A Compact Phase-Modulated Transmissive Metasurface Convergence Lens for OAM Antenna

A compact phase-modulated transmissive metasurface (MS) lens is proposed to enhance the propagation performance of orbital angular momentum (OAM) vortex beams. The system integrates a compact circular ring phased array antenna (CRPAA) operating in the X-band to generate OAM beams and an MS lens designed using an optical converging axicon theory. The MS lens, measuring 4.17λ₀ × 4.17λ₀ × 0.169λ₀ (λ₀: centre frequency wavelength), modulates the phase distribution to focus OAM beams generated by the CRPAA, which has dimensions of 2.67λ₀ × 2.67λ₀ × 0.0372λ₀. When placed 60 mm (2λ₀) from the CRPAA, the MS lens significantly reduces the divergence angle of the OAM beam (mode l = −2) from 72° to 30° and increases the gain by 22%–89% across the operating frequency range. Additionally, the system maintains high OAM purity at a propagation distance of 150 mm (5λ₀), ensuring stable vortex wave emission. The compact design demonstrates significant improvements in beam focusing, miniaturisation and communication reliability, making it highly suitable for applications requiring precise OAM beam control and enhanced transmission performance. Simulation and experimental results validate the effectiveness of the proposed MS lens in reducing beam divergence and improving gain, highlighting its potential for advanced OAM-based communication systems.