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Dual-band bandstop filters using open-circuited stub-loaded resonators are presented. The first and second resonant frequencies of the opencircuited stub-loaded resonator can be flexibly controlled by tuning the length and position of the open-circuited stub. To illustrate the concept, a dual-band bandstop filter has been designed, fabricated and measured. The prototype of the dual-band bandstop filter achieved attenuation of 42 and 44 dB at 2.4 and 5.2 GHz.

Compact microstrip low-pass filters (LPFs) with ultrawide stopband based on a defected ground structure (DGS) and their design procedures are presented in this paper. A key merit of the filter configuration is that the selectivity of the LPFs can be conveniently controlled, whereas the bandwidth is fixed. The LPFs are realized using open-circuited uniform impedance stubs and stepped-impedance stubs (SISs). The position of the transmission zeros can be freely controlled by tuning the impedance ratio of the SISs. To expand the stopband, the dumbbell DGS is used to suppress the spurious passbands of the LPFs. Two demonstration filters with a cutoff frequency at 1 GHz have been designed, fabricated, and measured. The measured results indicate good performance: broad stopband (>15 f c or 20 f c ), low passband insertion loss (<0.3 dB), compact size, and sharp skirt characteristic.

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