Broadband OAM vortex beams generating through transmitarray for millimeter wave applications

In this paper, we propose a wideband transmitting metasurface to generate orbital angular momentum (OAM) vortex beams for millimeter-wave communication applications, such as advanced wireless systems and 5G/6G networks. Our proposed element comprises four dielectric layers and four conductor layers, with a separation between layers set at 0.2 mm and the thickness throughout of 2.6 mm (0.26λ₀ at 30.0 GHz). The symmetrical arrangement and identical layers of the unit cell facilitate its usage in scenarios involving both polarization. We fabricate and test our proposed metasurface for OAM modes with l = +1 and l = +2 to validate its simulation performances. Experimental findings confirm that the proposed designs effectively achieve broad-frequency performance for vortex beams in both modes. The overall mode purity exceeds 85% for l = +1 and 75% for l = +2 across the sweep of frequencies ranging from 25.0 to 35.0 GHz, representing approximately 33.3% of the OAM bandwidth. Additionally, we achieve a maximum gain of 22.8 dBi for l = +1 and 21.1 dBi for l = +2 with aperture efficiencies of 15.1% and 10.2%, respectively.