Mitigation of Signal Attenuation in Radio Wave Transmission in ALISSA

A Laboratory Investigation in Signal Scattering and Attenuation (ALISSA) experiment is a newly constructed plasma experiment designed to study high frequency (>1 GHz) radio wave transmission through plasma. At speeds exceeding Mach 5, vehicles reentering Earth’s atmosphere generate enough heat to ionize the atmosphere as they move through it, creating a layer of plasma around the leading surface of the vehicle. This plasma layer interacts with radio transmissions between ground stations and the vehicle and can lead to signal attenuation and, in the worst case scenario, total loss of communication (referred to as blackout). ALISSA produces a steady state plasma with electron densities as high as 5*1016 m-3. Experiments have been performed in which S21 measurements were made between two antennas - a transmitter antenna inside of the plasma in ALISSA and a receiver antenna outside of the plasma in an anechoic chamber - that characterized the signal attenuation caused by the plasma. A novel antenna design which uses permanent magnets to alter plasma properties was then tested, with results showing a significant change in the attenuation pattern. Reduction in attenuation as large as 10 dB at certain frequencies was observed. The results of these experiments are presented.