Rigorous CAD of Rectangular Aperture Coupled Circular Waveguide Multi Mode Filters

A rigorous field theory analysis is presented for the accurate design of rectangular iris, coupled circular waveguide single, double and triple mode filters. The multi mode coupling between the orthogonal modes is achieved by a new technique using additional asymmetric slots in the Aperture plane by multiport coupling techniques. This avoids the traditional 45° coupling screws. The theorie is based on the full-wave mode-matching method for the key-building block discontinuity circular waveguide to smaller rectangular waveguide. Arbitrary iris location, different cross sections, unequal E-and H-plane cross size and finite thickness are rigorously taken into account. The efficency of the design method is demonstrated at two new filter examples; one rectangular-iris coupled one-resonator dual mode and one triple mode circular filter in the P-band (10-15 GHz). The theorie is verified by comparison with mesurements.