Design of microstrip quint-band lowpass–bandpass filters with flexible passband allocation

In this paper, newly proposed microstrip quint-band lowpass–bandpass filters are presented. The lowpass filter structure comprises a meandered transmission line and modified T-shaped resonators. Based on such an lowpass filter (LPF) structure, a quad-band λ/4 inductively-coupled dual-mode stub-loaded stepped impedance resonators BPF with one pair of a coupled I/O feeding line are parallelled on both sides of LPF to construct a quint-band lowpass–bandpass filter. By incorporating the parallel I/O coupled line and utilising multiple coupling paths, four bandpass bands can be fully controlled and designed independently. Specially, two distinct types of bandpass filters with different generating mechanisms are investigated and then combined to provide the quad-band responses. To validate the design concept, two types of quint-band lowpass–bandpass filters are designed, fabricated, and tested, where simulation results agree well with measurement results.