Generation of Multi-mode Vortex Electromagnetic Waves Based on Helical Antenna

In this paper, we present a novel and a practical method to generate multi-mode orbital angular momentum (OAM). The OAM modes of l=1, 2, 3, 4 can be generated at 2.5GHz by using four helical antennas arranged like concentric circles which is called the helical antenna array (HAA). To prove the generation of the OAM waves further and evaluate the quality of OAM modes, the purity of each mode is calculated by the Fourier transform at 2.5GHz. As a result, the purity of four modes is all above 65%. The generation of OAM waves is very important for the application of OAM in the field of radio frequency. And the generation of multi-mode OAM waves also lays a foundation for the multiplexing of vortex waves.