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This paper describes the design analysis and measured performance of an antenna composed of an -plane array of parallel waveguides having quarter-wavelength-thick transverse slots extending completely across the array. Each relatively wide nonresonant slot in this array radiates only a small amount of power, and the dimensions of the slots are relatively uncritical. The radiated field from this antenna lies parallel to the transverse slots. The cosine of the angle between the direction of maximum radiation and the plane of the antenna is equal to the velocity of light divided by the phase velocity of propagation along the array. An experimental antenna was built with a radiating aperture 9 inches wide and 20 inches long. The antenna was fed from a hog horn which yielded an approximately sinusoidal -plane illumination over the 9-inch aperture width. The power coupled from the transverse slots was varied along the 20-inch length of the aperture to achieve a Taylor aperture distribution with -25-db -plane sidelobes. At the design frequency of 11 kmc, the -plane and -plane beamwidths were and , while the -plane and -plane first-sidelobe levels were -24.7 db and -24.2 db, respectively, in close agreement with theoretical expectations. The direction of maximum radiation was within of the design value at 11 kmc. Good radiation patterns were obtained with the antenna from 7.0 kmc, which is slightly above the cutoff frequency of the guides, to 11.4 kmc, which is slightly below the frequency at which spurious lobes are generated by the widely-spaced slots.