Wideband OAM Vortex Beam Generation Through a Thin Transmitarray in the Ka-Band

This letter proposes a thin transmitarray for generating wideband orbital angular momentum (OAM) vortex beams in the Ka-band. Our proposed element achieves a 360
$^{\circ }$
transmission phase within a 3 dB transmission magnitude, with a total thickness of 1.8 mm (0.18
$\lambda _{0}$
at 30.0 GHz). The symmetrical configuration of the unit cell enables its applicability in dual-polarized applications. We investigate the transmitarray performance across four OAM modes (
$l = +1, +2, +3, +4$
), specifically focusing on the +1 mode during fabrication and measurement. Our experimental findings validate the successful generation of a wideband vortex beam with OAM mode
$l = +1$
. The performance of the beam is observed over a broad frequency range from 27.0 to 39.0 GHz (about 40% OAM bandwidth). Moreover, the proposed structure exhibits a peak gain of 21.9 dBi with a corresponding aperture efficiency of 12.3%, while the overall mode purity exceeds 71%.