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In this paper, a 20-GHz-band 64 x 64 hollow waveguide Butler matrix is proposed. By using two-plane short slot couplers and a modified diagram, a short-axis two dimensional Butler matrix is realized. The design method of the diagram and components is presented. The Butler matrix is composed of two plane hybrids, cross couplers and phase shifters. The designed Butler matrix is fabricated by milling and screwing together nine aluminum plates. The dimensions and weight are 86.40 mm x 74.19 mm x 396.34 mm and 7.0 kg, respectively. Transmission characteristics as a beam switching circuit are characterized by measurements, with a low insertion loss of less than 1.8 dB and an excitation error of 4-dB and 40-deg. standard deviation. Radiation characteristics as a 64-beam antenna are measured, and a wide coverage area of 40% of the hemisphere and high directivity of more than 18.7 dBi were determined by measurements.