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An antenna is described which consists of a broadside array of half-wavelength slots backed by a single cavity containing a filter that allows resonance with a mode suitable for the excitation of the array. A formula for the input resistance at resonance is derived using a mathematical model which is valid only if the cavity is highly underdamped and if the individual slot spacings exceed half a free-space wavelength. By proper choice of dimensions it is shown that the antenna can be matched to an external power source. Measurements were made on two arrays at 10 GHz, and results are consistent with calculations. The observed radiation patterns agree with expected field distributions provided there are no metallic projections on the plane of the array.