The radiation pattern of a microwave horn and a plasma layer

Theoretical and experimental studies have been made of the performance of a microwave horn terminated in a ground plane overlain by a plasma layer. The purpose was the determination of the effect of the plasma layer on the radiation pattern of the horn. Electron currents in the plasma were calculated on the basis of the assumption that a plane wavefront emanated from the mouth of the horn. The radiation from the horn was considered to be due to a magnetic current sheet that covered the horn aperture. The electron currents and the magnetic currents together were considered to constitute the source currents of an antenna. The experimental program consisted of measuring the influence of a transient gaseous discharge and afterglow on the radiation pattern of a horn. The operating frequency was 9745 Mc. The results indicate a diversion of energy to the off-axis region, as compared to the pattern of the horn in free space. The effect is significant for electron densities of the order of one-tenth of the critical electron density, for the geometry employed. The study is of value in considering the problem of communicating with a hypersonic vehicle during the re-entry phase.