What is "balun"

n. An electronic device for connecting a balanced transmission line to an unbalanced one.

A balun is an electrical device that converts between a balanced signal (two signals working against each other where ground is irrelevant) and an unbalanced signal (a single signal working against ground or pseudo-ground). A balun can take many forms and may include devices that also transform impedances but need not do so. Transformer baluns can also be used to connect lines of differing impedance. Sometimes, in the case of transformer baluns, they use magnetic coupling but need not do so. Common-mode chokes are also used as baluns and work by eliminating, rather than ignoring, common mode signals.

The origin of the word balun is “balanced to unbalanced”.

A variation of this device is the UNUN, which transfers signal from one unbalanced line to another.