A novel dual-band bandstop filter based on a square, symmetric dumbbell-shaped resonator and U-shaped slot defected ground structure is presented. First, the characteristic of the fundamental structure which adopts two dumbbell-shaped resonators and one U-shaped slot is analyzed. Simulated results demonstrate that the proposed structure induces two transmission zeros within 2-8 GHz. Then, the structure adopting four dumbbell-shaped resonators and one U-shaped slot is analyzed. Simulated results point out that the characteristic of dual stopbands is better than the fundamental structure. Based on above implementation, a dual-band bandstop filter based on eight proposed dumbbell-shaped resonators and two U-shaped slots is proposed and fabricated. Two center frequencies at 4 and 6.5 GHz are reported, corresponding to the attenuation levels of 41.9 and 26.1 dB. The return losses of center frequencies are 0.04 and 0.20 dB, respectively, and the dual stopband bandwidths with 10 dB signal attenuation are 690 MHz and 250 MHz. In addition, two transmission poles at each stopband are induced for better selectivity. Owing to the symmetric dumbbell shape, the size of the filter gets reduced. It is simple to design and quite compatible with planar construction fabrication.
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