A loop antenna for a VLF (Very Low Frequency) satellite transmitter

The Space Sciences Laboratory of The Aerospace Corporation is currently defining an experiment to test a loop antenna configuration as a very low frequency (VLF) transmitter in the ionosphere. The experiment is sponsored by the Naval Air Systems Command. The primary objectives of the experiment are to validate existing models for radiation by a loop antenna and to study the performance of the antenna in the ionospheric plasma. The antenna will be carried into orbit in the payload bay of the Space Shuttle. During the radiation tests it will be deployed above the payload bay by the remote manipulator system. A VLF receiver aboard a subsatellite will be used to map the radiation pattern of the antenna by measuring the field intensities at distances of 1 to 100 Km from the transmitter. Calculations predict that the antenna impedance will only be slightly modified by the plasma and that the link to the receiver can be closed at distances well beyond 100 km. A one-third scale model of the antenna has been constructed. Impedance measurements have been made on the model in a 5-m diameter space plasma simulation chamber at NASA Lewis Research Center. The measurements confirm that the reactance of the antenna in an ionospheric plasma is essentially identical to its free space self-inductance. The effective series resistance of the circuit increases with frequency. The losses are attributed to power transferred to plasma turbulence.