Computer-aided design of slit-coupled H-plane T-junction diplexers with E-plane metal-insert filters

The rigorous field-theory-based design of a class of rectangular waveguide slit-coupled H-plane T-junction diplexers utilizing E-plane metal-insert filters is described. The filter elements allow low-cost manufacturing by accurate and inexpensive metal-etching techniques. The design method is based on a field expansion in suitably normalized eigenmodes which yields the modal scattering matrices of the three key building blocks used: the E-plane metal-insert section, the H-plane iris discontinuity, and the H-plane T. The theory includes the finite thickness of the diaphragms as well as the higher order mode interaction of all discontinuities within the complete diplexer structure. Computer-optimized design data for diplexer examples in Ku-band (12-18 GHz) and E-band (60-90 GHz), designed for more than 23-dB common port return losses, are given. The theory is verified by measured results for a five-resonator Ku-band diplexer.<>