Microstrip bandpass filters using triple-mode circle-shaped ring resonator with/without stub perturbation

In this letter, microstrip bandpass filters (BPFs) using circular triple-mode ring resonators with/without stub perturbation are designed and analyzed. The first BPF, with a wide passband and a sharp band rejection, is proposed as a prototype, and stubs are attached to two additional BPFs to introduce perturbation. The effects of the stub perturbation are analyzed and evaluated using both the equations and the equivalent circuits presented in this study. The BPF prototype is equipped with both a circular outer resonator, which operates at one wavelength, and a half-wavelength open-ended coupled line with a central connected quarter-wavelength shunt open stub. Each side of the passband has two transmission zeros, which provides the filter with both good selectivity and good out-band suppression. Based on this BPF prototype, two additional BPFs are designed with stub perturbation for in-band (BPF 1) and multi-band (BPF 2) adjustment.