Abstract
AbstractHigh‐power transmission experiments in the very low frequency (VLF) mode have been conducted by the US Air Force Research Laboratory’s Demonstration and Science Experiments (DSX) satellite in the radiation belts using a novel transmitter that automatically tunes to find the resonance frequency of the transmitter circuit including the antenna. The resulting voltage–frequency curves are used to derive antenna impedance at the resonance. The analysis shows that the antenna reactance is far less than that of a dipole antenna in free space. The derived radiation resistance is up to several tens of kilo Ohms. Most interestingly, it is found that the radiation resistance is inversely proportional to the square of transmission wave frequency. The transmitted power can be up to 80 W for the DSX transmitter with an 82‐m long tip‐to‐tip antenna, showing that the high‐power VLF transmission is feasible. Whistler wave transmission inside the higher‐density plasmasphere is more efficient. Data analysis indicates that the antenna impedance does not vary systematically with the antenna orientation angle relative to the ambient magnetic field. The previous dominant theoretical studies yield not only incorrect values of the impedance but a completely different frequency dependence than that derived from DSX experiments. Instead, the recent theories correctly capture both the antenna impedance magnitude and the frequency dependence.
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