A Novel Coupling Structure for Inline Realization of Cross-Coupled Rectangular Waveguide Filters

This paper presents a new cross-coupling structure for introducing transmission zeros in waveguide filters with inline configuration. The cross-coupling is realized by an internal bypass metallic loop structure (wire), short-circuited to the filter top plate and passing through the waveguide cavities. Positive and negative cross-couplings are realized by only varying the length of the structure suitably. Fullwave electromagnetic (EM) simulations of the coupling structure are performed for extracting the coupling coefficient sign and value (k). The relationship of the values of k with the coupling structure dimensions is presented graphically. The loading effect of coupling structure on the resonance frequency of the waveguide cavities is also discussed, as well as the possibility of post-manufacturing tunability. The feasibility of the proposed coupling structure is demonstrated by the design of three test filters. Two filters verify the inline design of a triplet configuration using the proposed coupling wire to introduce a transmission zero below or above the passband. These filters have been also manufactured and measured for an experimental validation of the new structure. The third filter demonstrates the use of the wire loop to create a negative coupling in a cross-coupled configuration. Power handling capability of this configuration has been also thoroughly analyzed by full-wave EM simulation.