Scattering of electromagnetic waves from moving surfaces

Electromagnetic wave scattering from moving objects is considered. Use is made of the reformulated current method in which the object is replaced by a current distribution radiating into an unbounded media. Cylindrical and spherical profiles in nondeterministic motion are considered. Results are obtained for far-field and steady-state cases when the radii of curvature are large compared to the wavelength.     

Scattering of acoustic surface waves from periodic grating filters

New data are presented on the scattering of the z-propagating Rayleigh wave on y-cut LiNbO3 by a metallic grating into the x-propagating Rayleigh wave, a pseudo-surface-wave, and bulk shear waves. Estimates of the magnitudes of the scattering coefficients are given     

Scattering of surface magnetostatic waves by periodic slot gratings

Scattering of the magnetostatic surface waves generated in thin films by the gratings formed from slots in a metal screen is theoretically analyzed. A boundary-value problem for the Maxwell equations is formulated and solved in the static approximation. An algorithm of numerical solution of the boundary-value problem is proposed. Scattering of magnetostatic surface waves by 2D periodic slot gratings that are infinite along one coordinate and finite along the other coordinate is studied for an arbitrary orientation of slots and an arbitrary angle of incidence of the magnetostatic wave. The band structure of gratings is studied. The possibility of formation of magnonic crystals based on slot gratings is demonstrated.     

Scattering of volume magnetostatic waves by periodic slot lattices

Scattering of volume magnetostatic waves excited in thin films by the lattices composed of metal ribbons and slots in a metal screen is analyzed. A boundary value problem for the Maxwell equations is formulated and solved in the static approximation. An algorithm for the numerical solution of this boundary value problem is proposed. Scattering of volume magnetostatic waves by 2D periodic lattices of slots that are infinite along one coordinate and finite along the other coordinate is studied for an arbitrary orientation of slots and an arbitrary angle of incidence of the magnetostatic wave. The energy-band structure of lattices is analyzed. The possibility of formation of magnon crystals based on the slot lattices is demonstrated.     

Scattering of electron stream waves on metal-dielectric periodic structures

The results of the theoretical and experimental research of the surface waves scattering of an electron stream and a planar dielectric waveguide on metal-dielectric structures of the dielectric prism — band diffraction grating-type are presented. Within the approximation of the given current there has been carried out a numerical analysis of energy characteristics of diffraction-Cerenkov radiation for a wide interval of the main system parameters change. The results of numerical analysis are confirmed by the data of experimental simulation.     

Scattering of obliquely incident plane waves from a finite periodic structure of ferrite cylinders

For the scattering of waves obliquely incident on many homogeneous gyrotropic cylinders simultaneous matrix equations are first derived from the reciprocity theorem where the auxiliary electric and magnetic current sources are specially selected. As a numerical example we examine the scattering properties of a finite periodic structure composed of many circular ferrite cylinders, with attention to 1) the frequency characteristics of the scattering cross section where the parameters are the number of cylinders, the incident angle, and the applied static magnetic field; 2) the patterns of the scattered power density; 3) the distributions of the total field in the vicinity of the cylinders; 4) the role of the periodicity of the spacing of the scatterers on the scattering cross section; and 5) the effect of the interactions among the cylinders. The extension of the present method may make it possible to analyse guided modes along an array of gyrotropic cylinders.     

Scattering by periodic metal surfaces with sinusoidal height profiles--A theoretical approach

A theory of scattering by periodic metal surfaces is presented that utilizes the physical optics approximation to determine the current distribution in the metal surface to first order, but modifies this approximate distribution by multiplication with a Fourier series whose fundamental period is that of the surface profile (Floquet's theorem). The coefficients of the Fourier series are determined from the condition that the field radiated by the current distribution into the lower (shielded) half-space must cancel the primary plane wave in this space range. The theory reduces the scatter problem to the familiar task of solving a linear system. For certain basic types of surface profiles, including the sinusoidal profile considered here, the coefficients of the linear system are obtained as closed form expressions in well-known functions (Bessel functions for sinusoidal profiles and exponential functions for piecewise linear profiles). The theory is thus amenable to efficient computer evaluation. Comparison of numerical results based on this theory with data obtained by recent numerical schemes shows that for depths of surface grooves less than a wavelength and for unrestricted groove widths, reliable and comparable, if not more accurate, data is obtained, in many cases at considerably cheaper computational cost.     

Scattering from periodic arrays of crossed dipoles

An analysis is presented for calculating the scattering from periodic arrays of symmetrical crossed dipoles. It is shown that in general the reflection coefficient versus frequency of symmetrical crossed-dipole arrays exhibits two distinct resonances, which occur when the dipole elements are on the order of a half-wavelength long. The analysis reveals also the presence of an antiresonance, which occurs at a frequency between the two resonances. At the antiresonant frequency the array reflection coefficient is zero. Thus, within a relatively narrow frequency band, the array reflection characteristics traverse the extremes of complete reflection followed by no reflection, and finally by complete reflection again. The anomalous reflection versus frequency behavior in such arrays is shown to be directly attributable to two distinct resonances excited in the crossed elements. For an isolated crossed dipole, it is shown that the total induced current can be approximately represented by two uncoupled current components. The current components induced on the same elements in an array, however, are coupled to one another through interaction with neighboring elements. The coupling results in complete cancellation of the total bistatic array scattering, at a frequency which lies between the two component current resonant frequencies.     

Scattering from a periodic array of resistive strips

The problem of scattering from a resistive strip grating is formulated in the spectral domain. Results of numerical calculations of the reflection coefficient are presented for perfectly conducting strips and strips with resistivities up to750Omega/square.     

Scattering from breaking gravity waves without wind

Scattering experiments from breaking gravity waves conducted at a wave tank facility at small grazing angles in the absence of wind are analyzed. Breaking gravity waves are studied using a fully plane polarimetric horizontal (HH), vertical (VV), vertically transmitted and horizontally received polarization (VH), and horizontally transmitted and vertically received polarization (HV) pulse-chirped X-band (8.5-9.6 GHz) radar in conjunction with optical instruments: the plane polarimetric optical specular event detector (OSED) and side-looking camera (SLC). Spatially and temporally resolved radar backscatter has been measured and temporally correlated to the data obtained from the optical diagnostics. The experiments yield the following results: (1) enhanced scattering compared to Bragg scattering levels occurs throughout the evolutionary process of wave-breaking, i.e., the radar scatters strongly from both the unbroken and broken surfaces; (2) an explanation is found for the observation that the scatterer Doppler frequency is slightly less than the Doppler frequency corresponding to the fundamental wave phase speed; (3) a representative non-Bragg cross section of a breaking wave can be obtained; and (4) a breaking wave surface is found to be an efficient depolarizer     

Scattering from irregular-edged planar and curved surfaces

An electric field integral equation (EFIE) formulation is used to describe the electromagnetic scattering from finite planar and curved perfect electrical conducting surfaces truncated by an irregular edge. The edge can have an arbitrary form if it satisfies certain differentiability requirements. Similarly, the generating curve describing the surface can be convex, concave, or a combination of both. An edge-dependent entire domain Galerkin expansion is used for the current variation along the surface in the direction of translation. A subdomain expansion is used along the orthogonal direction. The backscatter cross sections obtained from the method of moments are compared with experimental data     

Scattering from periodic arrays of cylinders by Dirichlet-to-Neumann maps

A simple and efficient numerical method for computing the transmission and reflection spectra of periodic arrays of cylinders is developed. For each unit cell containing a cylinder, only the wave field on the edges of the unit cell is computed. For multilayered structures, a marching scheme based on a pair of operators is developed.     

Reflection of electromagnetic waves from oscillating surfaces

The problem of reflection of electromagnetic waves from oscillating surfaces is discussed. Using the induction theorem, the interface is replaced by equivalent current sources that radiate into an unbounded medium. Spatial movement is ascribed to these sources to account for oscillations of the surface. The general solution for the far-field due to any arbitrary surface motion is developed. A few deterministic and random functions for surface motion are considered. Most of the initial discussion pertains to normal reflection from planar surfaces, but the solution is also obtained for arbitrary incidence and for an oscillating cylinder.     

Scattering from periodic strip gratings via the secant-spectraliterative method

The secant method is used in an iterative algorithm for calculating the electromagnetic scattering from planar, periodic gratings. Results are compared with the moment method and the contraction-corrector spectral-iteration technique (SIT) methods. The secant approach does not depend on the evaluation of numerical derivatives to achieve convergence like the contraction-corrector SIT method. Suggestions for applying this method to more complicated structures are included     

Scattering of electromagnetic waves from dielectric coated conducting spheres

Several series of rigorous numerical calculations of the backscatter cross section of a conducting sphere with a thin lossless dielectric coating were carried out. The ratio of the radius to wavelength was varied from about 0.02 to 10.0; the dielectric constant of the coating was taken to be 2.56, 4.0, or 6.0; and the thickness of the coating was 0.1 or 0.05 times the outer radius of the coated sphere. Curves of the results are presented which indicate that the backscatter cross section of a coated sphere may be increased by as much as a factor of ten over that of an uncoated sphere of the same size, and, due to interference effects, an even greater decrease may be obtained. Further, small changes (less than one per cent) in the thickness or dielectric constant of the coating, or in the wavelength, may bring about large changes in the cross section. The numerical results are also compared with some experimental measurements, and with predictions of a "creeping-wave" type of analysis carried out by Helstrom.     

Scattering of electromagnetic waves from nonlinear thin dielectric coatings

In this paper the reflection and diffraction properties of nonlinear thin dielectric coatings are investigated. A quite simple geometry consisting of two semi-infinite homogenous linear spaces separated by a non-linear coating is considered. An incident plane wave propagating along the z direction towards the nonlinear thin coating is taken. The nonlinear thin coating is divided into elementary sectors, within each one of which one propagating and one reflected wave exists. The boundary conditions are then applied in order to develop an efficient numerical alogirthm to study the properties of the nonlinear thin coating.     

Scattering from periodic array of grounded parallel strips, TEincidence

The field distribution between strips in a periodic array of parallel strips was modeled. The electric field integral equation (EFIE) together with method of moments (MoM) was employed to solve the problem. The incident plane wave was assumed to be transverse electric (TE). The effect of the incident angle on the field between strips was investigated. For comparison, an array of the same configuration but containing a finite number of parallel strips was also modeled using the EFIE and solved via MoM. Good agreement was found even when the number of strips was small. The results are of interest for mobile radio     

Scattering by non-Gaussian surfaces

A two-dimensional density is constructed from given marginals and a given correlation coefficient by using an expansion in orthogonal polynomials. The results are used to derive a general method of finding the field scattered by a rough surface by both physical and geometrical optics when the surface is generated by a stationary random process which is not necessarily normal. The example of an exponential surface is calculated in more detail, particularly for backscatter.     

Scattering by S-shaped surfaces

When an S-shaped surface possesses no derivative discontinuities, techniques such as the geometrical theory of diffraction are not applicable. However, if the radius of curvature is relatively large at every point on the surface, the physical optics approximation may be employed. The authors present a uniform physical optics (UPO) solution which remains valid at caustics occurring when two or more specular points coalesce at the inflection point of the S-shaped surface. The solution is developed by approximating the surface with a localized cubic expansion, leading to exact expressions in terms of Airy integrals. In contrast to other solutions, the one given here requires only a knowledge of the stationary phase points and the first three derivatives of the surface-generating function at those points. A major effort is devoted to the validation of the UPO solution, and this is accomplished with numerical models of the S-shaped surface. It is found that the given UPO solution is quite accurate in the specular and nonspecular regions     

Scattering from dielectric structures above impedance surfaces andresistive sheets

Interest in understanding of electromagnetic interaction with rough surfaces has prompted the study of scattering from typical dielectric humps over impedance surfaces. It is shown that the Green's function of the problem for a resistive sheet resembles that of the impedance surface. Hence both problems are considered here. A numerical solution for the scattered field of a two-dimensional dielectric object, possibly inhomogeneous, with arbitrary cross section above the impedance surface or resistive sheet is sought. First the Green's function of the problem is derived based on the exact image theory. This form of the Green's function is amenable to numerical computation. Then the induced polarization currents are calculated by casting the integral equations into a matrix equation via the method of moments. Numerical problems in calculation of the Green's function when both source and observation points are close to the surface are discussed. Comparison of numerical results with a perturbation solution shows excellent agreement between the two methods