An Arbitrarily Multi-Feed VLF Linear Antenna in an Anisotropic Plasma

Abstract

For very-low-frequency (VLF: 3–30 kHz) space-borne transmit antennas, the conventional single-feed design often causes the current to concentrate near the feed point. As an alternative to increase the current moment without damaging the antenna, a multi-feed antenna provides a possible solution for enhancing the efficiency of VLF space-borne transmitting systems. In this article, we investigate a VLF linear antenna of arbitrary multi-feed in an anisotropic plasma and propose an analytical approach to evaluate its current distribution and input impedance. Both the effects of the ordinary wave and extraordinary wave under an anisotropic condition are considered, and the current distribution on the multi-feed antenna is derived and calibrated with the help of equivalent network theory and the moment method. Computations show that the currents on a multi-feed antenna are more uniformly distributed than a single-feed antenna, and the node input impedance can be optimized by setting a larger feed voltage in the middle or deploying the feed locations more averagely. Despite the high refractive index of the ionosphere in the VLF range, the input impedance of each node can still be small values by selecting a proper electrical length. The theoretical results are also compared with those simulated by commercial software, and the good consistency verifies the accuracy of the proposed method.