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In this paper, a novel compact bandstop filter having low insertion loss and low radiation loss is presented. The proposed filter uses a quarter-wavelength open-stub resonator with direct-connected microstrip transmission line. Due to the narrow gap between a resonator and a ground, we can realize low mismatch and the bandstop filter shows low insertion loss. In addition, this structure can reduce the radiation loss at the stopband due to the strong coupling between the resonator and the signal line. The fabricated bandstop filter with a single resonator has insertion loss of better than 0.5 dB in the passband, rejection of greater than 30 dB at stopband. The operating frequency of the proposed resonator can be varied by using the shunt capacitors between the resonator and the ground. Moreover, we will show several filter applications using this resonator.