Compact Dual-Band Branch-Line Balun Using Only Two Impedances With High Band-to-Band Isolation for Cellular/WLAN Applications

A compact balun design for arbitrary dual-band operation is presented using transmission line based shunt open stub unit. The present structure is at least 50% more compact compared to previously published configurations, has wider bandwidth and better return/insertion loss, uses lower impedance lines and is easy to fabricate. Closed form design equations are derived using lossless transmission line model. The open-stub lengths are designed to have quarter-wavelength long, provide a transmission zero at mid-band frequency, <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</i><sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> and hence the proposed dual-band design achieves high isolation between the bands. Measurements on two fabricated prototypes demonstrate wide balance bandwidths that more closely satisfies the GSM and WLAN standards and also validates theoretical predictions.