Circumferential distribution of scattering current and small hole coupling for thin finite cylinders

The scattering current induced on a thin finite conducting cylinder immersed in a"theta"-polarizedE-field is studied. Particular attention is paid to the circumferentially nonuniform mode as theE-field angle of incidence varies. This nonuniformity is shown significant (peak-to-average ratio of 3 dB at cylinder midlength) at certain incidence angles for wavelength long cylinders with diameters as small as0.067lambda. Also investigated is the relationship between scattering current and cavity response patterns for narrow thin-walled cylindrical cavities with small holes through which energy is coupled. It is demonstrated theoretically, with experimental verification, that the circumferential variation of scattering current strongly affects the fields within thin cylindrical cavities having apertures with small circumferential extents. It is noted, however, that for most thin-body radiation and scattering problems (in contrast with aperature coupling) only the uniform current mode is significant.     

散射目标近区口径面天线与目标相互影响的研究

本文用FDTD方法在散射目标近区对口径面天线与目标的相互影响进行了分析研究,在圆柱坐标系下以轴对称数值模型为例,用FDTD方法得出了散射目标电尺寸、目标与口径面天线的距离大小对天线口径面的场分布之影响的数值结果;同时获得了散射目标表面物理光学近似计算散射电流的应用范围.用圆波导中模式理论验证了方法的有效性     

High frequency scattering by a conducting ring

Asymptotic formulae are derived for the Fourier coefficients of the thin wire kernel in the integral equation for the electric current on an electrically large, thin circular loop. The total current induced on the ring by a plane electromagnetic wave is approximated by a modified physical optics term proportional to the incident field, plus resonant terms of lossy circulating waves. Numerical evaluation of the dominant poles and residues of the ring transfer function provides the amplitudes and complex propagation constants of these natural modes.     

Electromagnetic plane wave excitation of an open-ended conductingfrustum

A Chebyshev-Galerkin solution of the electric field integral equation for the surface current induced on a conducting frustum by an incident plane wave is presented. The physically motivated mathematics takes proper account of the static singularity in the kernel function and of the edge conditions at both apertures, to yield complete and convergent current expansions. Coupling of the electromagnetic field into the tapered interior of the open scatterer can be substantial, even in electrically narrow cross sections, due to the focusing action of the conical transmission line     

Transient response of the infinite cylindrical antenna and scatterer

The transient response of an infinite cylindrical antenna and scatterer is obtained by time gating a time-dependent electric-field integral equation for thin wires of finite-length. The driving point current and broadside radiated field approximate the driving point voltage in the antenna case, while the current excited in the scattering case approximates the time integral of the incident field. This approximate integral relation is used to estimate currents excited on cylindrical structures without employing the complete integral equation solution procedure.     

Mutual admittance between slots in cylinders of arbitrary shape

A numerical formulation involving the Fourier transform and the combined-field integral equation is presented to calculate the mutual admittance between axial slots or circumferential slots in arbitrarily shaped cylinders. Two cases are considered: coupling between two circumferential slots and coupling between two axial slots. The slots are assumed to be waveguide-fed apertures excited with the TE₁₀ waveguide mode, with no higher-order modes included in the model. The combined field-integral equation (CFIE) formulation was used in conjunction with the magnetic field integral equation (MFIE) to solve the resulting two-dimensional integral equation. Formulations for the axial slot case are discussed. Results concerning the discretization and truncation of the spectrum, as well as the accuracy of the procedure, are presented for both axial and circumferential cases. The accuracy of the procedure is demonstrated by comparison with eigenfunction data for circular cylinders     

Hemispherically capped thick cylindrical monopole with a conical feed section

The axis boundary condition scalar potential integral equation is used to study the hemispherically capped thick cylindrical monopole with a conical feed section. The monopole is perfectly conducting and is fed by a coaxial line whose outer radius is connected to an infinite perfectly conducting ground plane. One approximation in the equation is that the field across the coaxial aperture is approximated by the fundamental TEM coaxial line mode. The integral equation is numerically solved by the moments method using entire domain basis functions with delta weights. A simple way for checking the convergence of the solution that needs no new integrations is given. The theoretical current distributions, input admittances, and radiation patterns are given for a thin, moderately thick, and thick monopole with and without a conical feed section. This feeding section simplifies the matching of the antenna to conventional signal generators and improves the high frequency characteristics of the monopole.     

Plane-wave diffraction by a wedge--A spectral domain approach

The canonical problem of plane wave diffraction by a wedge in the context of the spectral domain approach which exploits the relationship between the induced current on a scatterer and its far field is investigated. It is shown how the exact solution to the wedge diffraction problem can be manipulated in a form which enables one to interpret the far scattered field as the Fourier transform of the physical optics (PO) current on the two faces of the wedge augmented by the fringe current near the tip of the wedge. A uniform asymptotic expansion for the total field which slightly modifies the Ansatz in the uniform asymptotic theory of electromagnetic edge diffraction is constructed.     

Analysis of Crossed Wires in a Plane-Wave Field

The currents and charges induced in a pair of electrically thin crossed wires by a normally incident plane electromagnetic wave are derived by analytical methods. The boundary conditions at the ends and at the junction are explained. The solution of a new integro-differential equation for the currents is obtained in terms of trigonometric and integral-trigonometric functions. Depending on the electrical lengths of the crossed elements and location of their junction a variety of quite different distributions of current and charge obtain. These determine the scattered near and far fields. Graphs of computed currents and charges per unit length on the four arms of several important cases are displayed. The accurate determination of the induced currents and charges on a mathematically tractable structure-the thin-wire cross-is an early step in a study that will proceed to electrically thick cylinders, wide strips, and their junctions in crossed configurations in an effort to gain a meaningful approximate understanding of the currents and charges induced on an aircraft by an electromagnetic pulse.     

Low-frequency scattering from two-dimensional perfect conductors

Exact expressions are obtained for the leading terms in the low-frequency expansions of the far field scattered by an arbitrarily shaped cylinder with finite cross section, an arbitrarily shaped cylindrical bump on a ground plane, and an arbitrarily shaped cylindrical dent in a ground plane. By inserting the low-frequency expansions of the incident plane wave and Green's function into exact integral equations for the surface current, integral equations are obtained for the leading terms in the low-frequency expansions of the surface current. Simple integration of these leading terms of the current expansion yield the leading terms in the low-frequency expansions of the scattered fields. The general low-frequency expressions are confirmed by comparing them to low-frequency results obtained from exact time-harmonic eigenfunction solutions     

Model of a dipole antenna in free-space and near a perfectly electrical conducting plate

In this paper, an accurate method is developed to model dipole antennas in free-space or near an infinite perfectly electrical conducting plate. The magnetic field integral equation is implemented and point-matching is used to construct the interaction matrix. The edges of the cylinder, which can cause numerical instabilities in the simulated current distribution, are getting a special treatment by choosing a specific set of basis functions. The excitation is modeled based on the equivalence principle and leads to the application of a magnetic surface current over the gap region. The exact kernel of the integral equation is applied and all possible current components are taken into account. When the antenna is close to a conducting plate, a strong modification of the surface current distribution is observed. It is demonstrated that for these configurations, the incorporation of the circumferential component of the surface current distribution is needed. At larger distances, the circumferential surface current can be neglected in the total distribution. The only approximation (or rather assumption) we made is that we impose a longitudinal directed electrical field over the boundary of the gap region. The results are verified experimentally.     

Analysis of electromagnetic scattering from conducting bodies ofrevolution using orthogonal wavelet expansions

The wavelet expansion method has been extended to study the electromagnetic scattering from conducting bodies of revolution. The magnetic field integral equation (MFIE) is solved by this approach. By expanding the induced surface currents in terms of Fourier series of uncoupled azimuthal cylindrical modes, a simplified MFIE is attained for each unknown mode current that varies along the curved profile of the scatterer. By applying the boundary element method (BEM), the curved profile is mapped into the definition domain of the orthogonal wavelets on the interval. The unknown mode currents are then expressed using multiscale wavelet expansions. The simplified MFIE is converted into a sparse, multilevel matrix equation by the Galerkin method. Numerical examples are provided to illustrate the merits of this wavelet approach     

Gaussian beam scattering from a conducting cylinder partially buried in a ground plane

The problem of scattering from an infinitely long conducting cylinder that is partially buried in a perfectly conducting ground plane due to an obliquely incident gaussian beam is solved by an exact procedure based on the method of images by first adopting a simplification originally proposed by Kozaki. The incident and the specularly reflected fields are expressed in terms of cylindrical vector wave functions multiplied by a weighting function which involves the beam parameters like the radial distance of the source and beam width. The scattered fields originating from the cylinder and its image in the ground plane are expressed in terms of cylindrical vector wave functions. The boundary conditions on the surface of the cylinder are then imposed and this procedure leads to a coupled infinite system of equations for the even and odd mode expansion coefficients of the scattered field. These equations are solved numerically for the case of cylinders having electrical radius in the Rayleigh and resonance regions. Both the transverse magnetic and transverse electric polarizations of the incident beam wave are considered and some representative numerical results for the scattered far-field are presented in graphical form. The magnitude of the induced current for the TM polarization is calculated and compared with the corresponding case of plane wave incidence.     

Electromagnetic Coupling to an Infinite Cable Placed Behind a Slot-Perforated Screen

We present an analysis of the electromagnetic coupling from an external illuminating field to an infinitely long thin cable placed behind the conducting screen that is perforated by a narrow slot. By using a Fourier transform representation for the unknown electric current on the cable, we obtain an integro-differential equation for the distribution of the electric field in the slot that accounts completely for the coupling. The transfer admittance function so obtained, is in a form very convenient to estimate the electric current on the infinite cable. Results of the slot electric-field distribution and the current induced on the infinite cable are given for a few typical cases. Not surprisingly, the axial distribution of induced voltage on the slot is markedly affected by the presence of the cable.     

Scattering and mode conversion of guided modes by an arbitrary cross-sectional cylindrical object in an optical slab waveguide

The scattering and mode conversion of guided modes caused by a cylindrical scatterer of arbitrary cross section embedded in, or placed on, a dielectric slab waveguide are investigated theoretically. The method employed for the analysis is the surface current integral equation technique. The total scattered power, mode-conversion coefficients, and scattering patterns for the typical numerical examples are shown.     

A Hallen-type integral equation for symmetric scattering from lossycircular disks

A rigorous technique is presented for calculating the current induced on a thin lossy disk by rotationally symmetric sources, and the resulting scattered field. A Hallen-type integral equation is developed for the current using the magnetic vector potential, and it is solved by the method of moments. It is shown that the diffraction lobes usually associated with radiation above a finite circular ground plane can be reduced dramatically by the addition of loss. Application to a quarter-wave monopole radiating above a finite circular perfectly conducting ground plane shows good agreement with experiment     

Cylindrical cavity-backed suspended stripline antenna-theory andexperiment

The problem of a cylindrical cavity-backed suspended stripline (SSL) antenna is viewed as a transition of the SSL to a circular cylindrical waveguide opening into an infinite ground plane. The fields in the waveguide are expanded in terms of TE and TM modes. The effect of the radiating aperture on the modal expansion of the fields is taken into account by introducing reflection coefficients for each mode. The current on the SSL probe is assumed to have sinusoidal distribution. These simplifications reduce the original problem to that of a known radially oriented current residing on a dielectric sheet inside a circular-cylindrical cavity whose top wall has known impedances corresponding to different modes. The Green's function for this modified structure is found and is used to obtain a general expression for the input impedance. This expression is specialized to the case where the SSL probe and the radiating aperture are coupled through the dominant TE ₁₁ mode only. This input impedance is translated to the measurement plane of the antenna. The computed and measured results are found to be in good agreement     

Determination of the natural frequencies of a thin wire elliptical loop

The natural frequencies of a thin wire scatterer formed into an elliptical loop are determined theoretically by solving the homogeneous transform domain electric field integral equation (EFIE). Comparison with frequencies extracted from measurements of the time domain scattered field responses of elliptical and circular loops at various aspects validates the assumption of a natural mode expansion of the late-time response of conducting radar targets.     

The exact kernel for cylindrical antenna

In the numerical calculation for a cylindrical antenna, the thin-wire kernel is widely used as a convenient approximation to the exact kernel of the electric-field equation. However, this thin-wire approximation is valid only if the radius of wire is much less than its length and the wavelength; furthermore, arbitrarily using this approximation may not obtain the convergent outcomes. The expression of the exact kernel for the cylindrical antenna, which would be useful in the numerical work, is presented     

Incremental diffraction coefficients for planar surfaces

Exact expressions for incremental diffraction coefficients at arbitrary angles of incidence and scattering are derived directly in terms of the corresponding two-dimensional, cylindrical diffraction coefficients. The derivation is limited to perfectly conducting scatterers that consist of planar surfaces, such as the wedge, the slit in an infinite plane, the strip, parallel or skewed planes, polygonal cylinders, or any combination thereof; and requires a known expression (whether exact or approximate) for the two-dimensional diffraction coefficients produced by the current on each different plane. Specifically, if one can supply an expression for the conventional diffraction coefficients of a two-dimensional planar scatterer, one can immediately find the incremental diffraction coefficients through direct substitution. No integration, differentiation, or specific knowledge of the current is required. Special attention is given to defining ambiguously all real angles and their analytic continuation into imaginary values required by the incremental diffraction coefficients