Generation of Continuously Variable-Mode Vortex Electromagnetic Waves With Three-Dimensional Helical Antenna

A three-dimensional helical antenna is proposed to generate the orbital angular momentum (OAM) electromagnetic waves with continuously variable-mode (CVM) OAM modes at different resonant frequencies, in which with increasing the resonant frequency, CVM OAM electromagnetic waves can be produced efficiently. We have analyzed the principles of the single-arm and double-arm helical antennas to generate CVM OAM electromagnetic waves with a higher purity of OAM states, and an improved double-arm helical antenna has been designed and proposed. First, a novel feeding network is designed with the simplified feeding structure. Then in order to reduce the height of the structure, a circular-ring conductive disk is loaded on the top of the helical arm. Besides, a cylindrical cavity has also been supplemented outside the structure to reduce the back radiation without affecting the performance and size of the antenna. We have also fabricated and measured the proposed antenna, from which we can confirm our design is efficient for generating the CVM OAM electromagnetic waves.