This article presents a novel 4 Chi 4 Butler matrix implemented in the grounded co-planar waveguide (GCPW) technology, compactly integrated with a highly efficient and broadband 1 Chi 4 air-filled substrate integrated waveguide (AFSIW) cavity-backed patch antenna array (AA), giving rise to a broad operational frequency range 23.75, 31 GHz covering the n257, n258, and n261 fifth-generation (5G) bands. Three novel quadrature hybrid couplers and two crossovers are designed and compared to obtain the optimal building blocks for the Butler matrix. Within each of the supported 5G bands, the measured excess insertion loss of the optimized Butler matrix remains smaller than 3.5dB with a maximal amplitude imbalance of +/- 0.9dB. Isolation between input ports is higher than 16.4dB. A maximal measured realized gain of 12.3dBi is obtained for the Butler matrix with integrated 1 Chi 4 AA while ensuring a -3-dB beamwidth coverage of 110 degrees. The main beamsteering directions of [-40 degrees, -14 degrees, 14 degrees, 40 degrees] exhibit measured deviations that stay within +/- 3 degrees. The fabricated Butler matrix with AA features a very compact footprint of 21.4 mm Chi 46.0 mm Chi 2 mm [2 lambda(0) x 4.3 lambda(0) x 0.2 lambda(0)].