Superconducting Ultra-Wideband (UWB) Bandpass Filter Design Based on Quintuple/Quadruple/

This paper analyzes the coupling modes when mul- tiple hairpin resonators are coupled. Three kinds of current distri- bution and direction states exist in the coupled hairpin resonators: the current along the clockwise direction, the current along the counterclockwise direction, and no current density distribution. Differentcombinationsofthesecurrentstates leadtodifferentcou- plingmodes.Inordertodistinguishthesemodes,auxiliarylinesare introduced between each two coupled resonators to denote each coupling mode, and this method is theoretically based on the anal- ysis of the eigenvector of the coupling matrix. A simple and ef- fective technique to construct multi-mode resonators (MMRs) is then proposed. An -mode resonator can be constructed by con- necting hairpinswithsmallpatches.Resonantmodes ofthis class ofMMRswereanalyzedindetailusingtheproposedauxiliarylines method. Single stage and four stages of quintuple-mode resonators wereusedtodesignhigh-temperaturesuperconductingultra-wide- band (UWB) filters by properly allocating the first five resonant peaks.TwoUWBfiltersbasedonquadruple-modeandtriple-mode resonators are also given. Parallel-coupled three lines were used to provide tight external couplings, which also generated two trans- mission poles. Measured results are in good agreement with the simulated ones without any tuning. Index Terms—Auxiliary line method, coupling mode, current direction, high temperature superconducting (HTS) ultra-wide- band (UWB) filters, multi-mode resonator (MMR), odd and even method.