Experimental characterization of plasma antennas

Plasma antennas use plasma elements instead of metal conductors. They are constructed by an insulating tube filled with low pressure gases: the plasma is rapidly created and destroyed applying proper radio frequency (RF) power pulses to the discharge tube so that the antenna is switched on and off. When the antenna is off, the plasma is non conducting, and therefore the tube is practically transparent, so if the antenna is used into an array configuration, its presence does not affect the radiation pattern of the other elements. When the plasma is on, it exhibits a high conductivity, providing a conducting medium for the applied RF signal. The main advantage in using plasma antennas instead of metallic elements is that they allow an electrical rather than mechanical control. In particular, for military applications the possibility to have conducting elements only when the useful signal needs to be transmitted make difficult the antenna detection by hostile radars. Moreover, a plasma antenna array can be rapidly reconfigured, to change the radiation pattern, without suffering perturbation from the unused element. Finally, the antenna effective length can be changed controlling the applied RF energy, allowing to vary its resonance frequency and therefore the useful bandwidth is increased. Recently, the physical properties of plasma antennas were investigated [1], showing the relationship between the plasma density and the applied power. The noise generation was also investigated. In the present work, a plasma antenna is characterised for the application in the VHF range. A test bench is set up to control both the delivered power at the exciting frequency and the power used to transmit the useful signal. The problem related to the load change passing from the “on” to the “off” condition is highlighted, together with the issue of the crosstalk between the pump and the signal circuits. The efficiency of the antenna is determined comparing its radiation pattern with that of a metallic antenna.