Balun is a balance to unbalance transformer. This is a circuit element which is used for the connection of a balanced line to an unbalanced line. This is also preferred for antennas. The most common application is in the connection of an unbalanced coaxial line to a balanced antenna. For this an ordinary tuned transformer is used which is operated at low frequencies. Here primary is unbalanced and the balanced antenna is connected to centre-tapped secondary. Adverse effects due to stray capacitances can be minimized by the usage of an earthed electrostatic shield. For high frequency operations, a bazooka balun is used. It is also known as choke or sleeve and is used for narrow band applications. Here a quarter-wavelength sleeve is placed around the outer conductor of the coaxial line.
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