Design of Wideband 8×8 Butler Matrix using Composite Right/Left-handed Transmission Line for Multi-mode OAM Generation

In this paper, a wideband Butler matrix for the uniform circular array antenna (UCA) generating multi-mode orbital angular momentum (OAM) vortex wave is designed. Firstly, the novel network topology of a Butler matrix is proposed. For the purpose of design and optimization convenience, the 8×8 Butler matrix is separated into two different sub 4×4 Butler matrix modules and one connection-output module. Then several wideband microwave components used in a Butler matrix, such as 3 dB directional coupler and stable phase shifter with composite right/left-handed (CRLH) transmission line, are designed. To demonstrate the effectiveness of the design process, a Butler matrix working in 5-7 GHz is designed and fabricated. It is found that the simulation results are in good agreement with the measured data. The constant amplitude distribution and progressive phase differences of ±45^◦, ±90^◦ between the output ports are observed, hence the ±1, ±2 mode OAM waves can be generated by the proposed Butler matrix.