Design of a novel harmonic-suppressed wideband microstrip bandstop filter

A systematic procedure for designing harmonic-suppressed wideband microstrip bandstop filter (BSF) based on the frequency-selecting coupling structure (FSCS) is presented. This filter is added with shunt open stubs to introduce several attenuation poles to achieve a deeper rejection in the first and second stopband. The simulation result shows that the BSF has a wide 3 dB fractional bandwidth which is more than 120% at the center frequency of 11.5 GHz. From 5.5 to 17 GHz in the first stopband, the proposed BSF has a rejection of better than 20 dB and the maximum rejection level of 50 dB, from 27.5 to 36 GHz in the second stopband, the proposed BSF has a rejection of better than 20 dB and the maximum rejection level of 40 dB. Application of this filter is also investigated. Obviously, it can be used to suppress the unwanted fundamental frequency from 11 to 12 GHz and the third harmonic from 33 to 36 GHz caused by a doubler, and let the corresponding second harmonic from 22 to 24 GHz pass through.