The paper presents a critical review of available solutions and design techniques. It then outlines three new and efficient algorithms, dedicated to the Jauman problem, which synthesize absorbers with Butterworth, equiripple or Chebyshev absorption characteristics'. Upper limits exist on the spacer dielectric constant for these solutions to be realizable, and are tabulated. An example of the results obtained, in the form of a design table for practical Chebyshev absorbers, is also presented. These designs are optimal in the sense of maximum bandwidth for fixed passband reflection coefficient magnitude. >

The design of Jauman absorbers

The present paper deals with a new efficient approach in order to assess the simulation of scattered fields from arbitrary metallic objects. The basic idea is to combine a ray tracing algorithm with the principles of physical optics (PO) and the physical theory of diffraction (PTD). The ray tracing algorithm stochastically launches discrete rays and uses a ray density normalization. In order to perform simulations at finite objects the PO/PTD formulation is required. Thus, fast intersection routines can be implemented, while the ray density formulation reduces the PO and PTD integrals to a pure sum of ray contributions. Simulation results obtained with this model are verified by comparison with both exact simulations using a method of moments (MoM) code and measurement results, proving an excellent accuracy and fast computation even at complex objects. With this asymptotic approach, scattering properties of large objects that are too complex for exact methods can be analyzed with rather moderate computation efforts. Typical applications include the simulation of low observability (LO) designs as well as the generation of databases for identifying unknown aircraft by their radar signature.

Ray tracing with PO/PTD for RCS modeling of large complex objects

New expressions are derived for the fringe current components of the equivalent edge currents. They are obtained by asymptotic endpoint evaluation of the fringe current radiation integral over the "ray coordinate" measured along the diffracted ray grazing the surface of the local wedge. The resulting expressions, unlike the previous ones, are finite for all aspects of illumination and observation, except for the special case where the direction of observation is the continuation of a glancing incident ray propagating "inwards" with respect to the wedge surface (the Ufimtsev singularity).

Elimination of infinities in equivalent edge currents, part I: Fringe current components

The bidirectional analytic ray tracing (BART) method is developed to rapidly calculate composite scattering from three-dimensional (3D) electrically large complex targets above a randomly rough surface. Ray tracing is carried out both along the incident (forward) direction and converse direction of scattering (backward) recording different orders of ray illumination on each facet or edge of the target and surface. Once a pair of forward and backward rays meet on a facet/edge, a scattering term is constructed using the diffused scattering/diffraction of this facet/edge and all reflections occurred on the tracing paths. The rough surface is modeled with ldquorough facetsrdquo including coherent scattering and diffused incoherent scattering, which can be directly calculated according to the IEM (integral equation method) of a randomly rough surface. Analytic tracing of polygon ray tubes is developed to precisely calculate the illumination and shadowing of facets, which exempt large patches of the target from any finer meshing. It significantly reduces the complexity relevant to the target electric-size. Higher orders of scattering and, in particular, interactions between the target and rough surface are then taken into account. The accuracy and performance of BART is validated and evaluated by comparing with exact computational electromagnetic methods for electrically small targets. Numerical examples of angularly composite scattering from a three-dimensional electrically large, e.g., a ship-like target over a randomly rough surface are presented and discussed.

Bidirectional Analytic Ray Tracing for Fast Computation of Composite Scattering From Electric-Large Target Over a Randomly Rough Surface

Physical optics (PO) and the physical theory of diffraction (PTD) are used to determine the backscatter cross sections of dihedral corner reflectors in the azimuthal plane for the vertical and horizontal polarizations. The analysis incorporates single, double, and triple reflections; single diffractions; and reflection-diffractions. Two techniques for analyzing these backscatter mechanisms are contrasted. In the first method, geometrical optics (GO) is used in place of physical optics at initial reflections to maintain the planar nature of the reflected wave and subsequently reduce the complexity of the analysis. The objective is to avoid any surface integrations which cannot be performed in closed form. This technique is popular because it is inherently simple and is readily amenable to computer solutions. In the second method, physical optics is used at nearly every reflection to maximize the accuracy of the PTD solution at the expense of a rapid increase in complexity. In this technique, many of the integrations cannot be easily performed, and numerical techniques must be utilized. However, this technique can yield significant improvements in accuracy. In this paper, the induced surface current densities and the resulting cross section patterns are illustrated for these two methods. Experimental measurements confirm the accuracy of the analytical calculations for dihedral corner reflectors with right, acute, and obtuse interior angles.

Backscatter analysis of dihedral corner reflectors using physical optics and the physical theory of diffraction

Michaeli recently derived a set of equivalent edge currents for scattering directions not on the Keller cone. Some years ago, Mitzner developed his incremental length diffraction coefficient (ILDC) for the same purpose. It is shown that Michaeli's results relate to Mitzner's for arbitrary directions in identically the way Keller's relate to Ufimtsev's for directions on the Keller cone.

The relationship between Mitzner's ILDC and Michaeli's equivalent currents

Radar absorbent materials have been classically regarded as "resonant" when they are a quarter-wavelength thick. However, the actual thickness that minimizes the reflectance of a material is hardly ever a quarter-wavelength. If the material is dominantly magnetic in character, the required thickness for minimum reflection may even exceed a half-wavelength.

The thickness criterion for single-layer radar absorbents

The deformation of metallic flat plates into cylindrical segments, or the shielding of flat plates by such segments, reduces the large specular echo but may not necessarily reduce the mean cross section by more than 1 dB or so. A useful reduction in the mean return can be achieved only if the angle subtended by the cylindrical segment is significantly greater than the angular base interval over which the mean is taken. For all save very small intervals, the radar cross section reduction is independent of frequency.

Radar cross section reduction using cylindrical segments

The normal-incidence flat-plate performance of radar absorbers can seldom be realized when the absorbers are used on cylindrical structures. This is also true of Jaumann absorbers, which consist of alternating layers of resistive sheets and light-weight spacers. A method based on Richmond's stepping theory [4] is used to predict the performance more accurately for cylindrical structures.

Performance degradation of Jaumann absorbers due to curvature

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Papers

The relationship between Mitzner's ILDC and Michaeli's equivalent currents

The thickness criterion for single-layer radar absorbents

Radar cross section reduction using cylindrical segments

Performance degradation of Jaumann absorbers due to curvature