Four-OAM-Mode Antenna With Traveling-Wave Ring-Slot Structure

Based on metallic traveling-wave ring-slot structure, an easily realized four-orbital angular momentum (OAM)-mode antenna capable of generating four coaxially propagating waves with OAM modes of $l$ = $\mathbf {-3}$ , $\mathbf {-2}$, $\mathbf {+2,}$ and $\mathbf {+3}$, respectively, is experimentally demonstrated at 10 GHz. The feeding network is simply implemented using two $\mathbf {90^{\circ }}$ hybrid couplers. A ring-focus double-reflector with a favorable focus-diameter-ratio is specially designed to gain a high directivity. The main characteristics of the antenna—such as $S$-parameters, directivity, and near-field radiation—are presented both in simulation and experiment. Employing a pair of four-OAM-mode antennas, the experiment shows that the near-field OAM-based radio communication channels are naturally isolated.