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.

Comments on diffraction principles and limitations of RCS reduction techniques

In this paper, a general systematic procedure is presented for defining incremental field contributions. They may provide effective tools to describe a wide class of scattering and diffraction phenomena at any aspect, within a unitary, self-consistent framework. This procedure is based on a generalization of the incremental theory of diffraction (ITD) localization process for uniform cylindrical, local canonical problems with elementary source illumination and arbitrary observation aspects. In particular, it is shown that the spectral integral formulation of the exact solution for the local canonical problem may also be represented as a spatial integral convolution along the longitudinal coordinates of the cylindrical configuration. Its integrand is then directly used to define the relevant incremental field contribution. For the sake of convenience, but without loss of generality, this procedure is illustrated for the case of local wedge configurations. Also, a specific suitable asymptotic analysis is developed to derive new closed form high-frequency expressions from the spectral integral formulation. These expressions for the incremental field contributions explicitly satisfy reciprocity and are applicable at any incidence and observation aspect. This generalization of the ITD localization process together with its more accurate asymptotic analysis provides a definite improvement of the method.

Incremental theory of diffraction: a new-improved formulation

The electric energy in a lossless or lossy spherical particle that is illuminated with a plane wave or a Gaussian beam is investigated. The analysis uses a combination of the plane-wave spectrum technique and the T-matrix method. Expressions for the electric energy in any mode as well as the total electric energy inside the particle are given. The amount of energy coupling into the particle for different beam illuminations is also investigated. The high-Q (low-order) resonant modes can dominate the electric energy inside a spherical particle many linewidths away from the resonance location, particularly if the beam is focused at the droplet edge or outside the droplet. If the sphere is lossy, low-order modes can still dominate the electric energy if the beam is focused far enough outside the sphere. As the absorption coefficient of the particle increases, the energy in a high-Q mode decreases much faster at the resonance frequency than it does at near or off-resonance frequencies. The effects of the absorption on the dominance of the internal fields by a high-Q mode decreases as the beam is shifted farther away from the particle. As the beam is shifted farther away from the particle the fraction of the incident energy coupled into the sphere at resonance first increases and then decreases. Although the coupled energy decreases as the beam is shifted farther from the sphere, most of that energy is in the lowest-order mode.

Internal electric energy in a spherical particle illuminated with a plane wave or off-axis Gaussian beam

Exact expressions are obtained for the leading terms in the low-frequency expansions of the far field scattered by an arbitrarily shaped cylinder with finite cross section, an arbitrarily shaped cylindrical bump on a ground plane, and an arbitrarily shaped cylindrical dent in a ground plane. By inserting the low-frequency expansions of the incident plane wave and Green's function into exact integral equations for the surface current, integral equations are obtained for the leading terms in the low-frequency expansions of the surface current. Simple integration of these leading terms of the current expansion yield the leading terms in the low-frequency expansions of the scattered fields. The general low-frequency expressions are confirmed by comparing them to low-frequency results obtained from exact time-harmonic eigenfunction solutions. >

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Low-frequency scattering from two-dimensional perfect conductors

In this study, a multilevel fast dipole method (MLFDM) is proposed for the electromagnetic scattering from arbitrarily shaped three-dimensional (3D) perfect electric conducting targets to improve the equivalent dipole-moment method (EDM) The EDM views the basis functions as equivalent dipoles models and gives very extremely simplified forms for the impedance element However, it is incapable of saving memory and matrix-vector multiplication (MVM) time In the MLFDM, the multilevel grouping scheme is employed, and a simple Taylor-s series expansion of the distance between two interacting equivalent dipoles in far-group pair transforms the impedance element into an aggregation-translation-disaggregation form naturally at each level, which speeds up the MVM significantly Further more, the memory requirement can be reduced to O ( N ), where N is the number of unknowns Numerical results are presented to validate the merits of the MLFDM

Multilevel fast dipole method for electromagnetic scattering from perfect electric conducting targets

Incremental length diffraction coefficients (ILDCs) for the half-plane are integrated around the rim of a paraboloid reflector antenna to obtain well-behaved far fields of the nonuniform current for all angles of observation. These far fields, when added to the physical optics far field, produce a more accurate total far field of the reflector. Excellent agreement with the far fields obtained from a method-of-moments solution to the electric field integral equation applied to a 20-wavelength-diameter reflector shows that the cross polarization, farther-out sidelobes, and fields near nulls of reflector antennas can be appreciably modified by the fields of the nonuniform currents. ILDCs are also used to investigate the effect of cracks on the surface of reflectors that can result from the imperfect fitting together of panels to form large reflectors. Three models of cracks are studied. Significant pattern effects are found, depending on the model and orientation of the cracks. >

Correction to "Application Of Incremental Length Diffraction Coefficients To Calculate The Pattern Effects Of The Rim And Surface Cracks Of A Reflector Antenna"

A.D. Yaghjian

Papers

Correction to "Application Of Incremental Length Diffraction Coefficients To Calculate The Pattern Effects Of The Rim And Surface Cracks Of A Reflector Antenna"