Study on divergence quantification of radio frequency orbital angular momentum beams through transmission properties

Microwave photonic (MWP) radars based on radio-frequency orbital angular momentum(RF-OAM) beams have been extensively researched recently, for their potential to provide high-azimuth and high-range resolution for radar imaging at the same time. However, the inherent divergence effect of RF-OAM beams limits the effective detection distance, which has become the main technical bottleneck of the long-distance MWP OAM radar. In this paper, the mathematical model of the MWP-controlled circular antenna array (CAA) is constructed, and the basic transmission properties of generated RF-OAM beams are investigated. Then each factor in the model is extracted for numerical simulation to explore its influence on the generation of RF-OAM beams, and the Bessel function term is found to be the factor leading to beam-divergence. Within the Bessel function term, the role that each physical element plays is analyzed, which is just consistent with our experimental experience and previous research results. Finally, the quantization equation of the RF-OAM beam divergence angle is fitted based on the properties of the Bessel function.