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Direct-Coupled Realizations for Microwave Bandstop Filters

This paper presents some new ideas for the design and implementation of microwave bandstop filters realized with waveguide, coaxial, dielectric resonators, or in a planar technology. The new methods represent an advance over present methods in that the resonators are direct-coupled, thus avoiding the need for transmission line phase lengths between resonator stubs which tend to degrade performance due to their dispersion, and are difficult to adjust during tuning. Two bandstop configurations are presented, both of which are very similar to regular bandpass filters in their design and realization. The first will accommodate even or odd characteristics and also asymmetric responses, although some negative or diagonal cross-couplings will be needed. The second resembles the cul-de-sac configuration for bandpass filters, and needs no diagonal or negative couplings even for asymmetric characteristics. A simulation of a 4th degree design in this novel configuration is included, and a practical model in waveguide is in the course of being tuned and measured.