This paper proposes two low divergence angle orbital angular momentum (OAM) Fabry-Perot (F-P) antennas with nonuniform superstrates. There are two steps to design the proposed two F-P antennas. First, two primary array antennas (a slot array antenna and a patch array antenna) are designed. Both antennas can generate OAM vortex beams with a mode of -1. Second, two F-P resonator cavity antennas are formed by loading a nonuniform partially reflective surfaces (PRS) superstrates above the two primary antennas in order to increase the antenna gain and reduce the divergence angle. The PRS is designed non-uniform for increasing the aperture efficiency. The measured results indicate that the F-P OAM antennas can obviously improve the performance of primary OAM antennas: (1) for the slot array antenna, the divergence angle reduces from 27° to 18°, and the maximum gain increases from 5.2 dBi to 7.5 dBi; (2) for the patch array antenna, the divergence angle decreases from 30° to 18°, and the peak gain increases from 3.4 dBi to 7.2 dBi.
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