Scattering of Toroidal Waves by Semitransparent Reflectors of Revolution in Application to Omnidirectional Antennas

Problems of radiation pattern synthesis, sidelobe suppression, or back radiation suppression appear when using antennas with reflectors or ground planes. The use of a semitransparent surface allows the required shape of a radiation pattern to be synthesized efficiently through the variation of reflection and transmission coefficients. Effective radiation pattern synthesis is possible when using analytical formulas designed for the radiation patterns of antennas with semitransparent reflectors. In this article, we present a model for the scattering of a toroidal wave of a general form by means of a semitransparent reflector of revolution. The model allowed us to simulate the radiation patterns of various kinds of axisymmetric antennas with semitransparent reflectors and ground planes. Two asymptotic expansions of the radiation pattern of the model were found in the physical optics approximation. The expansions allow the radiation pattern for the entire space to be determined. As an example of its application, we used the model to calculate the radiation patterns of a plane with Archimedean two-wire spiral antenna and a loop antenna above concave and convex reflectors, as well as an Archimedean slot spiral antenna with a ground plane. We synthesized the optimal profiles of perfectly conducting and semitransparent reflectors and ground planes to improve the front-to-back ratio of the antennas.

I. Introduction

The use of semitransparent surfaces is a novel trend in antenna engineering for applications in which special shapes of radiation patterns are required. Semitransparent ground planes provide back radiation suppression of omnidirectional antennas [1]–[4]. Reflectors with semitransparent edges provide a desirable sidelobe level and reduce a backscattering main lobe [5], [6]. Semitransparent disk ground planes and reflectors of revolution are often used with axisymmetric antennas, such as loop antennas, circular patch antennas, monopoles, or conical horns. The optimization of the transparency distribution on the surface of a semitransparent reflector is the main issue in the synthesis procedure for obtaining the required shape of a radiation pattern. Therefore, a simple model that allows the radiation patterns of various omnidirectional antennas with semitransparent reflectors to be calculated using analytical formulas is useful for the optimization issue.