Electromagnetic imaging of two-dimensional perfectly conducting cylinders with transverse electric scattered field

Electromagnetic imaging of two-dimensional perfectly conducting cylinders using measured transverse electric scattered field is studied in this paper. The contours of cylinders are denoted by local shape functions /spl rho//sub i/=F/sub i/(/spl theta//sub i/) in local polar coordinates which are then approximated by closed cubic B-splines instead of trigonometric series. By using the boundary condition of vanishing tangential electric field on surfaces of perfectly conducting cylinders, a set of electric field integral equations governing the scattering problem is derived. The scattering problem is solved by a point-matching method with pulse basis and Dirac delta testing functions. The inverse problem is reformulated as an optimization problem and solved by a real-coded genetic algorithm with closed cubic B-splines local shape function. Numerical examples show good agreement between the true profiles and the reconstructed results.     

Microwave imaging of multiple conducting cylinders

Inverse scattering for multiple conducting cylinders is investigated. It is assumed that a plane wave is incident upon separate perfectly conducting cylinders of unknown shapes and the scattered field is measured outside, then, using prior knowledge of the rough positions of the scatterers, the shapes of the conducting scatterers can be reconstructed. The Newton-Kanotrovitch method is employed to solve nonlinear integral equations and the pseudoinverse technique is used to overcome the ill-posedness. Numerical examples are given to demonstrate the capability of the inversion algorithm. Good reconstruction is obtained even when the multiple scattering between two conductors is serious. The effect of noise on the reconstruction result is also investigated     

Electromagnetic imaging for complex cylindrical objects

Electronic imaging of complex cylindrical objects with arbitrary cross sections was investigated, assuming an incident wave upon both penetrable inhomogeneous dielectric cylinders and perfectly conducting cylinders with known shape, and external measurements of the scanned field. By properly processing the scattered held measurements, the dielectric permittivity distribution of the scanned object can be reconstructed. A theoretical formulation was based on proper arrangement of the incident field directions resulting in a set of integral equations derived and solved by the moment method and the unrelated illumination method. Numerical results demonstrate the capability of the imaging algorithm. Good reconstruction results were obtained even in the presence of additive random noise. In addition, noise effects on the reconstruction results were investigated.     

Scattering from perfectly conducting and resistive strips on agrounded dielectric slab

The scattering properties of perfectly conducting and resistive strips are predicted for strips which are located on a dielectric slab backed by a perfectly conducting ground plane. The spectral domain Green's function is used to relate the currents and fields on the strip, and the resulting integral equation is solved using the method of moments. Both TE and TM strips are examined using piecewise linear and pulse subdomain basis functions, respectively, to model the current on the strip. Calculated results are compared with results measured at the NASA Langley Research Center     

Electromagnetic inverse scattering of multiple perfectly conducting cylinders by differential evolution strategy with individuals in groups (GDES)

The differential evolution strategy with individuals in groups (GDES) is proposed to solve the electromagnetic inverse scattering of multiple perfectly conducting cylinders with transverse magnetic wave incidence. The inverse problem is to locate the cylinders and reconstruct their contours, besides the determination on the number of cylinders. The governing electric field integral equations for the scattering problem are expressed as surface integrals over the cylinder contours. The scattering problem is solved using point-matching method with pulse basis and Dirac test functions. The inverse problem is cast into an optimization problem and solved using the GDES. Numerical reconstruction results show that both the quality of the reconstructed profile and the convergence performance are significantly improved, as compared to the original DES.     

Electromagnetic interaction between two parallel circular cylinderson a planar interface

Multiple interaction between two parallel and infinitely long circular cylinders on a planar interface separating two different media is analyzed theoretically. The scattering equations are derived from the so-called extinction theorem applied to this particular geometry. For simplicity, the surface is considered to be perfectly conducting although the method can be extended for any material. The equations, solved numerically by means of an appropriate discretization of the surface, provide the electric surface current density from which the scattered intensity can easily be calculated. Scattering of the transversal-magnetic and transversal-electric incident wave is studied as a function of the cylinder separation for cylinder diameters from 0.2λ to 4λ (λ being the incident wavelength). The effects of the interaction between cylinders are shown in the scattering cross section and in the surface current density of the planar substrate and of the cylinders     

High-frequency asymptotic solutions for backscattering by cylinders with conic section directrixes

High-frequency asymptotic expansions are derived for electric and magnetic fields backscattered from a perfectly conducting smooth two-dimensional surface illuminated by a plane incident wave in two cases of TE and TM linear polarizations. Diffraction corrections up to the second order of the inverse large parameter p=ak (where a is a curvature radius at the specularly reflecting point, and k is a field wave number) to the geometrical optics fields, and specifically to their phases, backscattering cross sections (HH and VV for TE and TM polarizations, correspondingly), as well as the polarization ratio HH/VV, are derived for the specular points of a general form. These general results are applied to backscattering from cylinders with conical section directrixes (circle, parabola, ellipse, and hyperbola), and a number of new compact explicit equations are derived, especially for elliptic and hyperbolic cylinders illuminated at an arbitrary incidence angle relative to their axes of symmetry.     

A heuristic UTD slope diffraction coefficient for rough lossywedges

Heuristic wedge diffraction coefficients for computing propagation path loss over finitely conducting earth are extended to include slope diffraction, with the assumption that propagation of energy through the wedge is negligible. The slope diffraction terms for the lossy wedge are obtained in an analogous manner as for the perfectly conducting case, except that special care must be taken with the factor multiplying the incident field for grazing incidence. Results given show that the slope diffraction term produces continuous results that behave reasonably when compared with results for perfectly conducting wedges     

Electromagnetic diffraction from a dielectric-filled slit-cylinderenclosing a cylinder of arbitrary cross section: TM case

A simple moment solution to the problem of the diffraction of a TM plane wave from an infinite, perfectly conducting slotted cylinder of an arbitrary cross section is summarized. The slit cylinder encloses a smaller perfectly conducting cylinder of an arbitrary cross section, and the space between the cylinders is filled with a dielectric material. The equivalence principle is used to obtain a set of coupled integral equations for the induced/equivalent surface currents on the cylinders, and the method of moments is used to solve numerically the integral equations. The electric field integral equation formulation is used. The advantages and the limitations of the method are discussed. Sample results for the induced current, aperture field, internal field, and scattering cross sections are given. These are in good agreement with some of the available published data     

Low frequency scattering by a resistive plate

For a plane wave incident on a resistive plate, the first two terms in the low frequency expansion of the far field are determined. Since the magnetic dipole is not excited if the resistivity is nonzero, the leading term is simply the electric dipole contribution, and is identical to that for a perfectly conducting plate. The resistivity appears explicitly in the next term which can be expressed in terms of potentials. Some implications of the results are discussed, and the method is illustrated in the case of a circular disk.     

A hybrid technique for combining the moment method treatment of wire antennas with the GTD for curved surfaces

This hybrid technique is a method for solving electromagnetic problems in which an antenna is located near a conducting body. The technique accomplishes this by casting the antenna structure in a moment method (MM) format, then modifying that format to account for the effects of the conducting body via the geometrical theory of diffraction (GTD). The technique extends the moment method to handle problems that cannot be solved by GTD or the moment method alone. Wire antennas are analyzed to find their input impedance when they are located near perfectly conducting circular cylinders, although the methods used are not restricted to circular cylinders. Three orthogonal orientations are identified, and antennas to match them are analyzed. For each case, the hybrid solution is checked with one of three independent solutions: an MM-eigenfunction solution, image theory, or experimental measurement. In almost all cases, excellent agreement is obtained due in large part to the fact that the moment method near fields are, for the first time, cast into a ray optical form.     

Transient interactions of an EM pulse with a dielectric sphericalshell

The authors study the scattering interaction of short electromagnetic pulses with a spherical target. The target is assumed penetrable and they model it as an air-filled dielectric shell. The radar cross-section (RCS) of such a target is obtained and its resonance features are analyzed. A dielectric composition makes the resonance features become very prominent compared with the case of a perfectly conducting sphere. When the interrogating waveform is a pulse of short duration, the resonance features of the RCS can be extracted within the frequency band of the spectrum of the incident pulse. To verify their theoretical predictions they illuminate spherical targets with short, broadband pulses using an impulse radar system. The actual shape of the pulse that is incident on the targets is theoretically modeled using a digital filter design technique together with pulse returns from a reference target. They verify that the shape of the predicted, backscattered pulse that results from their design method agrees well with the experimental findings using three additional targets of different sizes and materials. They investigate in the combined time-frequency domain the development in time of the various frequency features of the spectra of backscattered pulses using time-windowed Fourier transforms. The methodology developed can handle broadband pulses of any sufficiently smooth spectrum, interacting with (lossy or lossless) dielectric scatterers, and can extract resonance features within the frequency band of the spectrum of the transmitted pulse. Accordingly, this method could be also used for assessing the performance of high-power impulse radar systems     

Modeling of cylindrical objects by circular dielectric andconducting cylinders

The scattering of an incident plane wave from an array of parallel circular dielectric and/or conducting cylinders is derived rigorously using a boundary value approach. Both transverse electric (TE) and transverse magnetic (TM) polarized incident plane waves are considered. The validity and accuracy of the method are verified by comparing the numerical results with those based on other available methods. The advantage of the proposed analysis is the simplicity and efficiency in computation. The modeling of two-dimensional objects of arbitrary cross section and composite material is outlined and sample numerical results are presented to illustrate the versatility of the method     

Three-dimensional diffraction by infinite conducting and dielectric wedges using a generalized total-field/scattered-field FDTD formulation

We extend the generalized total-field/scattered-field formulation of the finite-difference time-domain method to permit efficient computational modeling of three-dimensional (3-D) diffraction by infinite conducting and dielectric wedges. This new method allows: 1) sourcing a numerical plane wave having an arbitrary incident angle traveling into, or originating from, a perfectly matched layer absorbing boundary and 2) terminating the infinite wedge inside the perfectly matched layer with negligible reflection. We validate the new method by comparing its results with the analytical diffraction coefficients for an infinite 3-D right-angle perfect electric conductor wedge obtained using the uniform theory of diffraction. Then, we apply the new method to calculate numerical diffraction coefficients for a 3-D infinite right-angle dielectric wedge, covering a wide range of incident and scattering angles. Finally, we show means to compactly store the calculated diffraction coefficients in a manner which permits easy interpolation of the results for arbitrary incidence and observation angles.     

Self-consistent GTD formulation for conducting cylinders with arbitrary convex cross section

A user-oriented computer program has been developed for high frequency radiation and scattering from infinitely-long perfectly. conducting convex cylinders. The analysis is based on the self-consistent geometrical theory of diffraction (GTD). The cylinder is modeled as anN-sided polygon. Two cylindrical waves with unknown amplitudes are assumed to travel in opposite directions on each face of the polygon. The boundary conditions for the corners are applied to set up a matrix equation for2Nunknowns (the amplitudes associated with the traveling cylindrical waves). Crout's method is used to solve the matrix equation. Once the amplitudes for the traveling waves are determined, the radiation or scattered field is readily obtained via the usual GTD techniques. Numerical results are presented for radiation and scattering from rectangular, semi-circular, circular, and elliptic cylinders for both principal polarizations. The results show excellent agreement with GTD, moment, and eigenfunction solutions.     

二维多导体柱电磁散射特性的特征基函数法分析

特征基函数法是近两年提出来的一种求解电磁散射问题的有效方法,该方法使用的特征基函数不受传统矩量法离散尺寸的限制,因而可以大大减小要求解的矩阵方程。应用特征基函数法分析了二维多导体柱的电磁散射特性,计算了多个无限长导电椭圆柱和方柱的雷达散射截面,结果表明特征基函数法的计算结果与传统矩量法的计算结果吻合良好,而计算量却大为减少。     

Optimization of a conical antenna for pulse radiation: an efficientdesign using resistive loading

The conical monopole antenna with a section of continuous resistive loading is considered as a radiator for temporally short, broad-bandwidth pulses. The geometrical details of the coaxial feed and the resistive loading are varied to optimize this structure for pulse radiation. Compared with the perfectly conducting cone, the optimized resistive cone radiates a better reproduction of the pulse excitation with no loss in amplitude, and has internal reflections that are much smaller in amplitude. Graphical displays of the field surrounding the antenna are used to give insight into the physical processes for transient radiation from this antenna. Experimental models were constructed to verify the optimization and demonstrate the practicality of the design. Measurements of both the reflected voltage in the feed line and the time-varying radiated field are in excellent agreement with the theoretical calculations     

Scattering of an Electromagnetic Pulse by a Moving Wedge

The paper is concerned with the analysis of 2-D scattering of an electromagnetic (EM) pulse by a perfectly conducting wedge moving with a relativistic velocity in a free space. The incident signal is described by a Dirac delta function. Analytical solution to this scattering problem is found, and its physical interpretation is given. The field representation, valid for all scatterer velocities, is then simplified to the case of moderate and low velocities, appropriate for practical applications.     

Solution of electromagnetic scattering problems using time domaintechniques

New method are developed to calculate the electromagnetic diffraction or scattering characteristics of objects of arbitrary material and shape. The methods extend the efforts of previous researchers in the use of finite-difference and pulse response techniques. Examples are given of the scattering from infinite conducting and nonconducting cylinders, open channel, sphere, cone, cone sphere, coated disk, open boxes, and open and closed finite cylinders with axially incident waves     

Cylindrical transmitting antenna: Tapered resistivity and multiple impedance loadings

A general formulation is presented which treats with accuracy the perfectly conducting antenna, the uniformly resistive antenna, the tapered resistive antenna, and the multiple impedance loaded antenna. The current distribution is obtained in analytic form that is easily used to obtain the effective length and radiated field. The presented theory has application in traveling-wave antenna design. Traveling-wave antennas are of interest for broadband and directional communication. Extensive numerical results are obtained, and comparisons are made with experimental data. The results indicate the feasibility of cylindrical traveling-wave antennas.