Broadband Vortex Beams Generation With Narrow Divergence Angle Using Polarization Insensitive Metasurface

In this paper, we propose a polarization insensitive metasurface to generate broadband vortex beams with narrow divergence angle. The metasurface is composed of 30 x 30 I-shaped metal elements printed on the F4B substrate with metal ground plane. The wideband is realized by optimizing the parameters of I-shaped unit cell to adjust the resonant frequency. A 360 degrees phase range is achieved by rotating the element along the central axis. The phase superposition method is used to design the metasurface to generate vortex beams with narrow divergence angle. The proposed metasurface can generate broadband vortex beams under circular polarization (CP) and linear polarization (LP) wave excitation. The far-field radiation patterns and near-field characteristics of the metasurface are simulated to verify the proposed design. The gain of vortex beams is better than 15 dBi within 12-20 GHz (relative bandwidth of 50%), and the maximum gains are 23.2 dBi, 25.3 dBi for CP and LP cases, respectively. The broadband metasurface is fabricated and measured, and the measured results agreed well with the simulated ones. The proposed design provides a convenient and flexible way to realize broadband vortex beams with narrow divergence angle.