Excitation of surface waves on a unidirectionally conducting screenby an arbitrary phased line source parallel to it

An exact solution is obtained for the scattering problem of a phased line source in front of a screen and parallel to it. The direction of perfect conduction forms an arbitrary angle Ω with the normal projections of the filament on the screen. The coupled surface waves and scattered far field are investigated, and alternative expressions suitable for numerical calculations are presented together with numerical results. It is found that the energy is highly localized in the neighborhood of the screen, and the closer the filament is to the screen, the larger the excitation of the surface wave. In addition, it was found that the surface waves travelling to the right and to the left of the filament are different and become identical only when either Ω=π/2 or the propagation constant γ=π/2. For the special cases γ=Ω and Ω+γ=π, a surface wave is excited in only one side of the screen. In general, the surface waves are slow waves which are circularly polarized with opposite polarization on both sides of the screen. The ordinary end fire source was found to be characterized by 100% launching efficiency     

Guidance of waves by a moving unidirectionally conducting screen

The scattering problem for a moving, unidirectionally conducting screen excited by a conical wave is analyzed using the exact relativistic integral representation previously derived by the author (see ibid., vol.37, no.1, p.64-70, Jan. 1989). The screen is in uniform motion in its own plane and in a direction perpendicular to that of conduction. The screen is found to respond with a conical scattered field and a certain type of surface waves which are of a different nature from that which a stationary screen allows. An equivalent image source is found, and low-velocity effects are show explicitly     

Radiation from an electromagnetic source in a half-space of compressible plasma-surface waves

The radiation characteristics of a line source of magnetic current are studied for the case in which the source is situated in a half-space of isotropic, compressible plasma which is bounded on one side by a perfectly conducting, rigid plane screen. In addition to the electromagnetic and plasma space waves, the line source excites a boundary wave. This boundary wave is a coupled wave. It has associated with it both a magnetic field component and the pressure term. This is in contrast to the space waves which can be decomposed into an electromagnetic (EM) mode with no pressure term and a plasma (P) mode with no magnetic field associated with it. The characteristics of this boundary wave are evaluated. The boundary wave propagates for all frequencies and the power carried by the boundary wave becomes smaller as the frequency is increased.     

Rigorous formulation of scattering at a randomly varying impedanceplane: general case of oblique incidence

The problem of scattering of electromagnetic plane waves at one-dimensional surfaces (random gratings) is solved in the general case in which the incident wave vector does not lie on the main section of the cylindrical surface (oblique incidence). The scatterer is simulated by a plane boundary characterized by a coordinate-dependent impedance that varies along one of the two coordinates on the surface. This representation could be regarded as a canonical model of one-dimensional surfaces with height corrugations. A rigorous electromagnetic formalism for calculating the fields scattered at the impedance plane is presented. The fields above the scatterer are represented by spectral domain expansions. It is shown that the wave vectors of the scattered waves lie on the surface of a cone containing the direction of specular reflection and whose axis coincides with the direction of the grooves of the random grating. The theory is exemplified by calculating the angular distribution of the mean intensity scattered from an ensemble of surfaces with similar statistical parameters     

Radiation by a corner reflector with dielectric loaded edges

The first part, of this paper deals with the electromagnetic scattering by a cylindrical dielectric shell with an azimuthal permittivity profile and an internal E-polarized line source or an externally incident plane wave. The integral equation for the resulting scattered electric field is solved approximately by the method of moments and the results for the echo width and polar radiation pattern are displayed graphically for typical geometrical dimensions, frequency and permittivity profiles.

The second part of this paper deals with the cylindrical dielectric shell terminating a conducting strip with one edge coinciding with the axis of the cylinder. The results for the scattered field due to a line source excitation are presented and extended to the case of two such strips whose unloaded edges intersect to form a corner reflector antenna. The resulting radiation pattern is shown to improve for specific dimensions and complex permittivity profiles of the dielectric caps.     

Time-domain upper bounds for signals on a multiconductor transmission line behind an aperture

A technique for bounding the maximum voltages at terminations of a multiconductor transmission line (MTL) located behind an aperture-perforated conducting screen excited by an electromagnetic field in the time domain is developed. The electromagnetic field is coupled through a small aperture as the excitation of a multiconduetor transmission line behind the aperture. A model is presented in terms of traveling waves and multiple refleetion phenomenon. These traveling waves transfer energy to the terminations. The energy at a terminatian is translated into voltages from which the upper bounds are determined. The upper bounds are obtained using vector norms and associated matrix norms.     

Excitation of Surface Waves on a Unidirectionally Conducting Screen

The excitation of plane surface waves by a line source on a unidirectionally conducting 1) infinite and 2) semi-infinite screen is considered. The conditions for the existence of the surface wave and the optimum location of the line source for obtaining the highest efficiency of excitation is determined.     

RCS reduction of a conducting cone-cylinder using an active method

This paper presents an active method for reducing the radar cross section (rcs) of a perfectly conducting cone-cylinder. The active elements are four microstrip patch antennas located symmetrically along the conecylinder axis. These elements radiate a field which permit the total scattered field in the direction of the receiver to be close to zero. In each time step, the hoarded system should determine some parameters of the incident wave: the incident direction, its frequency, amplitude, and phase. Then when antennas are fed with currents of suitable amplitude and phase, they radiate an electromagnetic field in the direction of the receiver that has the same amplitude but is opposite in phase compared to the scattered field, without feeding patches. Then the total field vanishes in the direction of the receiver.     

Plane wave reflection from a rectangular-mesh ground screen

The theory of scattering of electromagnetic plane waves of arbitrary incidence and polarization from an infinite rectangular-mesh ground screen is treated. The screen is composed of thin wires of circular cross section, and is parallel to the interface between two homogeneous media. The theoretical results for parallel-wire screens are obtained in the limit of large wire spacing for one dimension of the mesh screen. Results presented for incident parallel polarized plane waves indicate that both the parallel-wire and mesh screens may exhibit a change in reflecting properties as the plane of incidence is varied from the wire axis direction. In addition, it is shown that the parallel-wire screen can produce appreciably higher cross-polarized fields than a square-mesh screen of the same wire spacing.     

Surface Waves on an Anisotropic Plasma Sheath

A treatment of the excitation of unidirectional plane surface waves on a perfectly conducting screen covered with an anisotropic plasma sheath is given for the case in which the external magnetic field is oriented parallel to the screen but perpendicular to the direction of propagation. The dispersion relations for the surface waves and their dependence on the strength of the external magnetic field and the sheath thickness, are discussed. For sufficiently small sheath thickness, backward surface waves are found to exist. The powers carried by the surface waves and the space waves are evaluated, and the efficiency of excitation of the surface waves are determined as a function of sheath thickness for a typical set of parameters. The power carried by the forward and backward surface waves are compared for two cases in which, in a given direction, either one or both of these exist.     

Frequency-domain upper bounds for signals on a multiconductor transmission line behind an aperture

This paper develops a technique for bounding the maximum voltages and currents at terminations of a muiticonductor transmission line (MTL) located behind an aperture-perforated conducting screen excited by an electromagnetic field in the frequency domain. The electromagnetic field is coupled through a small aperture as the excitation of a multiconductor transmission line behind the aperture. A model is presented in terms of external and internal sources which in turn create traveling waves on the multiconductor transmission line. These traveling waves transfer energy to the terminations. The energy at a termination is translated into voltages and currents from which the upper bounds are determined. These upper bounds are obtained using vector norms and associated matrix norms. The formulation is presented in the frequency domain to obtain useful upper bounds for analysis of multiconductor transmission line geometries with aperture excitation.     

Fields in Planar Anisotropic Transmission-Line Metamaterials

An electromagnetic analysis of wave propagation in planar anisotropic transmission-line metamaterials is presented. It is shown that a planar square-celled grid, series-loaded with orthogonal inductors and capacitors and positioned over a ground plane, is magnetically anisotropic and may be described in terms of a diagonal permeability tensor. Resonance cone field concentrations form when two of the three diagonal elements of the permeability tensor are opposite in sign and the dispersion surface becomes hyperbolic. A theoretical treatment of an electric line current source excitation shows that the formation of resonance cones is a consequence of the singularity associated with the characteristic surface of a hyperbolic equation. The resonance cone angle, which also describes the direction of local power flow in the region between the grid and the ground plane, can be predicted to a good degree of accuracy. To the authors' best knowledge, the present work also verifies experimentally for the first time that current flow reverses direction across the resonance cone. Experiments, simulations, and analytical calculations of the cone angle are in good agreement.     

Electromagnetic coupling to a conducting wire behind an aperture of arbitrary size and shape

The electromagnetic coupling to a conducting wire behind an aperture in a plane conducting screen is analyzed. The aperture can be of arbitrary size and shape. The wire can be of finite length, with or without terminating loads, or of infinite length. The electric current on the wire and the equivalent magnetic current over the aperture region are calculated by the method of moments. An equivalent circuit for the effect of the aperture on the transmission line mode of the wire is derived.     

Scattering of electromagnetic waves by an anisotropic plasma-coated conducting cylinder

This paper considers the scattering of electromagnetic waves by an infinitely long anisotropic plasma-coated conducting cylinder. The source is assumed to be a magnetic current filament which gives rise to an incident magnetic field with only an axial component. Complete expressions for the scattered electric and magnetic fields are obtained. Scattering by an anisotropic plasma column and that by an isotropic plasma-coated conducting cylinder are special cases of the present problem.     

A note on the representation of scattered fields as a singularity expansion

The representation of the electromagnetic field scattered by a perfectly conducting finite-extent scatterer immersed in a lossless medium as a singularity expansion is considered. While the analytic properties of the temporal Laplace transform of the surface currents residing on such an object have received a great deal of attention, the properties of the scattered fields have not. It is shown that the representation of the transform of the scattered field must include an exponential entire function except for observation points in the forward-scattered direction. Explicit time domain representations that are counterpart to the Laplace domain representation are constructed and are shown to embody, in the early time, temporal variation besides that of the damped sinusoidal factors intrinsic to the singularity expansion. An important practical consequence of this more complicated time variation arises in connection with the application of the singularity expansion for target classification purposes and is commented upon herein.     

Shielding Properties of a Wire-Medium Screen

The shielding performance of a planar metamaterial wire-medium (WM) screen under plane-wave illumination is studied. The screen consists of a finite number of periodic layers of thin conducting cylinders embedded in a dielectric matrix; in the low-frequency limit and for waves with the electric field polarized along the wire direction, such a screen can be modeled as a homogeneous slab with a plasma-like dispersive permittivity. In particular, well below the plasma frequency, the effective relative permittivity is negative and very large in absolute value, giving rise to high values of the shielding effectiveness. A comparison with a conventional planar metal screen is reported, showing how the proposed structure can be designed to be advantageous in terms of low density and weight saving. In order to be effective against arbitrarily polarized waves, the original structure is modified, adding a second WM screen with wires orthogonally oriented with respect to the first one. Numerical results are provided that illustrate the validity of the homogenized model in predicting field levels both near to and far from the screen and show the shielding performance attainable with the proposed metamaterial screen in both single- and double-layer configurations at normal incidence.     

Numerical evaluation of dyadic diffraction coefficients andbistatic radar cross sections for a perfectly conducting semi-infiniteelliptic cone

In this paper, the scattering of electromagnetic waves by a perfectly conducting semi-infinite elliptic cone is treated. The exact solution of this boundary value problem in problem-adapted spheroconal coordinates in the form of a spherical multipole expansion is of poor convergence if both the source point and the field point are far away from the cone's tip. Therefore, an appropriate sequence transformation of these series expansions (we apply the Shanks transformation) is necessary to numerically determine the dyadic diffraction coefficients and bistatic radar cross sections (RCS) for an arbitrary elliptic cone. Our far-field data for an elliptic cone, a circular cone, and a plane angular sector are compared with some other results obtained with the aid of quite different methods     

线状散射体的有效电磁建模方法及其应用

首先引入直截锥体单元来模拟金属线状目标,可对横截面连续变化的导线做精确近似;再用简化核代替精确核,避免了源点与场点重合引起的积分奇异性;然后根据提出的混合域基函数,采用矩量法对目标模型的电磁散射特性进行分析计算.计算结果表明,该方法简单有效且有利于电磁散射问题的计算机快速求解.     

Asymptotic analysis of edge-excited currents on a convex face of aperfectly conducting wedge under overlapping penumbra region conditions

The edge-excited surface currents on a convex face of a perfectly conducting curved wedge are investigated in the asymptotic high-frequency limit for the case where the penumbra regions of the edge and surface diffractions overlap. The edge of the wedge is assumed straight, and the incident electromagnetic wave locally plane and normal to the edge. Both polarizations are considered. The surface field induced by the edge diffraction is synthesized in the spirit of the spectral theory of diffraction (STD): the solution for the edge-diffracted field is interpreted as a spectrum of inhomogeneous plane waves, and the surface field excited by each spectral plane wave is obtained by analytical continuation of the Fock (1965) functions into complex space. The main purpose of this work is to prove the reciprocity of a solution deduced previously for the problem of line source radiation from the wedge in question. As a by-product, useful identities for an incomplete Airy function and an Airy-Fresnel integral are developed     

Edge diffraction in the vicinity of the tip of a composite wedge

The electromagnetic field due to a line source radiating in the presence of a two-dimensional composite wedge composed of a number of conducting and dielectric materials is obtained. The Fourier transform path integral method (FTPI) is described and used to perform the numerical analysis. An important feature of the FTPI method is that it is based on a global solution to the Helmholtz scalar wave equation. As such the method avoids numerical enforcement of boundary conditions and the necessity of reformulating the analytical/numerical equations for each geometric configuration. The total scattered field is presented for several cases where one of the dielectric wedge sections is lossy, including examples of microwave scattering from a crested ocean surface and an air-ocean-sea ice interface