Design of Miniaturized Broadband Butler Matrix for 5G Beamforming System

This paper proposes a compact 4 × 4 Butler matrix (BM) with broadband characteristics within the 5G NR frequency range (3 GHz ~4.5 GHz). To achieve broadband performance, 3 dB broad-side coupler with optimal parameters was implemented, and the circuit was designed with a strip line based on the FR-4 four-layer structure to reduce the size. The average implementation loss was measured as −1.5 dB, and the reflection coefficient was measured as over 15 dB across the entire frequency range. Phase differences of −45°, 135°, −135°, and 45° were confirmed, with an average phase error of ±3°. When the antenna was mounted, beam steering angles were measured as 10.29° ± 1°, −32.41° ± 1.5°, 32.41° ± 1.1°, and −10.29° ± 2.5°. The size of the manufactured compact BM was 40×38.6×1.2 mm3, and it achieved a size reduction of 30% and a bandwidth improvement of 2.54 times compared with the legacy BM using the same frequency. Ansys HFSS was used to design and simulate the BM, and the results suggested that this wideband compact BM could be used for the miniaturization of 5G beamforming systems.