Design of the Microstrip Bandstop Filter With High Insertion Loss

A novel planar bandstop filter with high insertion loss is proposed. With the assistance of coupled lines with open stubs at the input and output ports, the newly proposed structure is different from the conventional bandstop filter. With the coupled lines mentioned above, selectivity and stopband rejection of the newly proposed planar bandstop filter can be effectively enhanced. Moreover, the attenuation level of the proposed structure can be controlled by adjusting the resonant frequency of each resonator. Design equations can be obtained with the assistance of the lossless transmission line model and parallel coupled-line model. In order to validate the analysis, a bandstop filter, with a fractional bandwidth of 14% for the measured stopband at 50 dB, is fabricated.