Design of Planar Wide-Stopband Bandstop Filters With Extra-High Attenuation

The novel microstrip bandstop filters with extra-high attenuation and wide stopband have been proposed. In order to realize the bandstop filter with broad stopband and great attenuation within the stopband, the coupled lines with single-ended ground have been adopted at both input and output ports. Moreover, for a greater fractional bandwidth, a higher impedance ratio
$Z_{ei}/Z_{oi}$
of two adopted coupled lines is required. Furthermore, by differentiating the even- and odd-mode impedances of coupled lines at the input and output ports, the attenuation within the stopband can be increased and the stopband’s bandwidth would be slightly narrower. Two bandstop filters, with the attenuations of 46 dB and 55 dB and a fractional bandwidths of 58.1% and 50.3%, respectively, have been developed to verify the proposed method.