Millimeter-Wave Three-Layer Substrate-Integrated9x9 Butler Matrix and Its Application to Wide-Angle Endfire Multibeam Metasurface Antenna

A novel three-layer substrate-integrated waveguide (SIW) 9 x 9 Butler matrix (BM) that can produce a broadside beam and wide-angle coverage is presented in this article. Unlike the traditional 8 x 8 BM, the proposed BM is realized by cascading three-way couplers. Connecting to the radiation structure and feeding the center input port, a phase difference of 0(degrees) is observed and a broadside beam can be produced. When exciting the side input ports, a maximum phase difference of +/- 160 (degrees) is obtained and wide-angle side beams are generated. The operation principle and design development of the 9 x 9 BM are illustrated in detail to determine the topology and the phase shifters. With the desired phase differences and amplitude distributions, a three-layer topology of the 9 x 9 BM is developed to reduce the footprint and improve the compactness, which is subsequently realized in SIW technology. To realize wide-angle coverage, a new endfire metasurface antenna derived from the traditional Vivaldi antenna is proposed, which achieves a fractional 10-dB impedance bandwidth of 72% and a half-power beamwidth (HPBW) of 128 (degrees) at 28 GHz. By integrating the three-layer SIW 9 x 9 BM with an endfire metasurface antenna array, a wide-angle endfire multibeam metasurface antenna is obtained, which achieves a wide HPBW coverage of +/- 113 (degrees) and a maximum gain of 12.1 dBi. The topology of an extended single-layer 18 x 18 BM based on the designed 9 x 9 BM is presented, and multilayer solutions are adopted to demonstrate the possibility of realizing higher order BMs.