenting a high impedance to currents flowing in the outer conductor.

Baluns are widely used in RF and communications contexts, especially at the feed points of antennas such as dipoles and folded dipoles, to preserve the intended radiation pattern and reduce feedline radiation. They are also used in audio and video systems to interface balanced lines with unbalanced inputs, and in some instrumentation to isolate circuits galvanically while preserving signal integrity.

Construction and performance depend on frequency range, bandwidth, core material, and physical form. A balun may provide only balance transformation, or both balance and impedance matching, depending on design.