Generating Vortex Electromagnetic Wave Using Cone-Shaped Helix Antenna

This paper proposes a cone-shaped helix antenna for generating multimode vortex electromagnetic (EM) waves of small divergence angle and high gain. Four-arm Archimedean helix lines are printed on a conical substrate and fed by four wave ports to generate vortex EM waves with orbital angular momentum modes (OAM)
$l= 0, 1, 2, 3$
. By changing the angle of the conical substrate, it can effectively reduce the divergence angle of the vortex EM wave. The divergence angle of vortex EM wave with OAM mode
$l= 1$
reduces from 38.4° to 14.6° when the cone angle changes from 1° to 30°. In addition, a conical metal reflector is placed
$\lambda_{0}/4$
away from the conical substrate to further reduce the divergence angle and improve the gain. When the cone angle is 30°, simulation results indicate that the divergence angle of vortex EM wave with OAM mode
$l= 1$
reduces about 2.5° and the gain increase about 3 dB after adding reflector. This paper can reduce the divergence angle of vortex EM wave, which is of significant importance for further applications of vortex EM waves.