The waveguide re-entrant or inverted re-entrant turnstile circulator relies for its operation on either a quarter wave long cylindrical or prism resonator mounted on either a circular or an equilateral platform with its open face separated from the top waveguide wall by a suitable gap. The adjustment of the prism geometry, the main endeavour of this paper, is characterized by two degrees of freedom. One degree of freedom is the orientation of the prism inside the junction with respect to a typical waveguide feed. The others are the aspect ratio of the gyromagnetic resonator and the choice of platform or piston. The agreement between some calculations based on a Finite Element (FE) engine and experiment is excellent. The work undertaken here indicates that the preferred geometry is that for which the platform has the crosshairs section of the resonator.
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