Calculating currents in a conductor from the measured scattered magnetic field

A theoretical method for calculating currents in a wire antenna when it is illuminated with an electromagnetic wave, is proposed. The current is determined from the scattered field observed near the antenna. The purpose of such calculations is to evaluate currents even when the wire antennas are not directly accessible to current measurement. The tlm method has been used to simulate the scattered field. Theoretical results obtained with our method and tlm simulations are in very good agreement.     

Moment-method modeling of short-pulse scattering from and theresonances of a wire buried inside a lossy, dispersive half-space

A frequency-domain method-of-moments (MoM) algorithm is used to model short-pulse plane-wave scattering from a wire buried inside a lossy, dispersive half-space with the time-domain scattered fields computed via the Fourier transform. Further, the complex resonant frequencies of such targets are also calculated via the MoM. The phenomenology associated with scattering from a buried wire is investigated in detail, and it is demonstrated that the time-domain scattered fields and the target resonances depend on the buried-wire orientation as well as the electrical properties of the half-space     

Currents induced in a wire cross by a plane wave incident at an angle

The currents induced in a thin-wire cross with equal mutually perpendicular arms by an incident plane electromagnetic wave are determined when the normal to the wave front is perpendicular to the horizontal wire and is at an anglethetawith respect to the vertical wire; the direction of the electric vector in the wave front is arbitrary. The analysis is formulated in general terms but explicit formulas are obtained only for the zero-order currents which are generally adequate to determine the scattered field of very thin wires. The relatively simple formulas consist of even and odd parts for both the vertical and horizontal wires; they include components due to mutual coupling as well as those excited directly by the incident field.     

Coupling through narrow slot apertures to thin-wire structures

Electromagnetic coupling through a narrow slot aperture in an infinite ground plane is considered when a thin wire is present in the coupled half-space. The effect of the wire on coupling is determined. The slot has width and depth. The method of moments is applied to determine equivalent aperture currents and wire currents. Then coupling is calculated from the resulting fields. Results are presented for high and low Q slots and on and off resonance     

Analysis of the currents induced in a general wire cross by a plane wave incident at an angle with arbitrary polarization

Earlier analyses of the wire cross under restricted conditions are generalized, and a complete first-order solution is provided for the currents and charges per unit length in all four arms. These may have different and unrestricted lengths. The angles of incidence and polarization of the exciting plane wave are arbitrary. Detailed graphs are provided for the currents in and scattered fields of an equi-arm cross with electrical arm lengthskhup to 3.5 when the angle of incidence is45degand the electric vector is perpendicular to the horizontal wire. The included three resonances and the condition for a three-arm type of oscillation are treated. The critical effects of changes in the length of one of the vertical arms are examined for normal and nonnormal incidence.     

A three-wave FDTD approach to surface scattering with applicationsto remote sensing of geophysical surfaces

A major difficulty in physical interpretation of radio wave scattering from geophysical surfaces is the lack of detailed information on the signatures of geologically plausible discrete objects. Although the aggregate response will never be dominated by any single object, differences in the population of discrete objects on or near the surface (their sizes and shapes, for example) can change the character of a radio echo markedly. When the average surface is modelled as a flat, homogeneous half-space, the field that “drives” the scattering process is a composite consisting of the incident plane wave and the reflected and transmitted plane waves, all of which are known quantities; the total field can then be defined as the sum of the driving field and the scattered field. When a discrete object is near the surface, the total field can be calculated using finite-difference time-domain (FDTD) techniques, and the scattered near field can be calculated accordingly. The Green's functions for electric and magnetic currents above and below the surface, obtained by Sommerfeld theory and employed in conjunction with Huygens' principle, transform the local scattered fields to the far field. The FDTD implementation accommodates discrete lossy dielectric and magnetic scatterers in the vicinity of a dielectric surface; extension to a lossy half-space is straightforward. Two-dimensional results for scattering from perfectly conducting circular cylinders above and below a dielectric surface agree with moment method solutions within a few percent. Results for scattering from a dielectric wedge exhibit expected forward diffraction and internal reflection phenomena     

Scattering by a finite set of perfectly conducting cylinders buried in a dielectric half-space: a spectral-domain solution

An analytical-numerical technique, for the solution of the two-dimensional electromagnetic plane-wave scattering by a finite set of perfectly conducting circular cylinders buried in a dielectric half-space, is presented. The problem is solved for both the near- and the far-field regions, for TM and TE polarizations. The diffracted field is represented in terms of a superposition of cylindrical waves and use is made of the plane-wave spectrum to take into account the reflection and transmission of such waves by the interface. The validity of the approach is confirmed by comparisons with results available in the literature, with very good agreement. The multiple interactions between two buried cylinders have been studied by considering both the induced currents and the scattered field diagrams. Applications of the method to objects of arbitrary cross-section simulated by a suitable configuration of circular cylinders are shown.     

On the Use of Surface Impedance in the Detection and Location of Buried Objects

A method to detect and locate the cylindrical objects buried in a half-space is presented. The method is based on the reconstruction of the appearing surface impedance of the half-space through remote field measurements of scattered field for a single plane wave illumination. The existence of the objects causes surface impedance to be a function of location. The inhomogeneous surface impedance is determined from the boundary condition itself which requires to know the scattered field and its derivative on the boundary. These values are calculated by analytically continuing the measured data to the boundary of the half-space. The method yields satisfactory results if (i) adjacent cylinders are not very close to each other, (ii) the cylinders are not deeply buried and (iii) the half-space of burial is not highly attenuative.     

Highrise Building Reradiation and Detuning at MF

An evaluation of the reradiation from highrise buildings in the MF broadcast band is presented. Scattered field measurements were made at MF on buildings of the order of one quarter wavelength in electrical height and at UHF on scale models. A computational model incorporating the effects of lossy ground and building materials was developed using moment-method techniques. Reduction of scattering is demonstrated, both for models and a full-scale building, using simple wire detuning stubs to modify the induced RF currents. The current distributions resulting in the decrease of scattering are described for a vertical stub like those used in power line detuning, and for two related new designs, a rooftop stub and the "umbrella" stub. The latter is shown to be the most effective, yielding scattered field reductions of the order of 6dB in fullscale tests.     

MLFMA analysis of scattering from multiple targets in the presence of a half-space

The multilevel fast multipole algorithm (MLFMA) is applied to the analysis of plane-wave scattering from multiple conducting and/or dielectric targets, of arbitrary shape, situated in the presence of a dielectric half-space. The multiple-target scattering problem is solved in an iterative fashion. In particular, the fields exciting each target are represented as the incident fields plus the scattered fields from all other targets. The scattered fields from each target are updated iteratively, until the induced currents on all targets have converged. The model is validated with an independent scattering algorithm, and results are presented for several example multitarget scattering scenarios.     

Scattering of TE-polarized EM wave by discontinuity in groundeddielectric layer

The two-dimensional problem of EM wave interaction with a dielectric discontinuity in an infinite grounded dielectric layer is studied. An electric field integral equation (EFIE) for TE illumination has been derived based on the Green's function for the electric field produced by induced polarization currents in the discontinuity region. Impressed electric fields consist of either plane waves incident from space above the dielectric layer or surface waves supported by that layer. Method of Moments (MoM) numerical solutions for the induced electric field in the discontinuity region are implemented. The amplitudes of surface waves excited by excess discontinuity-region polarization currents are calculated, as well as the pattern of the scattered field and the associated scattering width. It is observed that the excitation of a surface-wave mode reduces the back scattered radiation for TE-polarized plane wave incidence. The accuracy of the theory is verified by comparison of numerical results with those of existing studies     

A wire-grid model for scattering by conducting bodies

A point-matching solution is developed for scattering by conducting bodies of arbitrary shape. Numerical results are included for the backscatter echo area of circular and square wire loops, circular and square plates, spheres, and hemispheres. The results show good agreement with experimental data. An efficient calculation procedure is achieved by using a wire-grid model instead of a continuous conducting surface. A system of linear equations is generated by enforcing the boundary conditions at the center of each wire segment of the grid, and a digital computer is employed to solve these equations for the currents on the segments. Then it is straightforward to calculate the distant scattered field and the echo area.     

Asymptotic solutions for the scattered field of plane wave by a cylindrical obstacle buried in a dielectric half-space

Two-dimensional scattering of a plane wave by an embedded conducting strip is formulated rigorously using the concept of Kobayashi potential, in which potential or wave function is expressed in terms of infinite integrals including the Bessel function in the integrand. By imposition of the required boundary conditions at the interface of the dielectric half-space and on the strip, the problem is reduced to a dual integral equation (DIE). Using the discontinuous properties of Weber-Schafheitlin's integrals, DIE is transformed into a matrix equation with infinite unknowns whose elements are expressed by infinite integrals. Asymptotic solutions for the matrix elements are derived when the separation between the interface of the different media and the obstacle is large compared to the wavelength. Using these results, the expression for the scattered field is derived in a general form which can be applied to an arbitrary cylindrical obstacle. Some numerical results are given for conducting strip and circular cylinder to see the effect of inhomogeneity on the surrounding medium, size of the obstacle, and the angle of incidence on the scattered field.     

Electromagnetic Modeling for Microwave Imaging of Cylindrical Buried Inhomogeneities

Many diagnostic techniques in geophysics and civil engineering are based on the interaction of electromagnetic waves with objects buried in homogeneous or stratified media. Most of the investigations are concerned with the detection of buried objects, but a few papers have dealt with the problem of identifying the objects. The proposed method is based on the integral representation for a plane wave incident on a lossy half-space containing a cylindrical object of arbitrary cross section and electrical properties. The induced current distribution in the object is obtained from the backscattered field measurement in amplitude and phase. In order to improve the spatial resolution of the image, the scattered field is measured for different plane wave incidence and frequencies. Results of numerical simulations concerning the shape and size of the object for different values of soil electromagnetic parameters are presented in this paper.     

Electromagnetic response of two crossing, infinitely long, thinwires

The electromagnetic coupling of two crossed thin wires of infinite length is considered. Two coupled integral equations are obtained, given in terms of generalized impedance functions, for the spectral currents flowing in each wire. The wires may be in a homogeneous medium or over a half-space. The numerical implementation focuses, however, only on the former. The numerical solution may be obtained by either applying moment or multiple scattering methods. The solution obtained from the method of moments is applicable for any wire spacing. Obversely, the multiple scattering method leads to a convenient matrix series solution, which shows that the coupling between wires is proportional to 1/d ² (where d is the wire separation) plus higher order scattering terms     

Electromagnetic scattering from a chiral cylinder of arbitrarycross section

A simple moment solution is presented to the problem of electromagnetic scattering from a homogeneous chiral cylinder of arbitrary cross-section. The cylinder is assumed to be illuminated by either a TE or a TM wave. The surface equivalence principle is used to replace the cylinder by equivalent and magnetic-surface currents. These currents radiating in unbounded external medium produce the correct scattered field outside. When radiating in an unbounded chiral medium, they produce the correct total internal field. By enforcing the continuity of the tangential components of the total electric field on the surface of the cylinder, a set of coupled integral equations is obtained for the equivalent surface currents. Unlike a regular dielectric, the chiral scatterer produces both copolarized and cross-polarized scattered fields. Hence, both the electric and magnetic current each have a longitudinal and a circumferential component. These four components of the currents are obtained by using the method of moments (MoM) to solve the coupled set of integral equations. Pulses are used as expansion functions and point matching is used. The Green's dyads are used to develop explicit expressions for the electric field produced by two-dimensional surface currents radiating in an unbounded chiral medium. Some of the advantages and limitations of the method are discussed. The computed results include the internal field and the bistatic and monostatic echo widths. The results for a circular cylinder are in very good agreement with the exact eigenfunction solution     

The backscattered field of a thin wire loop for H-polarization

For a thin wire loop illuminated by a plane electromagnetic wave, the backscattered field is determined when the incident magnetic vector is parallel to the plane of the loop. The complete second-order geometrical theory of diffraction (GTD) solution is obtained and found to be in excellent agreement with numerical data at angles close to normal incidence on the loop. At wide angles, however, the data exhibit a significant lobe that is not predicted by GTD. Analysis of the data shows that the lobe is due to currents circulating around the loop and the properties of these currents are deduced. Using a simple model for the current, the corresponding contribution to the backscattered field is determined. When this is added to the GTD solution, the resulting expression for the backscattered field is in good agreement with the numerical data for all angles of incidence and all loop diameters greater than a wavelength.     

An equivalent model for the analysis of electromagnetic scatteringfrom a long, lossy tether structure

An equivalent lossy wire model is introduced for the analysis of electromagnetic scattering from a long lossy tether structure, which is used to suspend a satellite from a shuttle in free space. The equivalent model incorporates the electrical characteristics of the uniform TM₀₁ mode of a coated wire configuration and the geometrical properties of a lossy wire configuration. By considering residue contributions and the resulting surface wave quantities alone, and by adopting two additional simplifications in the calculation of induced currents and scattered field, the equivalent lossy wire model is used to determine the effect of a long, lossy tether on communications between its two ends. It is shown that for tether lengths of up to 30 km with the primary source being located close to one end of a lossy, straight tether, the presence of the tether does not have a significant effect on the near-zone fields in the neighborhood of its `receiving end'     

SEM representation of the early and late time fields scattered fromwire targets

In the communication, the singularity expansion method (SEM) is used to express the field scattered from an arbitrary thin-wire target. Explicit expressions are given for both the class-1 and class-2 representations of the scattered field due to step excitations. Numerical results are given for the early-time transient fields scattered from both a straight wire and a simple swept-wing aircraft model. The results of the SEM computations are compared to fields obtained by Fourier inversion techniques. It is shown that the class-2 representation yields a significant improvement over the class-1 result during the early-time interval, albeit at the expense of a more complex computation