baluns

A balun, short for balanced-unbalanced, is an electrical device that converts between balanced and unbalanced signals and provides impedance transformation when needed. In radio engineering, a balanced signal has two conductors with equal impedance to ground and equal and opposite currents; an unbalanced signal has a single conductor relative to ground. Baluns help feed balanced antennas from unbalanced feeders and prevent RF currents from flowing on the outside of coax.

There are several implementations. Current baluns (also called RF chokes) use a winding or coaxial choke to block common-mode currents, preserving balance. Voltage baluns employ transformers to convert impedance and enforce balance. Common forms include 1:1 baluns, 4:1 baluns, and sleeve or coaxial baluns. The construction often uses ferrite cores (toroidal or rod), air-core windings, or coaxial sleeves.

Applications: most widely used in amateur radio and TV antennas to connect dipoles to coax feeders, to suppress feedline currents, and to match impedances between antenna and transmitter. A 1:1 current balun keeps impedance the same but prevents common-mode currents; a 4:1 balun can transform impedances to better match typical 50-ohm coax with a higher-impedance balanced antenna. Bandwidth and frequency range depend on core material, winding, and construction; losses and imbalance can occur at frequencies outside the balun's design range.

Design considerations include operating frequency range, desired impedance transformation, power handling, core material, and physical form factor. Care should be taken to avoid overheating and to ensure the balun is installed to maintain the balanced nature of the load, with proper grounding practices where applicable.