A Miniaturized Wideband 4×8 Butler Matrix for Beamforming in Millimeter-Wave Applications

This article presents our study on a wideband miniaturized $4\times8$ Butler matrix (BM) beamforming network based on a stacked metallic waveguide layout in millimeter-wave band. The size and complexity of the $4\times8$ BM have been substantially reduced by introducing a double-layer structure to avoid eight cross-overs and by using reverse feeding to eliminate four band limiting 180° phase shifters. Additionally, by introducing two kinds of wideband hybrid couplers in $H$ -plane and $E$ -plane and a broadband 45° phase shifter realized by loading inductive and capacitive irises in both paths, the operating bandwidth of the proposed $4\times8$ BM has been substantially increased. Furthermore, a Chebyshev amplitude distribution among the eight outputs has been realized through the unequal power dividers to suppress the sidelobes. The beamforming network and magnetoelectric (ME)-dipole array have been verified in the experiment, showing an operating bandwidth of 60–80 GHz with $S_{11}< -15$ dB. The sidelobe of the radiation pattern is below −14 dB at 70 GHz and the peak gain is 18 dBi in the operating band. The beam switching angles are ±11° and ±35° for each port, respectively. Compared with the reported similar work on $4\times8$ BM, the bandwidth is increased by threefold and the size is reduced by more than 50%.