Peer Review Report For: Butler Matrix Based on Substrate Integrated Waveguide Without Crossover and Phase Shifter for Millimeter Waves Applications [version 1; peer review: 1 approved with reservations, 1 not approved]

Background A novel low-loss 4 × 4 Butler Matrix structure for 26 GHz operation is designed in this study. The proposed 4 × 4 antenna Butler matrix comprises an inscribed structure, which makes it fundamentally different from traditional beam-forming networks in 5G 26-GHz applications. Method This novel Butler matrix comprises four hybrid couplers without crossovers or phase shifters. To reduce the size, magnitude, and phase difference, and to increase the bandwidth, interdigital and non-metallic vias are used in the coupler design. The performance of the proposed matrix was simulated using the CST software and implemented on a Rogers 8085 substrate. It was also interfaced with four substrate-integrated waveguide slot antennas to demonstrate its distinct beam scanning capability. Results The matrix yielded an error magnitude loss of 1 dB and a phase error of 1.7° with a broad operational bandwidth of 3 GHz. The measured coupling factor of the 4 × 4 Butler matrix at 26 GHz is -6 ±1 dB at the output. The measured results validate that four phase-scanning states at -14.3°, 14.88°, -32.21°, and 31.33° can be realized with return losses of less than -10 dB. Conclusions The proposed design achieves a compact, efficient, and low-loss beamforming solution suitable for 5G applications at 26 GHz, demonstrating distinct scanning features without the need for crossovers or phase shifters.