Quasi-Elliptic Wideband Bandstop Filter Using Stepped-Impedance Coupled Line

A cross-coupled wideband bandstop filter (BSF) with a 5th-order quasi-elliptic response is proposed and investigated in this letter. The BSF is composed of stepped-impedance coupled line connected by an open-circuited shunt stub. An enhanced selectivity and stopband rejection can be achieved as a result of two new transmission zeros, generated from the stepped-impedance coupled line. Based on the derived rigorous correspondence between the BSF and its lumped prototype, design of the proposed BSF can be handled as the same to the lumped network. A BSF example is given with measured stopband attenuation of 34.4 dB over a fractional bandwidth of 113.4%.