Propagation characteristics of very low frequency vortex electromagnetic waves in an anisotropic ionosphere

We report very low frequency (VLF) orbital angular momentum (OAM) vortex electromagnetic waves propagation in an anisotropic ionosphere by proposing a Hertizan dipole array antenna. The circular array with linear insulated antennas is designed and demonstrated to generate vortex waves carrying OAM modes. To match the impedance of the ionosphere with high refractive index in the VLF band, electromagnetic characteristics of single linear antenna in the ionosphere is adjusted by selecting proper radii and electrical length. Effects of high and anisotropic refractive index of the time- and space-varying ionosphere on the OAM spectra and phase profiles generated by the array antenna are analyzed. Numerical results show that the propagation features present obvious differences if OAM vortex electromagnetic waves change their topological charges. The VLF vortex waves can provide an efficient avenue to increase the spectrum efficiency, more information will be offered by the OAM VLF waves for wireless communications and information transmissions.