Analysis and synthesis of in-line coaxial-to-waveguide adapters

An equivalent circuit for a class of in-line coaxial to waveguide adapters is presented. It takes into account the existence of two propagating modes in part of the transition, a phenomenon which had previously escaped attention. When treated correctly, it is possible to analyze the multi-step structure by cascading sections, enabling the design to be based on a synthesized prototype circuit, using minimal optimization. Broadband return losses of >40 dB have been obtained.