Physical Characteristics of Plasma Antennas

Abstract

This experimental and theoretical study examines the excitation of a plasma antenna using an argon surface wave discharge operating at 500 MHz with RF power levels up to 120 W and gas pressures between 0.03 and 0.5 mb. The results show that the length of the plasma column increases as the square root of the applied power and that the plasma density decreases linearly from the wave launcher to the end of the plasma column. These results are consistent with a simple global model of the antenna. Since noise is critical to communication systems, the noise generated by the plasma was measured from 10 to 250 MHz. Between 50 and 250 MHz the excess noise temperature was found to be 17.2/spl plusmn/1.0 dB above 290 K. This corresponds to an ohmic thermal noise source at 1.4/spl plusmn/0.3 eV, compared with an electron temperature of 1.65 eV predicted by the global model. Estimates of the electrical conductivity of the plasma column based on measured electron number densities lead to an antenna efficiency of about 65% at a transmission frequency of 100 MHz and an increase in total antenna noise of 1 dB due to the plasma. Theoretical modeling and experimental observations of the radiation pattern of the antenna show that the linear variation of conductivity and finite resistance of the column lead to a reduction in the depth of the nulls in the radiation pattern and a consequent increase in the width of the main lobe.