A window assembly for a hollow waveguide of circular cross- section, with improved bandwidth and cooling capability for handling high microwave power transmissions over wide frequency ranges, is disclosed. A plate or disc of dielectric of refractive index n.sub.1 extends sealingly across the waveguide and has two parallel faces which exhibit a pattern of corrugations. One of these faces is in contact with a dielectric fluid of refractive index n. sub.2, and means are provided for cooling and circulating the fluid over said one face. Each of the corrugations extends into the fluid a distance proportional to the inverse of the geometric mean of the product of the refractive indices n.sub.1, n.sub.2. In a preferred embodiment, a second plate is included, separated from the first by a region in which said dielectric fluid is circulated, and in which at least both the faces in contact with the fluid are corrugated. The corrugations not only result in greatly improved matching over a broad band, but also greatly improved fluid flow and surface contact thereof over said one face, for enhanced cooling and thus power handling capability of the window assembly.
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