Creeping wave propagation constants and modal impedance for a dielectric coated cylinder

The electromagnetic characteristics associated with the creeping waves supported by a dielectric coated cylinder are investigated. The propagation constants and wave impedances of the creeping waves are obtained numerically. Higher order modes which are significant for a thick coating are also investigated. The propagation constants and creeping wave modal impedance are compared with those obtained for a planar dielectric slab backed by a ground plane. It is found that, contrary to the planar configuration, no cutoff frequencies exist for the creeping waves associated with the coated cylinder. In fact, the coated cylinder supports an infinite number of modes. However, depending upon the thickness of the coating, only a few Elliott type creeping wave modes with low attenuation can exist. Furthermore, for each of the Elliott type creeping waves, there is a critical radius for the cylinder below which the Elliott type creeping wave cannot exist. The results are also compared with an impedance boundary cylinder, where the impedance is chosen to be purely imaginary.     

Excitation of creeping waves on a circular cylinder with a thick dielectric coating

A magnetic line source on a dielectric coated conducting cylinder excites a creeping wave field. The coating is not necessarily thin, so more than one mode can exist. It is found that as the coating thickness is increased, the surface field on the dielectric-air interface exhibits a distinctive beat pattern resulting from the interaction between two creeping-wave modes.     

Scattering of Electromagnetic Waves by a Coated Nihility Cylinder

An analytical solution for the scattering of electromagnetic plane waves from an infinitely long nihility cylinder, coated with a double positive (DPS), double negative (DNG), epsilon negative (ENG), or mu negative (MNG) layer of uniform thickness is presented. The solution is determined by solving the scalar wave equation in the cylindrical coordinates, for different regions and applying the appropriate boundary conditions at the interfaces. Both TM and TE polarizations as incident plane have been considered in the analysis. Comparison of behaviors of a coated nihility cylinder with a coated PEC cylinder has been made. It is noted that two situations are more closer for DNG coating as compared to DPS coating.     

Electromagnetic scattering from a dielectric-coated circular cylinder

The normalized backscattering width of an infinitely long, dielectric coated circular cylinder is obtained via a high frequency ray solution. The ray solution provides a physical picture of the scattering process in terms of a geometrical optics ray and two surface waves. It is shown that the surface wave resonance phenomena in the backscattering fields of the coated cylinder can be predicted in terms of the Regge poles of the coated cylinder. The numerical results for the backscattering widths of the cylinder obtained via the high frequency ray solution show excellent agreement with the eigenfunction results. The trajectories of the Regge poles associated with the coated circular cylinder are also presented.     

基于FE-BI的介质粗糙面上方涂覆目标电磁散射特性研究

采用有限元-边界积分（finite element boundary integral,FE-BI）方法研究了介质粗糙面上方涂覆目标的复合电磁散射特性,推导了一维介质粗糙面上方二维涂覆目标电磁散射的FE-BI公式.在仿真中,采用功能强大的有限元方法模拟涂覆目标内部场,对于涂覆目标与粗糙面之间的多重耦合作用则通过边界积分方程方法进行考虑.结合Monte-Carlo方法,数值计算了介质高斯粗糙面上方涂覆圆柱目标的电磁散射,分析了涂层材料介电常数、粗糙面粗糙度以及介质粗糙面介电常数变化对复合模型双站散射系数的影响.数值结果表明,相比于传统矩量法（method of moment,MoM）,本文方法虽然在处理理想导体模型时效率略低,但可以处理MoM难以处理的复杂媒质电磁散射问题,且计算精度较高.     

Scattering and shielding properties of a chiral-coated fiber-reinforced plastic composite cylinder

A state-equation approach is developed to tackle a fiber-reinforced plastic composite (FRPC) cylinder coated by chiral media. This cylinder can be treated as consisting of multilayered bi-isotropic and anisotropic materials. As the chiral coating is properly designed, the backscattering of the linearly polarized plane wave almost disappears in some specific frequency bands. Numerical results of the monostatic echo widths show that a high invisible characteristic of the cylinder can be obtained by appropriately selecting the chiral layer thickness, chiral impedance, chiral nonreciprocity, chirality, and cylinder radius. It is also found that, with the presence of the chiral coating, the bistatic echo widths may be greatly reduced except in the forward scattering direction. However, cross-polarized penetrated fields occur owing to the chiral coating. Adding an interior metal coating is suggested to enhance the shielding performance without sacrificing the invisible property of the cylinder.     

Radar cross-section study of cylindrical cavity-backed apertureswith outer or inner material coating: the case of E-polarization

A dual-series-based solution is obtained for the scattering of an E-polarized plane wave from a cavity-backed aperture which is formed by a slitted infinite circular cylinder coated with absorbing material. The material coating can be done on the inner or outer surface of the cylinder. For both cases, numerical results are presented for the radar cross section and comparisons are given for two different realistic absorbing materials. The radar cross-section results are also given for the aspect angle of the screen. Finally, the dependence of radar cross section on the thickness of the absorbing layer is presented     

Plane wave scattering by a material coated parabolic cylinder

An eigenfunction solution to the problem of transverse magnetic (TM) or transverse electric (TE) scattering by a coated parabolic cylinder is presented. Paralleling the well-known solution for the coated circular cylinder, eigenfunction expansions involving parabolic cylinder functions are obtained for the fields in the exterior and coating regions. Next, boundary conditions are enforced to obtain a pair of coupled equations for the unknown coefficients in the eigenfunction expansions for the fields. Unlike the corresponding solution for the coated circular cylinder, the eigenfunctions in the exterior and coating regions are not orthogonal, and an exact term-by-term solutions of these equations is not possible. Instead, the equations are solved by the method of moments. For thin coatings both an uncoupled-mode approximation and a surface-impedance model are described. In particular, for the TM polarization it is shown that a thin coating can be modeled by a specific nonuniform surface impedance for which an exact term-by-term solution is possible. Numerical data are presented, showing the convergence of the solution and comparing the solutions for the uncoupled-mode and surface-impedance models     

Diffraction by a cylinder with a dielectric shell. Surface waves and an antiradar coating

The field formed during diffraction by a circular cylinder with a dielectric shell can be represented in the form of a Watson series, i.e., as a set of azimuthal waves. The structure, velocity, and attenuation of these waves are found. It is shown that the process during which surface waves excited in the presence of diffraction by any guiding nonplanar surface carry away a portion of the incident energy and reduce the scattered wave can be used for creating a coating in the form of a dielectric layer.     

The RCS of a cylindrical array antenna coated with a dielectric layer

The scattering properties of dielectric coated waveguide aperture antennas mounted on circular cylinders are investigated. Both the single element antenna and the array case are treated. The array antenna consists of 4 /spl times/ 32 rectangular apertures placed in a rectangular grid on the surface of an infinitely long circular cylinder. The problem is formulated in terms of an integral equation for the aperture fields which is solved with the method of moments using rectangular waveguide modes as basis and test functions. An efficient uniform asymptotic technique is used to calculate the excitation vector and the backscattered far-field. The asymptotic solution is valid for large cylinders coated with thin dielectric layers away from the paraxial (i.e. near axial) region. A similar asymptotic solution is used to calculate the mutual coupling in the nonparaxial region. For the self coupling terms and for the mutual coupling in the paraxial region a planar approximation is used with a corresponding spectral domain technique. Numerical results are presented as a function of frequency, angle of incidence, cylinder radius, and electrical thickness of the coating.     

A ray representation of surface diffraction by a multilayer cylinder

The exact expressions for the field produced by a point source radiating in the presence of an infinitely long cylindrical structure comprising layers of different materials is evaluated asymptotically for source and observation points that are widely removed from the cylinder surface and related so that the cylinder shadows the source from the observer. The diffracted field is interpreted in a ray-optic format. It is observed that surface-propagating waves along the layers can be slowed significantly as compared with those on an impenetrable cylinder of the same radius. This wave slowing requires that rays attaching to the cylinder refract into a layer-guided mode and refract a second time as the curved structure sheds energy into the scattered field. Consequently, rays attaching to and shedding from the cylinder are not tangent to it when the layer profile slows the surface-propagating waves. The analysis applies for both metallically backed and open-shell layers. Computed diffraction coefficient results are given for a coated metallic cylinder as a function of coating thickness.     

Propagation characteristics of a circular waveguide coated inside with a metamaterial

The problem of guided wave propagation of a circular waveguide that is coated on the inside with a metamaterial coating is studied by a boundary value approach and the propagation features cylindrical waveguide are studied by solving the resultant characteristic equation numerically. The results are compared to those with an inside dielectric coating. The variation of the normalized phase constant is studied as a function of the parameters of the coating including the permittivity and permeability, the coating thickness expressed as a ratio of b/a, and frequency. The behavior of the cylindrical waveguide is shown to be significantly different from that with a dielectric coating.     

Electromagnetic scattering model for a tree trunk above a tiltedground plane

An efficient and realistic electromagnetic scattering model for a tree trunk above a ground plane is presented. The trunk is modeled as a finite-length stratified dielectric cylinder with a corrugated bark layer. The ground is considered to be a smooth homogeneous dielectric with an arbitrary slope. The bistatic scattering response of the cylinder is obtained by invoking two approximations. In the microwave region, the height of the tree trunks are usually much larger than the wavelength. Therefore the interior fields in a finite length cylinder representing a tree trunk can be approximated with those of an infinite cylinder with the same physical and electrical radial characteristics. Also an approximate image theory is used to account for the presence of the dielectric ground plane which simply introduces an image excitation wave and an image scattered field. An asymptotic solution based on the physical optics approximation is derived which provides a fast algorithm with excellent accuracy when the radii of the tree trunks are large compared to the wavelength. The effect of a bark layer is also taken into account by simply replacing the bark layer with an anisotropic layer. It is shown that the corrugated layer acts as an impedance transformer which may significantly decrease the backscattering radar cross section depending on the corrugation parameters. It is also shown that for a tilted ground plane a significant cross-polarized backscattered signal is generated while the co-polarized backscattered signal is reduced     

Scattering of electromagnetic pulses by simple-shaped targets withradar cross section modified by a dielectric coating

We study the scattering interaction of electromagnetic pulses with a spherical target. The target is a perfectly conducting sphere coated with a thin dielectric layer. Two different hypothetical materials are specified: a lossy dielectric and a dielectric that also has magnetic losses. The monostatic radar cross section (RCS) is computed in each case and we examine the influence of the coating on the RCS. In particular, we compare the RCS of the coated sphere with the (normalized) backscattered power when a large perfectly conducting flat plate coated with the same dielectric layer is illuminated at normal incidence by the same waveform. In particular, we find that except for frequencies below those within the efficiency band of the absorbent material, the normalized RCS of the coated sphere agrees well with the power reflection coefficient of the plate covered with the same kind of coating. For low-frequency incidences, the peaks and dips in the RCS are more prominent for the coated target than they are for the bare one. Analyzing the response of the spherical targets in the combined time-frequency domain we demonstrate that the coating itself, although reducing the RCS could introduce additional resonance features in the target's signature at low frequencies that could be used for target recognition purposes. This observation is also confirmed by a study of the bistatic RCS of these coated objects, which we have displayed in various color graphs     

An efficient formulation to determine the scatteringcharacteristics of a conducting body with thin magnetic coatings

Two new and efficient surface integral equations, derived from corresponding volume integral equations, are developed to calculate the scattering of electromagnetic (EM) waveform from an arbitrarily shaped conducting body coated with thin lossy magnetic film. Their numerical solutions by the method of moments (MM) for two-dimensional structures with full or partial coatings are presented. It is shown that the radar cross-section of a conducting body can be significantly reduced by coating it with a lossy magnetic film. To verify the validity and accuracy of the proposed formulation, another method based on the expansion of cylindrical harmonic functions with real arguments is also developed to calculate the scattering of a plane EM wave from an electrically large coated circular cylinder. The same problem was also solved by the proposed formulation, and excellent agreement between the two approaches was achieved. In addition, numerical results of the scattering from a rectangular coated cylinder is shown to be consistent with that obtained by a modified finite-difference-time-domain (FDTD) method     

Numerical Solution of Steady-State Electromagnetic Scattering Problems Using the Time-Dependent Maxwell's Equations

A numerical method is described for the solution of the electromagnetic fields within an arbitrary dielectric scatterer of the order of one wavelength in diameter. The method treats the irradiation of the scatterer as an initial value problem. At t = 0, a plane-wave source of frequency f is assumed to be turned on. The diffraction of waves from this source is modeled by repeatedly solving a finite-difference analog of the time-dependent Maxwell's equations. Time stepping is continued until sinusoidual steady-state field values are observed at all points within the scatterer. The envelope of the standing wave is taken as the steady-state scattered field. As an example of this method, the computed results for a dielectric cylinder scatterer are presented. An error of less than /spl plusmn/10 percent in locating and evaluating the standing-wave peaks within the cylinder is achieved for a program execution time of 1 min. The extension of this method to the solution of the fields within three-dimensional dielectric scatterers is outlined.     

Scattering from a chirally coated DB elliptic cylinder

An exact analytic solution to the problem of scattering of a plane electromagnetic wave from a chirally coated elliptic cylinder defined by a DB boundary has been obtained, by expanding the different electromagnetic fields associated with the problem in terms of suitable elliptic vector wave functions and a set of expansion coefficients. The incident field expansion coefficients are known, but the expansion coefficients associated with the fields scattered outside the coated cylinder and the fields transmitted inside the coating are unknown. These unknown coefficients are obtained by imposing appropriate boundary conditions at the two boundaries. Results have been presented as normalized bistatic and backscattering widths for a variety of admittances, permeabilities, and permittivities of the chiral materials used for the coating, to show their effects on scattering from the chirally coated cylinder.     

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.     

Study of different realizations and calculation models for softsurfaces by using a vertical monopole on a soft disk as a test bed

We study a vertical monopole on a circular metallic disk which is loaded in different ways to provide a soft boundary condition, e.g., by transverse corrugations or by dielectric or lossy material coatings. The different realizations are calculated by the uniform geometrical theory of diffraction (UTD), the incremental theory of diffraction, the moment method (MM) with an impedance boundary model, and MM using equivalent currents on all material interfaces. The calculated results are compared with measured results for specific geometries. Some general characteristics of the different realizations of the soft surfaces are extracted from the results. It is verified that the artificially soft boundary condition can be realized by different surface loadings. In particular, a nearly soft boundary condition can be realized over a large bandwidth by coating a conductor with a thin layer of lossy magnetic material. It is found that the surface impedance model works very well for modeling corrugations. It may also work for material coated surfaces provided the surface wave is prohibited from radiating. The bandwidth of the different soft surfaces are given, including corrugations with different cross-sectional shapes     

Current induced by a plane wave on a thin infinite coated wire above the ground

The current induced on a thin infinite coated wire above the ground when an electromagnetic plane wave is incident on it is calculated. The final result is presented in a form which exhibits clearly the various contributions to the total impedance of the wire-ground system. Numerical examples are given which demonstrate the dependence of the induced current on the conductivity of the ground, the thickness of the coating, and the polarization and direction of propagation of the incident wave.