Measurements of dipole antenna impedance in an isotropic laboratory plasma

The impedances of relatively long dipoles in an isotropic laboratory plasma have been measured at 9.2 GHz ( band). A helium plasma was generated by discharging a capacitor between electrodes in a cylindrical container ten free-space wavelengths in diameter. Impedance measurements were made in the decaying afterglow plasma at electron densities both above and below the critical electron density ( cm^-3). The dipole antennas were fed from miniature solid-jacketed coaxial cables attached to a waveguide on which the voltage reflection coefficient was measured. The effect of plasma on the impedance of the dipole antenna was derived theoretically by treating the dipole as a dissipative transmission line. The theory described in a qualitative fashion the observed impedance variation with electron density for all the dipoles investigated. The measurements indicated certain antenna impedance properties, due to the plasma, which were not accounted for by the simple theory.