The ridged waveguide consists of a rectangular waveguide of 4 GHz band and a T-shaped aluminum ridge. Three kinds of gap spacing G, 2 mm, 4 mm and 6 mm between the waveguide wall and the plane plate on the ridge were provided. Cutoff frequency for respective waveguide is calculated by the equivalent circuit method (2) as 0.47 GHz, 0.66 GHz and 0.81 GHz. The slotted coupler (coupler II) is compared with the coupler I which has a simple open end. As an other parameter of the slotted coupler, gap spacing g at the mouth were varied for each waveguide. The VSWR generally increases in the absence of water which corresponds to the
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