Design Methodology of Transmission-Line Quadruplet Filters Supported by Rigorous Circuit Transformations

This article presents a novel and rigorous technique to design transmission-line quadruplet filters. Unlike the prevailing approaches relying on electromagnetic (EM) simulations for coupling tests, the proposed design technique completely counts on rigorous circuit transformations. As all design parameters (lengths and line impedances) can be rigorously calculated from the design equations provided in this work, a quadruplet filter can be designed without conducting EM simulations to find the dimensions delivering the desired coupling coefficients. Our design approach will be demonstrated with the fourth- and fifth-order filter examples, both of which were fabricated and measured for validation.