Orbital Angular Momentum Antennas: Understanding Actual Possibilities Through the Aperture Antennas Theory

In this article, we propose a method based on the aperture antennas theory to understand the limitations of orbital angular momentum (OAM) antennas in far-field links. Additional insight is also given by analyzing the properties of the operators relating source and farfield distributions for a given order of the vortex and emphasizing additional drawbacks. The degrees-of-freedom (DoF) of the fields associated with the different orders of the vortices are also discussed.