Balun The balun ("balanced-to-unbalanced") is used to couple balanced and unbalanced transmission lines. The balanced lines have two conductors of equal potential with a 180-degree phase difference. In an unbalanced line, current flows through a grounded element such as a conducting shield.
Lumped
A lumped version of a rat race coupler is a useful as a balun. The phase of ports #2 and #3 with respect to the input are +90 degrees and -90 degrees, respectively. Therefore, the phase difference between the two output ports is 180 degrees, as desired. At the isolated port the signals from various paths cancel.