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 2 GHz 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