Effect of the Orientation of the Array Elements of Uniform Circular Antenna Arrays on Orbital Angular Momentum (OAM) Modes

In this paper, the effect of the orientation of the array elements of a uniform circular antenna array (UCA) on orbital angular momentum (OAM) communication is investigated. For the first time, UCAs are studied in which the elements are rotated in a systematic way around their center by a linearly increasing angle of rotation. It is shown here that these UCAs provide a smaller decrease in transmission over distance for some modes compared to other configurations used, but at the cost of increased mode conversion. For that reason the mode conversion and the decrease of the received mode power over distance are of particular interest. U CAs with dipoles normal and tangential to the array surface are compared. For the latter, the dipoles are rotated around their center with a rotation angle increasing linearly around the U CA. The transmission of the OAM modes is evaluated on a sphere, showing that the direction of the strongest transmission of the OAM modes varies with the rotation of the array elements. It is shown that the U CAs with radial (or azimuthal) dipoles provide the best mode transmission with little mode conversion. All investigations have been carried out using a tool based on the method of moments (MoM).