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This paper describes the design and measured performance of an antenna made up of an array of dielectric-loaded rectangular waveguides with common narrow walls. Longitudinal slots are cut in the center of each broad wall. The preferred form of this antenna uses a slab of dielectric placed over all the slots, although it can operate when dielectric is placed within each guide. An approximate theory is discussed, and empirical design data are presented from which an experimental model of this antenna is designed. Empirical data are also given for cases in which dielectric is placed inside the waveguides. The radiated field from this antenna is parallel to the antenna aperture, and the main beam is directed up from the aperture at an angle equal to the arc cosine of the velocity of light divided by the slotted-waveguide phase velocity. An experimental antenna, constructed using standard 0.900- by 0.400 in waveguide, had an aperture 9 in wide by 20 in long. The antenna was fed from a hog horn, which yielded an approximately sinusoidal plane illumination over the 9-in aperture width. The slot width was varied over the 20 in length to obtain a Taylor aperture distribution yielding a theoretical radiation pattern with -25.5- db -plane sidelobes and with a beamwidth. At the design frequency of 10 kMc, the -plane and -plane beamwidths were 5.4 and 8.0 degrees, respectively, and the -plane and -plane first sidelobe levels were -22.0 db and -23 db, respectively. Good radiation patterns were obtained from 8 to 11 kMc.