Design of a High Return Loss 4 × 4 Butler Matrix Without Crossover for 5G Base Station

This paper presents a compact broadband 4 × 4 Butler matrix (BM) with high input return loss and without crossover components. The design employs four broadband 90-degree hybrids, each achieving 49% fractional bandwidth at a center frequency of 3.5 GHz with input return loss greater than 30 dB. The complete BM has a 20 dB return loss bandwidth of about 37% from 3 to 4.4 GHz and an insertion loss of less than 0.5 dB at the center frequency. Such high input return loss and low insertion loss are highly desirable in base station applications. A prototype of this structure is fabricated, and the measurement results are compared with the simulations. The measurement results show that this BM can cover 5G bands of N77 (3.3–4.2 GHz) and N78 (3.3–3.8 GHz) as well as the LTE bands 42 (3.4–3.6 GHz) and LTE band 43 (3.6–3.8 GHz) with 0.5 dB insertion loss, ±9° phase variations, and stable beamforming across 3.0–4.4 GHz. These features make the design highly suitable for 5G base station applications.