Korada R. Umashankar

A Novel Method to Analyze Electromagnetic Scattering of Complex Objects

The finite-difference time-domain (FD-TD) method is proposed as a means of accurately computing electromagnetic scattering by arbitrary-shaped extremely complex metal or dielectric objects excited by an external plane wave. In the proposed method, one first uses the FD-TD method to compute the near total fields within a rectangular volume which fully encloses the object. Then, an electromagnetic-field equivalence principle is invoked at a virtual surface of this rectangular volume to transform the tangential near scattered fields to the far field. To verify the feasibility of this method, the surface currents, near scattered fields, far scattered fields, and radar cross section of two canonical two-dimensional objects are presented. For these cases, it is shown that the FD-TD method provides magnitude of current and field predictions which are within ± 2.5 percent and further phase values within ± 30 of values predicted by the method of moments ( MOM) at virtually every point including in shadow regions.

Finite-difference time-domain modeling of curved surfaces (EM scattering)

The finite-difference-time-domain (FDTD) method is generalized to include the accurate modeling of curved surfaces. This generalization, the contour path CP), method, accurately models the illumination of bodies with curved surfaces, yet retains the ability to model corners and edges. CP modeling of two-dimensional electromagnetic wave scattering from objects of various shapes and compositions is presented. >

Electromagnetic scattering by arbitrary shaped three-dimensional homogeneous lossy dielectric objects

The recent development and extension of the method of moments technique for analyzing electromagnetic scattering by arbitrary shaped three-dimensional homogeneous lossy dielectric objects is presented based on the combined field integral equations. The surfaces of the homogeneous three-dimensional arbitrary geometrical shapes are modeled using surface triangular patches, similar to the case of arbitrary shaped conducting objects. Further, the development and extensions required to treat efficiently three-dimensional lossy dielectric objects are reported. Numerical results and their comparisons are also presented for two canonical dielectric scatterers-a sphere and a finite circular cylinder.

Calculation and experimental validation of induced currents on coupled wires in an arbitrary shaped cavity

An efficient numerical technique is presented for the calculation of induced electric currents on coupled wires and multiconductor bundles placed in an arbitrary shaped cavity and excited by an external incident plane wave. The method is based upon the finite-difference time-domain (FD-TD) formulation. The concept of equivalent radius is used to replace wire bundles with single wires in the FD-TD model. Then, the radius of the equivalent wire is accounted by a modified FD-TD time-stepping expression (based on a Faradays law contour-path formulation) for the looping magnetic fields adjacent to the wire. FD-TD computed fields at a virtual surface fully enclosing the equivalent wire are then obtained, permitting calculation of the currents on the wires of the original bundle using a standard electric field integral equation (EFIE). Substantial analytical and experimental validations are reported for both time-harmonic and broad-band excitations of wires in free space and in a high- Q metal cavity.

Review of FD-TD numerical modeling of electromagnetic wave scattering and radar cross section

Applications of the finite-difference time-domain (FD-TD) method for numerical modeling of electromagnetic wave interactions with structures are reviewed, concentrating on scattering and radar cross section (RCS). A number of two- and three-dimensional examples of FD-TD modeling of scattering and penetration are provided. The objects modeled range in nature from simple geometric shapes to extremely complex aerospace and biological systems. Rigorous analytical or experimental validations are provided for the canonical shapes, and it is shown that FD-TD predictive data for near fields and RCS are in excellent agreement with the benchmark data. It is concluded that, with continuing advances in FD-TD modeling theory for target features relevant to the RCS problem, and with continuing advances in vector and concurrent supercomputer technology, it is likely that FD-TD numerical modeling will occupy an important place in RCS technology in the 1990s and beyond. >

Radar Cross Section of General Three-Dimensional Scatterers

Two disparate approaches¿the finite-difference time-domain (FD-TD) method and the method-of-moments (MOM) surface -patch technique-which permit highly realistic modeling of electromagnetic scattering problems are compared. New results of induced surface currents and radar cross section are presented for an important three-dimensional canonical cube scatterer. It is shown that a high level of agreement for the two modeling approaches is obtained for this scattering example.

A hybrid moment method/finite-difference time-domain approach to electromagnetic coupling and aperture penetration into complex geometries

An approach is presented for the direct modeling of electromagnetic penetration problems which involves a hybrid technique combining the frequency-domain method of moments (MM) and the finite-difference time-domain (FD-TD) method. The hybriding is based upon a novel use of a field equivalence theorem due to Schelkunoff, which permits a field penetration problem to be analyzed in steps by treating the relatively simple external region and the relatively complex internal region separately. The method involves first, determination of an equivalent short-circuit current excitation in the aperture regions of the structure using MM for a given external illumination. This equivalent current excitation over the aperture is next used to excite the complex loaded interior region, and the penetrating fields and induced currents are computed by the FD-TD method. A significant advantage of this frequency domain/time domain hybriding is that no Greens function need be calculated for the interior region. This hybrid approach takes advantage of the ability of MM to solve exterior problems using patch models and also takes advantage of the ability of FD-TD to model in great detail localized space regions containing metal structures, dielectrics, permeable media, anisotropic or nonlinear media, as well as wires.

The Finite-Difference Time-Domain Method for Numerical Modeling of Electromagnetic Wave Interactions

This paper succinctly reviews the background and formulation of the finite-difference time-domain (FD-TD) method for numerical modeling of electromagnetic wave interactions with arbitrary structures. Selected 3-D results are given showing comparisons with both measured data and other numerical modeling approaches. An assessment is made of the present horizon of FD-TD modeling capabilities, and possible future directions.

Electromagnetic excitation of a wire through an aperture-perforated conducting screen

Integro-differential equations are formulated for the general problem of a finite-length wire excited through an arbitrarily shaped aperture in a conducting screen. The wire is assumed to be electrically thin and perfectly conducting, and it is arbitrarily oriented behind the perfectly conducting screen of infinite extent. A known, specified incident field illuminates the perforated-screen/wire structure. The integro-differential equations fully account for the coupling between the wire and the aperture/screen. They are specialized to the case of the wire parallel to the screen with the aperture a narrow slot of general length. These special equations are solved numerically and data are presented for wire currents and aperture fields under selected conditions of wire/slot lengths and orientation. Data indicative of the coupling between the wire and slot are presented.

Numerical analysis of electromagnetic scattering by electrically large objects using spatial decomposition technique

Apparent computational difficulties with the direct integral equation and method of moments have prompted an alternative numerical solution procedure based on the spatial decomposition technique. Using rigorous electromagnetic equivalence, the spatial decomposition technique virtually divides an electrically large object into a multiplicity of subzones. It permits the maximum size of the method of moments system matrix that needs to be inverted to be strictly limited, regardless of the electrical size of the large scattering object being modeled. The requirement on the computer resources is O(N), where N is the number of spatial subzones and each subzone is electrically small, spanning on the order of a few wavelengths. Numerical examples are reported along with comparative data and relative error estimation to expose the applicability and limitations of the spatial decomposition technique for the two-dimensional scattering study of electrically large conducting and dielectric objects. >