Coupled stepped-impedance-resonator bandstop filter

This study presents a novel design of bandstop filter with cancellation of first spurious mode by using coupled three-section step impedance resonators (SIRs). This cancellation occurs when two transmission poles coincide with the first spurious mode (transmission zero) by properly choosing the step impedance ratio and the gap between the SIR and the main transmission line. A stripline design of the proposed bandstop filter exhibits an upper passband width of up to six times the fundamental bandstop centre frequency at 1.0 GHz with 3 dB fractional bandwidth of 60%.