Design of the wide stopband microstrip bandpass filter by cascading stepped coupled lines

This study develops a microstrip bandpass filter with a broader stopband. The proposed bandpass filter is constructed by cascading three stepped coupled lines and two short-circuited stubs. Moreover, quarter-wavelength electrical lengths and weak- strong-weak couplings are adopted for the stepped coupled line. In this way, a compact circuit size and a wide stopband can be achieved readily. In addition, the short-circuited stubs connected to the input/output port can result in a steeper passband skirt and a broader passband. Finally, a design example is provided to show the availability of the proposed structure.