Compact Dual-Band Bandpass Filters Using Open-/Short-Circuited Stub-Loaded $\lambda /4$ Resonators

Two types of modified open- or short-circuited stub-loaded λ/4 resonator and their applications to second- order dual-band bandpass filters are presented in this letter. Based on the resonance characteristics analysis, it is found that the first two resonant frequencies of the two resonators can be independently controlled. Furthermore, different frequency ratios can be achieved by loading different stubs. Finally, to illustrate the proposed idea, two second-order dual-band filters with controllable bandwidths are designed, fabricated and measured. The measured results are in good agreement with the full-wave simulation results.