New broadband balun

The letter describes a new balun composed of a modified `rat race¿ with an open-circuit impedance at the uncoupled port. The advantage of this device is the great bandwidth, together with its very small dimensions. The performance of a layout on an alumina substrate is compared with theorotical predictions. For a midband frequency of 2.5 GHz, a bandwidth ratio of 52% and dimensions smaller than 1 cm2 on a 10 mil-thick alumina substrate are typical.