Radiation Pattern of a VLF Linear Antenna in an Anisotropic Magnetoplasma

As the importance of spaceborne antennas keeps increasing in contemporary very low frequency (VLF: 3–30 kHz) transmitting systems, figuring out the radiation patterns of VLF spaceborne antennas of different lengths has become a critical issue that needs urgent solution. In this article, we propose an analytical method for calculating the radiation pattern of a VLF linear antenna immersed in an anisotropic magnetoplasma. The core idea of the approach is: 1) to consider the influence of current inhomogeneity to the radiation pattern so that the method is suitable for antennas of arbitrary length and 2) to derive the correction factor of the antenna under the far-zone condition. The results show that the radiation patterns for the extraordinary wave (E-wave) and ordinary wave (O-wave) are quite different, but in the near zone, both modes will contribute to the total field collectively. The radiation in the far zone mainly depends on the E-wave, but unlike isotropic cases, the radiation pattern in the anisotropic environment is always oriented along the geomagnetic field, with the shape of the pattern not influenced by the antenna length anymore. This is because the radiation field of a linear antenna can be regarded as the field excited by an electric dipole multiplying the correction factor, and variations on the correction factor only change the magnitude of the field, instead of the pattern.