Miniaturized Reconfigurable Dual-Band Bandstop Filter Utilizing a Novel Hybrid Resonator for Enhanced Stopband Suppression

A novel varactor-loaded microstrip resonator with a hybrid structure of microstrip and parallel-coupled lines has been proposed. Stopbands are constructed utilizing the first-and second-mode of the resonator, respectively. The independent control of the two modes of this resonator was theoretically analyzed and validated. The introduction of parallel-coupled lines was employed to enhance stopband attenuation, mitigating the impact of low- $Q$ value varactor diodes to some extent. Miniaturization is also achieved through the introduction of two sections of parallel-coupled lines. A 2nd-order dual-band tunable bandstop filter was designed, fabricated, and measured, with the measured results revealing high attenuation levels of 36.4 dB and 27.85 dB for the two stopbands, respectively, in addition to a compact size of $0.23\lambda_{g}\times 0.07\lambda_{g}$ (where $\lambda_{g}$ is the guided wavelength in the substrate at 2.2 GHz).