Three-dimensional profiling of collimated radio frequency orbital angular momentum beams

The use of orbital angular momentum (OAM) modes in radio communication is thought to enhance capacity. This work focuses on using the l = +1 mode transmitted from a 180 mm diameter, 8-element circular antenna array. The transmitted OAM beam was collimated by using a spherical mirror and the intensity and phase were investigated. A xyz scanning stage was used to profile the propagating OAM beam in three dimensions, resulting in a detailed investigation into the effects of collimation on the OAM beam. The proposed system was shown to reduce the beam divergence from 36.6° to 1.2°, without affecting the OAM mode purity of the beam for a frequency range of 4–6 GHz. This investigation showed a step towards realising practical control over the divergence of OAM-carrying beams.