Waveguide-based Butler matrix beamforming network for millimeterwave applications

Abstract

The current wireless cellular system may suffer from congestion and spectrum shortage issue. Thus, higher frequency spectrum is introduced for wireless cellular system. However, at high frequencies, a higher propagation loss is expected. With smaller antenna element at millimeterwave band, more elements can be packed creating arrays making beamforming possible by controlling the signal phase. The Butler matrix beamforming network is adopted in this thesis due to its simplicity with capability to form the beam in desired direction by having different phases at the outputs. However, at millimeterwave the massive network can introduce significant losses on the components as well as the interconnections. Therefore, this thesis proposes a low loss waveguidebased structure where the signal is governed within the walls. Components of Butler matrix beamforming circuit are designed using waveguide structure prior to the integration with the antenna. The components are the 3-dB coupler, 0-dB crossover, and 45° phase shifter. The components are implemented using rectangular cavity resonators with iris coupling k-value control method. This iris coupling k-value controls the coupling and the phase shift of the Butler matrix components. By using the analytic technique of tuning k-value, the required coupling and phase difference at outputs can be obtained. The antenna is basically a very directive waveguide slots antenna. The slots are symmetrically distributed on both sides of the broad wall of the waveguide structure. This enables a dualbeam property. The structures are simulated using CST microwave software before fabricated using direct metal laser melting (DMLM) and selective laser melting (SLM) 3-dimensional (3D) printing techniques and measured using standard vector network analyser (VNA). The printed 4 × 4 Butler matrix has been measured and analysed. The measured reflection and isolation coefficients are observed to be less than -10 dB, with transmission coefficients ranging between -7 to -9 dB. The phase differences of - 42.02°, 42.02°, -130.95°, and 133.3° are observed at the outputs. The matrix has been integrated with four waveguide slots antennas. The measured results show the highest gain of 15.21 dB with scanning angles between 20° to 30°. Overall, the waveguide Butler matrix beamforming network shows good performance and has great potential for millimeterwave wireless systems applications.