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A radial line slot antenna (RLSA) with a quasi-elliptical aperture to create an elliptical beam is discussed in this paper. A simple feeding structure consisting of a coaxial feeder and two parasitic pins is designed to produce the feed pattern required for uniform illumination over the aperture. The slot length distribution in the radial direction is designed to be similar to conventional RLSAs for specified radial dimensions associated with the elliptical aperture. A 22 GHz quasi-elliptical RLSA with major and minor axes of 300 mm and 150 mm, respectively, was designed. The radiation pattern predicted by a method of moments (MoM) had 3 dB beam-widths of 2.46 and 4.9 degrees in the 0- and 90-degree cut-planes, and the directivity of 33 dBi corresponds to an aperture efficiency of 71.5%. The measured 3 dB beam-widths and the directivity agree well with the predictions. The measured antenna gain of 31.8 dB at 22.0 GHz corresponds to an antenna efficiency of 56%, while the axial ratio is 1.74 dB.