Baluns are an essential component of a ham radio system as they help to prevent RF currents from flowing on the outside of the shield of the coax feeding the antenna. Baluns are used to match the impedance of the antenna to the coaxial feedline and to prevent common-mode currents from flowing on the feedline. They can also be used to feed dipole antennas with 50-ohm coax and to transpose from ladder line feeding a multi-band antenna to the unbalanced input of an antenna tuner. The most common baluns are 1:1, 4:1, 6:1, and 9:1 and they can be constructed using different designs such as transformer, choke, and unun. Baluns come in different shapes and sizes, and their selection will depend on the required bands it must cover as well as the power level it must handle. Proper selection and installation of a balun are essential for effective operation of a ham radio antenna system.