Tunable absorptive bandstop filter with an ultra-broad upper passband

This paper presents a new broadband external coupling structure for tunable evanescent-mode cavity resonator-based bandstop filters. This coupling method has low levels of parasitic reactances, which when combined with the wide spurious-free range of evansecent-mode cavities enables the implementation of a 3 to 6 GHz tunable bandstop filter which has a measured upper passband with less than 3-dB of insertion loss up to 28.5 GHz.