OAM-Generating Transmitarray Antenna With Circular Phased Array Antenna Feed

A transmitarray antenna (TAA) with a small-scale circular phased array antenna (PAA) feed is proposed in this article to generate orbital angular momentum (OAM)-carrying radio beams. After analyzing the operating mechanisms of the OAM-generating PAA-fed TAA, its numerical model is further developed to determine the TAA element arrangement and the PAA feed excitation coefficients. Furthermore, a three-layer element with polarization-conversion property and the traditional patch antenna element are identified as the elements of the TAA and PAA feed, respectively. Benefiting from the intrinsic multibeam capabilities of the TAA, the steering of the zero-OAM-mode beam is firstly discussed, where a conical scanning across an angular range of ±25° is realized. Meanwhile, the creation of multiple pure or mixed OAM beams with helical phase fronts is also presented, where the superposition of multiple OAM states provides more possibilities to increase the channel capacity and spectrum efficiency. To facilitate the experiments, an 8\times 8 Butler matrix with a bandwidth over 30% is designed, which integrated with the array-fed TAA also provides an attractive solution to simultaneously generate multiple OAM modes without requiring any active modules. The experimental results agree reasonably well with the numerical ones, validating the feasibility of our idea to design a high-gain OAM-generating PAA-fed TAA.