A Reflectarray for Generating Wideband Circularly Polarized Orbital Angular Momentum Vortex Wave

A reflectarray for generating wideband circularly polarized (CP) orbital angular momentum (OAM) vortex wave in the X-band is proposed. First, a dual linearly polarized (LP) element consisting of two rhombus-shaped metal patches etched on two dielectric substrates is designed. The element has high isolations between two polarizations and preferable linear phase responses in wide frequency band. Then, the phase distribution of the reflectarray carrying the OAM vortex wave with mode l = 1 is designed, and dimensions of the elements are determined by their reflective phases. The far-field radiation patterns and near-field phase distributions are simulated to verify the proposed design. Finally, prototype of a 313-element reflectarray with an LP Vivaldi feed, which converts an incident LP wave into a reflective CP wave, is fabricated and measured. The results of measurement indicate that the reflectarray can generate CP OAM vortex wave with mode l = 1 in 9-11 GHz band (a fractional bandwidth of 20%). The reflectarray also achieves 20% 1 dB gain bandwidth, and its peak gain is 19.9 dB at 11 GHz. Except for a very narrow band (10.9-11.1 GHz), its measured 3 dB axial-ratio bandwidth is about 40%.