Low Loss Wideband 4×4 Butler Matrix Networks Based on Substrate Integrated Waveguide for 5G Applications

Abstract

Current wireless communications urgently need to provide huge data rates, high gain and high directivity radiation pattern beams. Therefore, beamforming networks (BFNs) are introduced to provide these needs. Butler matrix (BM) is a type of beamforming network, which can be realized using fixed network circuits and feeds the antenna array. BM at high frequency suffers from components loss and phase error for massive network, especially when it is implemented using common microstrip structures. Different transmission lines such as waveguide and substrate integrated waveguide (SIW) are studied and introduced to realize Butler matrix. SIW structures are good candidate for the implementation of BM due to its property of low loss transmission line which comprises of properties of microstrip and waveguide technology. However, SIW antennas and structures at millimeter waves have unwanted radiation loss coming from the vias holes. In addition, the vias separation distance is dependent on waveguide size, which leads to a more massive beamforming network at 26-GHz. Hence, this thesis is proposing a more size-friendly and optimal SIW antenna beamforming structure to reduce the vias loss and provides higher bandwidth and gain at 26 GHz. The BM components such as 3 dB coupler, 0 dB crossover, and 45-degree phase shifter are designed by implementing metallic vias determination method. Size and distance of vias are the most important factors in determining the coupling ratio and phase shifts at output ports. Hence, the coupler is designed with different vias width and distance to obtain the correct phase and coupling at output ports. Then, the designed coupler is cascaded to form a 0-dB crossover. The phase shifter is designed with alerting vias distance inside the coupling area of SIW structure. The last component in beamforming is the design of SIW slot antenna based on longitude slot distributions. All the components are integrated to form a $4\times4$ BM with four slot antennas attached to BM networks. Microtrip separation feedline used to for coupler, crossover, BM and BFN. The proposed designs are simulated using CST software and fabricated by PCB LPKF ProtoMat printer. The outcomes of wide bandwidth with more than 1-GHz and high directive gain of more than 10-dB for the beamforming network are achieved. The output power of the BM is between −6-dB to −8-dB at all four ports with phase difference error less than 5°. Four directive beams are achieved at beam scanning of −14°, −41°, 40°, and −14° at port 1 to port 4 respectively. Hence, this $4\times4$ -BM with four slot antenna theses have introduced a successful design of an antenna beamforming network based on SIW technology with significant characteristics at 26-GHz.

Keywords

• 5G
• Butler matrix
• coupler
• crossover
• beamforming network (BFN)
• millimeter waves