A compact quad-band bandstop filter using dual-plane defected structures and open-loop resonators

A novel approach for designing a quad-band bandstop filter is proposed by using dual-plane E-shaped defected structures and open-loop resonators coupled to the microstrip line. Through this design, the proposed filter features four conveniently adjusted stopbands, compact size and simple design procedure. The equivalent circuit model of the quad-band bandstop filter is derived before the design procedure is summarized. To facilitate the filter design, an approximate design rule for the resonant frequency of the E-shaped defected structure and the open-loop resonator is presented. As an illustrated example, a quad-band bandstop filter is designed and fabricated. The fabricated filter has four stopbands with center frequency at 2.37/3.52/5.20/5.76 GHz and the rejection levels of the stopbands are all better than 31 dB, verifying our proposed design concept.