A Dispersive Coupling Structure for In-line Helical Resonator Filters with Transmission Zeros

In this paper, a dispersive coupling structure for helical resonator filters is proposed for generating transmission zeros (TZs) in an in-line coupling topology. The coupling structure is composed of two interleaved helical resonators with a connecting wire between two helically wound arms. A linear frequency dependence of the coupling value makes the synthesis and design of such a filter straightforward. It is found that both the intercept and the slope of the dispersive coupling can be controlled independently in a wide range, leading to the flexibility of realizing TZs for an in-line helical resonator filter at the higher or lower rejection band. In addition to the parametric study, the proposed coupling structure has been validated by prototyping a 3-pole filter with one TZ.