Planar Quasi-Elliptic Filters with Inline EM Coupled Open-Loop Resonators

This paper describes the planar quasi-elliptic filters with EM coupled open-loops having an inline resonator alignment. For two coupled open-loops, the coupled sides with the maximum magnetic field are combined with their open gaps with the maximum electric field, creating the mixed electric and magnetic coupling. By adjusting the distance between this two loops and gap position of each loop, a desired coupling coefficient and a controllable transmission zero can be simultaneously obtained. The shapes of the open-loop can be either a rectangle or triangle, but the latter is preferred to the former since the coupling between the EM coupled triangular pairs is tighter and more controllable. A backed aperture can be etched on the ground plane underneath the coupled lines of two adjacent loops to further enhance both electric and magnetic coupling, and to help to realize a wider bandwidth and closer-to-passband transmission zero. High order filters can be constructed by cascading several EM coupled open-loop pairs to create multiple transmission zeros.