Generalized S-Matrix CAD of Aperture Coupled Bandpass, Bandstop, and Elliptic Function Waveguide Filters

The rigorous field theoretical design of rectangular and circular aperture coupled waveguide cavity filters is described. The design is based on the full-wave mode matching method applied for a few simple basic building block elements in combination with the generalized S-Matrix technique for composite structures. The theory includes the finite thickness of the coupling elements as well as the immediate higher-order mode interaction of all discontinuities. Critical filter characteristics, therefore, such as the stopband of bandpass filters, may be taken into account in the design optimization process to yield improved filter performance. The efficiency of the design method is demonstrated at typical design examples: symmetric or asymmetric rectangular and/or circular iris coupled waveguide filters with improved stopband characteristic, a narrow-band T-junction aperture coupled bandstop cavity filter, and a metal insert filter with additional bandstop cavities. The theory is verified by comparison with measured results.