10.1 INTRODUCTION

Designing a pilotless and sensorless vehicle with a low RCS might not be too difficult a task. However, useful platforms must always have sensors (and we could well include pilots in the category of sensors). Although the rules for shaping can often be applied to sensors, and RAM may sometimes be used to further reduce scattering into a threat region, designing low-RCS sensors is not an easy task. In this chapter we specifically focus on the scattering characteristics of antennas and on techniques for control of that scatter. More specifically, we concentrate on antennas that must operate in one or more of the threat radar bands, where simply covering the antenna with a RAM designed to provide attenuation out of the operating band is not an acceptable option.

For targets that have not been designed for low RCS and are dominated by specular scatter from large surfaces, the scattering from antennas is generally not felt to be of concern. Although that may often be true, antennas can also be the dominant scatterers for large RCS platforms. We show later that the RCS of an antenna due to its reradiation of an incident wave can be as large as that of a flat plate with an area equal to the antenna capture area. With that in mind, consider the fire control radar antennas aboard a platform such as a ship. Assuming a 2 m dish diameter, such antennas could have an RCS in K _u band (a...