What does a balun do?

A balun, short for balanced to unbalanced, is a transformer used to match a balanced antenna, such as a dipole antenna, to an unbalanced feedline, such as a coaxial cable. The balun ensures that the impedance of the antenna and the coaxial cable match and also minimizes the transmission of RF interference to the cable. This efficiently prevents the cable itself from acting as an antenna and changing the radiation pattern of the actual antenna or emitting unwanted radiation. Baluns are critical to the optimal performance of many types of antennas in amateur radio.