Chiral Metasurface Enabled Circularly Polarized OAM-Generating Folded Transmitarray Antenna With High-Gain Low-Profile and Broadband Characteristics

A broadband low-profile high-gain folded transmitarray antenna (FTA) is proposed to generate orbital angular momentum (OAM) with circular polarization. A chiral metasurface is employed as the top layer with the function of reflecting anticipant circularly polarized (CP) incident waves while transmitting the orthogonal ones. The geometric phase is then introduced to achieve 360° coverage of the transmission phase without affecting the reflection one. To obtain a high-gain and compact planar source, a $2\times2$ patch antenna array, integrated with the bottom metallic ground, is selected as the feed. Two proof-of-concept prototypes for vortex beam generation are constructed and experimentally measured. Both simulated and measured results demonstrate that $l=-1$ and +1 OAM mode beams can be generated from 8.7 to 9.5 GHz, and the measured maximum gains for $l=-1$ and +1 reach 18.1 and 18.3 dBi with the corresponding peak aperture efficiencies of 6.1% and 6.4%, respectively. Meanwhile, the axial ratios (ARs) of the peak of far-field patterns are less than 3 dB over the entire operating frequency range, providing broadband CP vortex beams. The proposed high-gain low-profile broadband FTA antenna shows a promising perspective in OAM-based wireless communication.