Dual-band bandpass filter using loop resonator with independently-tunable passband
A dual-band microstrip bandpass filter using the rectangular loop resonator (RLR) is proposed. A microstrip open stub is implemented to perturb modes of the RLR, and the two modes form the low-frequency passband. The stepped impendence microstrip lines are adopted to excite the RLR and the lines also work as the resonant stubs, i.e. one is 3/4-wavelength resonance stub and another is full-wavelength resonance stub. As a result, the second passband is introduced. Furthermore, each passband can be tuned independently through the corresponding resonant paths (parameters), which are discussed in detail. Without the connection stub between stepped impendence microstrip lines and RLR, this dual-band bandpass filter will degenerate to a dual-mode single-band bandpass filter and the corresponding results are also obtained and demonstrated. The measured results agree with the simulated results well.
