Millimeter-Wave Three-Layer Substrate-Integrated 9 × 9 Butler Matrix and Its Application to Wide-Angle Endfire Multibeam Metasurface Antenna

A novel three-layer substrate-integrated waveguide (SIW) 99 Butler matrix (BM) that can produce a broadside beam and wide-angle coverage is presented in this article. Unlike the traditional 88 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° is observed and a broadside beam can be produced. When exciting the side input ports, a maximum phase difference of ±160° is obtained and wide-angle side beams are generated. The operation principle and design development of the 99 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 99 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° at 28 GHz. By integrating the three-layer SIW 99 BM with an endfire metasurface antenna array, a wide-angle endfire multibeam metasurface antenna is obtained, which achieves a wide HPBW coverage of ±113° and a maximum gain of 12. 1 dBi. The topology of an extended single-layer 1818 BM based on the designed 99 BM is presented, and multilayer solutions are adopted to demonstrate the possibility of realizing higher order BMs.