Mutual Coupling of Two Thin Infinitely-Long Slots Located on a Perfectly Conducting Plane in the Presence of a Uniform Plasma Layer

The investigation of antenna coupling was motivated by a study of communication with a hypersonic vehicle which upon re-entry, introduces a plasma sheath over the antennas. The model chosen for analysis consists of two infinitely-long thin slots on a ground plane covered by a uniform plasma layer which is assumed to be a lossless gaseous dielectric slab having a dielectric constant less than unity but greater than zero. The coupling effects are described in terms of a mutual admittance parameter in an equivalent π network from which other admittance parameters are derivable. The problem is formulated by spatial Fourier transforms which, upon inversion, would yield the desired results. The transform integral for the case of a thick plasma layer is evaluated approximately by the method of steepest descent. The results are explained in terms of multiple reflections of rays by the sharply defined plasmaair interface. When the quantities are plotted as functions of slot separation, the perturbations show up in the form of ripples about the curve for the unbounded plasma. In a realistic situation, there is no well-defined boundary and the rippling may show up in a statistical manner. The coupling effect is found to be less serious in the presence of a plasma than in its absence. When the operating frequency is appreciably higher than the plasma frequency, the change in driving point admittance is small. Also, inside a thick plasma layer, no unattenuating pole waves are excited.