Design of triple-band bandpass filter using quad-mode stepped impedance resonator (SIR) with shorted stub

Abstract

A compact microstrip triple-band bandpass filter (BPF) using an improved quad-mode stepped impedance resonator (SIR) with shorted stub is introduced in this letter. Distinct mode splitting characteristics of the quad-mode SIR are investigated by adopting even- and odd-mode theory. Two pairs of even-odd modes are utilized to design the former two passbands with the help of loop-loop coupling for splitting two identical modes. Then, a tapped side-coupled feed-lines construct an additional passband avoiding occupying extra size. The bandwidth of each passband can be controlled independently. To validate the approach, a triple-band BPF centered at 1.4/2.7/3.57 GHz has been implemented. Good agreement is obtained between simulated and measured results.