Design of Stepped-Impedance Combline Bandpass Filters With Symmetric Insertion-Loss Response and Wide Stopband Range

An enhanced stepped-impedance combline bandpass filter employs an array of stepped-impedance resonators with tapped-transformer coupling at input and output is presented in this study. This filter has enhanced performance, including symmetric insertion-loss response around the passband and wider stopband range. The structure is compact and suitable for multilayer realization because it is free of lumped capacitors and has fewer via-hole grounds. The circuit is investigated with the characteristic mode theory of coupled lines to prove the existence of multiple transmission zeros around the passband. Numerous diagrams are given for circuit design purposes. The second- and fourth-order bandpass filters at 2.45 GHz were designed, measured, and compared with the conventional combline structure to demonstrate their performance enhancement.