Millimetre wave SIW diplexer circuits with relaxed fabrication tolerances

This study is concerned with the development of millimetre wave substrate integrated waveguide (SIW) diplexers, with an aim of relaxing fabrication tolerances. A method of designing SIW components initially in a dielectric filled waveguide (DWG) medium, and then translating a final optimised design to SIW is presented. Owing to the equivalence between SIW and DWG, the translated SIW components do not require a further optimisation which is a significant advantage as it speeds up the design process. The authors demonstrate this process by presenting the design of a Ka-band diplexer that consists of a highpass filter, hybrid coupler and low-order bandpass filter, where good agreement between the DWG and translated SIW cases can be observed. To investigate how the diplexer handles fabrication errors, the authors subjected the circuit to a tolerance analysis where simulated results suggest that the highpass filter in the diplexer is less sensitive to fabrication errors than the bandpass filter and its use in millimetre wave SIW diplexer circuits can help to relax sensitivity to fabrication errors. These observations are verified with the measurement of a fabricated Ka-band SIW diplexer, where the measured and simulated S-parameters are in very good agreement.