Comments on diffraction principles and limitations of RCS reduction techniques

Reduction of radar cross sections (RCS) is one of the major problems in stealth technology. Basic techniques used for RCS reduction are well known. They involve appropriate shaping, use of radar absorbing materials (RAMs), and passive and active cancellation of the scattered jield. Detailed description of these techniques and related bibliography have been presented in RCS handbooks. In this paper we discuss these techniques briefly from the physical point of view and summarize their limitations. Attention is concentrated on discussions of the physical structure of radar waves scattered from large objects (compared with the radar wavelength). These waves consist of ordinary and diffracted rays, thier beams, and shadow radiation. The latter is caused by the transverse diffusion of the radar beam in the vicinity of the shadow boundaries behind the scattering object, i.e., in the penumbra region. Shadow radiation creates strong forward scattering and is responsible for RCS enhancement at large bistatic angles. Possible passive and active techniques to control and reduce reflected beams, rays, and shadow radiation and potential limitations of these techniques are considered in the paper. In particular, it is emphasized that grazing rejlected rays and shadow radiation cannot be eliminated by absorbing materials.