Design of cross-coupled dielectric-loaded waveguide filters

The dielectric loading of waveguide cavities for the design of cross-coupled filters is described. A mode-matching program has been written to compute resonant frequencies and unloaded quality factors (Q(u)) of the fundamental TE011 mode supported by dielectric loaded cavities. Charts are presented at 2GHz that allow the design of a resonator with optimal Q(u)/volume ratio. Trade-offs between Q(u)/volume, spurious performance and maximum coupling available between adjacent cavities are investigated. The latter is obtained by mode-matching while the spurious variations are studied with a finite-element analysis package. The practical realisation of the filter is considered in detail. The focus is on a design that is practical for manufacturing, assembly and alignment. This includes particularly the layout of the tuning mechanisms, the positive or negative cross-couplings and the mounting of the dielectric slabs in the cavities. The effect of the external coupling and the frequency tuning on the performance of the resonator is evaluated. Cross-coupled degree-six bandpass filters are built and show a 2.0:1 improvement over conventional comb-line resonator filters.