using varactor perturbed dual-mode resonators

In this paper, an electronically switchable quad-band microstrip bandpass filter is designed using varactor perturbed dual-mode resonators. For this purpose, four meandered dual-mode square loop resonators (DMSLRs) having reference patches are nested and varactor diodes are used to serve as perturbation elements. The designed resonator is analyzed by deriving even/odd mode resonance conditions. S-parameters are also calculated according to the even and odd mode input impedances. Stub loaded feedlines are used to couple the DMSLRs to Input/Output (I/O) ports. According to the capacitances of varactor diodes, each passband can be independently turned on and off. Coupling scheme and coupling matrix of the designed filter are introduced to realize the coupling descriptions. Switching operations are also investigated over the frequency responses obtained from coupling matrix. Passbands of the filter are adjusted to 1.4, 1.7, 2.25, and 2.7GHz. The designed filter was also fabricated and measured results show a good agreement with the simulated and theoretical results.