Generations of Multiple Orbital Angular Momentum Modes by 2-D Fisheye-Lens-Based Beamformer Excited Antenna Arrays in a Spiral Configuration

This article presents the generations of multiple orbital angular momentum (OAM) modes in a spiral antenna array configuration. The feeds to the antenna elements are produced from a Fisheye-lens-based beamforming circuit (FL-BFC), where relatively linear phase variations of different slopes are generated. By selecting proper slope values, the feeding to planar circular arrays results in orthogonal OAM modes. In addition, selecting proper output port positions on the circumference of the FL-beamforming network (BFN) may produce phases to excite the antenna array in various configurations to reduce the nulling regions of radiation by different OAM modes. This article presents the characteristics of FL-BFC in its implementation to excite a planar array of antennas for OAM mode radiations. The orthogonality characteristics are quantized using an envelope correlation coefficient (ECC) for accounting for the mutual interaction. An orthogonalization procedure is also examined to obtain the orthogonal OAM modes. The radiation characteristics are compared to demonstrate the feasibility. Both full-wave simulation and measurement results are shown to validate the design concept.