Dependence of Beamforming on the Excitation of Orbital Angular Momentum Modes

It is shown that beamforming and especially beam steering must excite the so-called orbital angular momentum (OAM) modes in circularly symmetrical antenna arrays, and the claim that such modes have been only recently exploited is not correct at all. It is also demonstrated that discrete radiating structures, like antenna arrays, are not capable of exciting pure (uncorrupted) OAM modes. The corruption is caused by an infinite number of higher-order modes, which is a direct consequence of the sampling theory. However, the impact of higher-order modes decays as the number of array elements increases. This leads to the fact that pure OAM modes can only be generated in the (continuous) limit of an infinite number of array elements, and proposes using transmitting and receiving arrays with considerably different number of elements for increasing the measurement accuracy of the intended fundamental modes. Closed-form expressions for the radiation patterns as well as graphical simulations justify the above-mentioned assertions. The array interelement coupling is finally considered, which shows some practical challenges related to the matching of multiple OAM modes.