GENERALIZED IMPEDANCE-TRANSFORMING DUAL-BAND BRANCH-LINE COUPLERS FOR ARBITRARY COUPLING LEVELS

High-performance dual-band Doherty power amplifier and non-uniform circularly polarized antenna array require impedance- transforming unequal dual-band 90± branch-line couplers for power dividing and phase shifting in the feed networks. In this paper, an analytical design methodology of generalized impedance-transforming dual-band branch-line couplers for arbitrary coupling levels is proposed. The coupler features wide range of realizable frequency ratio, multiple flexible selections of open- or short-circuited and pi- or T-network topologies. For demonstration, four numerical examples with different parameters are presented. Furthermore, two microstrip couplers based on open-circuited pi- and T-network topologies were fabricated and measured. The measured results show good performance at dual 1.8/3.45 GHz bands. The fractional bandwidths defined by the fluctuation of the coupling level and the phase difference less than ±0:5 dB and ±5± are up to 17% and 18%, 18% and 2% for open-circuited pi- and T-network topologies, respectively.