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The conditions for endfire radiation from a traveling-wave slot are discussed. These are that the phase velocity of the traveling wave be equal to or less than the velocity of light in free space and that the aperture be excited with a strong longitudinal component of electric field. Practical ways of achieving these conditions are mentioned and an approximate analysis based on partially dielectric-filled waveguide theory is used to determine the aperture fields as a function of the antenna geometry. Far-field pattern characteristics are discussed. It is found that the actual pattern may differ considerably from that predicted by simple theory. This is attributed primarily to two things, a finite ground plane and the discontinuity produced by the abrupt aperture opening. Two practical discontinuity minimizers are described. The ground plane effect is analyzed by applying Huygens Principle, and a simple procedure is described for determining the first-order effect. The problem of obtaining pencil-beam radiation in the horizontal (plane of the ground plane) pattern is solved quite simply by using wide-aperture antennas which can be made to operate satisfactorily over better than a 2:1 band. The efficiency and vswr of a practical antenna, with sidelobes in the horizontal pattern at least 20 db down, have been measured over a 2:1 band. The efficiency varied from 65 per cent at the high-frequency end of the band to 55 per cent at the low end. The vswr was less than 1.4 over the band.