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

We propose a simple method based on the Aperture Antennas Theory to understand in a simple fashion limitations of OAM antennas in far-field links. Additional insight is also given by analyzing the properties of the operators relating source and far-field distributions for a given order of the vortex. The outcomes fully agree with the results recently achieved by Edfors, Craeye, and co-authors, and emphasize some additional draw-back. The "degrees of freedom" of the fields associated to the different orders of the vortices are also discussed.