The influence of complex waves on the radiation field of a slot-excited plasma layer

A study of the near field of a magnetic line source located inside an isotropic plasma layer reveals, in addition to a weakly-excited space wave, strong mode complex waves corresponding to poles in the integral representation of the field solution. Under these conditions, the radiation field may be calculated by means of a Kirchhoff-Huygens integration over the pole contributions in the near field. This result is in excellent agreement with the accurate steepest-descent evaluation whenever the complex waves are dominant. Depending on its location in the steepest-descent plane, each pole contributes either a broad peak at broadside or a sharp peak at an oblique angle. In the frequency range in which the plasma is opaque, the complex waves are spectral and yield a single radiation peak at broadside, and with small power. Also, surface waves may be present and contribute to the far field located near the plasma-air interface. When the plasma is transparent only leaky waves exist, the strongest of which accounts for a sharp major peak that closely corresponds to the critical angle obtained by geometrical optics. The weaker leaky waves may yield additional but minor peaks, or contribute at broadside only. The more practical case of a distributed source is also treated. Its solution is shown to be obtainable directly from that for the line source; the expression for the radiation pattern is then given by the product of the line source solution and that for the source distribution in the absence of the plasma layer.