Advanced Techniques for Designing Wideband and Highly Selective Bandpass Filters Through Cascaded Modules

This paper presents a novel design approach for the realization of highly-selective wideband bandpass filters (BPFs). The technique builds on a conventional filter topology, featuring shunt short-circuited stubs connected by transmission lines, in which the traditional λ_g/4 interconnecting lines and λ_g/4 short-circuited stubs are replaced by a building block comprising λ_g/8 lines separated by λ_g/8 short-circuited coupled-line sections. To enhance selectivity, λ_g/2 open- and short-circuited coupled-line segments are used at the filter's input and output. The paper provides design guidelines and formulas through ABCD analysis for practical filter implementations. Additionally, the design examples are demonstrated, along with their practical validation through the manufacturing and testing of a fourth, and a fifth-order microstrip-type BPFs as well as a fifth-order BPF with improved selectivity.