Fringing effects in re-entrant and inverted re-entrant turnstile waveguide junctions using cylindrical resonators

One waveguide network is a five-port turnstile junction consisting of three H-plane waveguides with a circular quarter-wave long resonator on its axis, open circuited at one end and short circuited at the other. It is readily converted to a turnstile three-port circulator by replacing the dielectric resonator by a gyromagnetic one and closing two of the five ports. Another network that may be reduced to a three-port circulator is a seven-port turnstile junction containing a pair of similar coupled resonators or a half-wave long arrangement open circuited at each end. The introduction of suitable symmetry planes in the latter two geometries suggests that all three have, in the absence of any fringing effects, the same centre frequency. In practice, however, this is not so. This study employs an finite element (FE) process to investigate this assumption in the cases of re-entrant and inverted re-entrant resonators. One conclusion of this work is that an FE or similar numerical tool is mandatory for the adjustment of such a degree-2 circuit.