Quad-Band Microstrip Bandstop Filter Design Using Dual-Mode Open Loop Resonators Having Thin Film Capacitors

This letter presents a quad-band microstrip bandstop filter based on open loop resonators having thin film chip capacitors. For this purpose, dual-mode properties of open loop resonators are used and four independently controllable resonance frequencies are obtained by two open loop resonators in different electrical length. Even/odd mode analysis of the proposed structure is introduced to clarify the design method. In order to obtain higher degree, two unit cells including two resonators are used. While center frequencies can be independently controlled, the same is not true for the bandwidths and rejection levels. The proposed filter was also fabricated for the experimental verification of the predicted results. Four stopbands at 1.46, 1.86, 2.3 and 2.79 GHz were obtained in a good agreement with the predicted results.