Design of loop resonator filters with a dual passband response

Design method is presented for loop resonator filters with a dual-passband response. The loop resonator is equipped with stepped-impedance junctions, so that its second passband can be tuned, relative to the fundamental frequency f₁, by its geometric parameters. Analysis of the resonant characteristics of a loop resonator is formulated by the transmission line theory. Through proper tapped input/output couplings, bandpass filters with a dual-passband response can be achieved. Measured results of two fabricated circuits show good agreement with the simulation.