Short-Through-Line Bandstop Filters Using Dual-Coupled Resonators

A new approach using dual-coupled-resonator bandstop sections to realize microwave bandstop filters with arbitrarily short through-line length is proposed. In contrast to our recent work in 2015, this approach does not require the resonator-to-through-line couplings to be comprised of both electric and magnetic coupling, i.e., mixed coupling. An exact transformation from a conventional inline bandstop filter topology to a dual-coupled-resonator bandstop filter topology is presented. A design procedure is given for both all-dual-coupled-resonator designs and mixed (single-coupled and dual-coupled-resonator) designs. A fifth-order elliptic dual-coupled-resonator microstrip prototype is presented with a center frequency of 500 MHz and a through-line length of 6.35 cm, 17% the length of a conventional design. A fifth-order elliptic mixed-resonator microstrip prototype is presented with a center frequency of 500 MHz and a through-line length of 5.01 cm, 13.7% the length of a conventional design.