Planar Vortex Beam Generator for Circularly Polarized Incidence Based on FSS

In this article, a new technique to design a low-profile planar vortex beam generator is proposed based on microwave frequency selective surface (FSS), which is capable of generating orbital angular momentum (OAM) mode under circularly polarized incidence. Such OAM generator is composed of transmissive units distributed over a planar surface. Each unit cell, behaving as a spatial phase shifter, is composed of a stack of patches and grids separated by thin dielectric substrates. In order to reduce the thickness of the device, a rectangular patch is embedded in the middle layer to introduce an additional resonance. Compared to other resonant unit cells, this structure presents nearly perfect efficiency to convert circularly polarized incident wave to its cross-polarized component. A simple equivalent circuit model, composed of transmission lines coupled together with shunt capacitors and inductors, is presented to analyze this structure. The prototype of the proposed planar OAM generator operating in x-band is designed, fabricated, and experimentally characterized. Good agreement is achieved between full-wave simulations and measurements. It is demonstrated that the generator can achieve an efficiency of 85% at 10.7 GHz with an overall thickness of 0.067 λ₀.