Further experimental study of plasma sheath effects on antennas

The plasma sheath simulation technique developed earlier has been used to investigate the effects of various types of sheath discontinuities and inhomogeneities on a slot-antenna radiation pattern and input impedance. The results are especially applicable to re-entry environments. In the case of semi-infinite and finite-extent homogeneous plasma sheaths, the radiation pattern and the impedance characteristic are investigated with regard to the geometry of the discontinuity and its proximity to the slot antenna. It is found that the radiation pattern is unaffected by the discontinuity as long as the ratio of the distance between the source and discontinuity to the sheath thickness is of the order of 20 or greater. When the sheath is made finite in extent, with a discontinuity on each side of the slot, the resulting pattern is seen to be a superposition of the effects of each discontinuity acting separately. Investigation of the input impedance of the slot shows essentially no variation with discontinuity separation for separation-to-thickness ratios of at least 4. For smaller separations, the impedance exhibits significant variations, strongly dependent on the geometry of the discontinuity. The effects produced by inhomogeneities are found to be similar to those of a homogeneous sheath with a certain equivalent dielectric constant. For both the homogeneous and inhomogeneous sheaths, favorable comparisons with the available theory is obtained.