A Technique to Suppress Harmonic Outputs in Coaxial Cavity Filters at the Excitation Port

A technique to improve the spurious performance of coaxial cavity filters is investigated. It involves incorporating elements/posts resonant at spurious frequencies within the coaxial input/output cavities. To perform suppression and control of harmonic outputs, the spurious elements are coupled directly to the excitation posts and prevent the excitation of harmonics at the fundamental elements. The spurious elements are physically significantly smaller than the fundamental elements that perform the basic filtering function (fundamental frequency); thus they impose only minimized degradation to the ohmic loss of the devices and miniscule additional design effort. Design guidelines of the proposed filters are given. Obtained results demonstrate significant improvement in the spurious performance, and experimental results verify the concept.