Current Distribution and Input Impedance of a VLF Linear Antenna in an Anisotropic Plasma

In this paper, the problem of a very low frequency (VLF) linear antenna in an infinite homogeneous anisotropic plasma is treated both analytically and numerically. Based on the current distribution formula in an isotropic medium, the analytical expression for the current distribution in an anisotropic plasma is derived. The current distribution under an anisotropic condition is composed of the contributions from both the ordinary and extraordinary waves, and thus has a more complicated form. Computations show that the amplitude coefficients for the ordinary wave are obviously greater than those for the extraordinary wave, and the current distribution in an anisotropic plasma is more sensitive to parameter changes. The input impedance of the antenna becomes much larger in this case because of the high refractive index of the ionosphere at the VLF range. Despite that, the impedance can still be adjusted to a small value by selecting a proper electrical length. Finally, the method proposed in this paper may help improve the efficiency of a space-borne linear antenna by determining the optimal antenna parameters.