Synthesis of open structures starting from closed-cross-section waveguide devices

This study presents a new methodology, based on the field modal expansion theory, to derive and analyse millimetre wave (mm-wave) ‘open’ cross-sections structures starting from standard closed cross-section design. The method is of great interest since W-band open devices offer many advantages over traditional closed structures in terms of risks for manufacturing, tuning and power handling in critical high-power devices. A hybrid scalar product, or ‘overlap’ integral, between the ‘open’ and ‘closed’ orthogonal basis of the open structure and the reference closed one has been defined, so introducing a figure of merit to evaluate if the derived open structure can have the same performances of the closed one in terms of fields confinement and distribution. As an example of the methodology, an ‘open’ version of a Gaussian horn and of a rectangular-to-circular WR10 waveguide transition have been successfully derived starting from classic ‘closed’ design. In order to fully test the method, the mm-wave rectangular-to-circular open-waveguide transition has been manufactured and measured (thanks to tapered close-to-open transitions and standard WR10 interfaces). The measurements well agree with full-wave numerical simulations and provide experimental validation of the proposed approach and fabrication technology. The method can be applied to other open microwave structures as well.