Design of Dual-Band Filtering Quadrature Coupler Using $\lambda /2$ and $\lambda /4$ Resonato

This letter presents, for the first time, a dual-band filtering quadrature coupler with high skirt selectivity. The proposed coupler employs two sets of resonators, i.e., four half-wavelength resonators and four quarter-wavelength resonators. The former not only operate at the upper passband but also feed the latter. The inherently realized source-load coupling for lower passband contributes two additional transmission zeros close to the two edges of lower passband. The tapped feeding position results in a pair of transmission zeros in the vicinity of the upper passband. The central frequencies and bandwidth of the two passbands can be independently designed. For verification, the simulated and measured results of a dual band filtering quadrature coupler operating at 2.77 and 3.55 GHz are presented showing good agreement.