摘要:

In a previous paper the momentum transfer expansion was introduced for scalar wave scattering. It was shown that, using this expansion, one could obtain wavelength dependent curvature corrections to the Kirchhoff approximation. In this paper we extend the momentum transfer perturbation expansion to electromagnetic waves. Curvature corrections to the surface current are obtained. Using these results, the specular field and the backscatter cross section are calculated.

ISSN:0048-6604    

DOI:10.1029/90RS01863

年代:1991

数据来源: WILEY

摘要:

The method of moments has found extensive applications in the solution of a wide class of electromagnetic problems. Additionally, there are a number of basis functions which have found favor and with them different types of testing methods. This work examines the selection of test functions, given a selected set of basis functions, which serve to minimize the mean square error. These test functions will guarantee equal or lesser error compared with any other test function. Additionally, these test functions provide a monotonic improvement of the approximation as the increase in the order of the system. Hence while these test functions require the application of the differential or integral operator one additional time, they provide an absolute lower bound to the mean square error in the approximation and a means to systematically improve accuracy with each increase in the order of the unknowns.

ISSN:0048-6604    

DOI:10.1029/90RS02334

年代:1991

数据来源: WILEY

摘要:

The electromagnetic fields inside and outside a steadily rotating, magnetized, conducting sphere are determined for the cases of nonprecession and precession. In both cases the spin rotational axis is aligned with the magnetic axis of the sphere. The field expressions are those measured in the laboratory reference frame. For a nonprecessing sphere the magnetic fields are identical to the fields of a stationary sphere, but in addition there is an induced induction of order υ2/c2whose lines of force radiate in loops above and below the equator. The electric and magnetic induction field expressions were derived in the laboratory frame when the sphere precesses in an external magnetic field. They can be divided into static and dynamic parts. The amplitudes of these parts were plotted as functions of the angle of inclination of the polar axis. The dynamic parts are circularly and elliptically polarized. The pivot point of the precessing sphere was chosen off center. The only two stable positions are at θ = 0° and 180° for a center pi

ISSN:0048-6604    

DOI:10.1029/90RS02429

年代:1991

数据来源: WILEY

摘要:

The natural frequencies of an object determine the late time response of a transient excitation and govern the time harmonic scattering in the resonance region. In this paper the natural frequencies and the corresponding eigencurrents of a perfectly conducting elliptic disk are treated using the null‐field approach. The elliptic disk is obtained as the zero thickness limit of a general ellipsoid. It is found that the dominant frequencies, i.e., the frequencies that stay in the finite complex frequency plane when the disk approaches the “wire limit,” line up in two series. The first series, closest to the real axis, can be identified as the resonances of a thin wire antenna, corresponding to quasi‐stationary current waves on the disk. The second series has more involved nonstationary current patterns. The theoretical results are illustrated with numerical calcu

ISSN:0048-6604    

DOI:10.1029/90RS02331

年代:1991

数据来源: WILEY

ISSN:0048-6604    

DOI:10.1029/90RS02079

年代:1991

数据来源: WILEY

摘要:

When boundary conditions of higher order than the first are applied to an edged structure such as a wedge or half plane, difficulties occur associated with the uniqueness and reciprocity of the solution. These are illustrated in the case of a plane wave incident on a half plane subject to arbitrary second order boundary conditions different on the two faces. Using an extension of the standard Maliuzhinets method, it is shown how reciprocity can be explicitly enforced, and a solution is then obtained involving an arbitrary constant. The constant is related to the surface value of the field at the edge, and this additional information is necessary for a unique solution.

ISSN:0048-6604    

DOI:10.1029/90RS00967

年代:1991

数据来源: WILEY

摘要:

The transverse aperture‐integral equation method provides a means of computation for diffraction coefficients at blunt edges of a broad class of stratified layers, including sheet‐anisotropy models for conducting composites. In this paper we concentrate on the application of the method when the material profile comprises layers of homogeneous, potentially lossy material. The method proceeds from defining an artificial aperture perpendicular to a semi‐infinite, planar, stratified region and passing through the terminal edge of the region. An integral equation is formulated over this infinite‐extent aperture, and the solution to the integral equation represents the influence of the edge. The kernel in the integral equation is a weighted sum of the Green's functions for the respective half spaces lying on either side of the aperture plane. The vector wave equation is separable in each of these half spaces, resulting in Green's functions that are expressible analytically. The Green's function for the stratified half space is stated in terms of a Sommerfeld‐type

ISSN:0048-6604    

DOI:10.1029/90RS01645

年代:1991

数据来源: WILEY

摘要:

A number of iterative algorithms to solve integral equations arising in field problems are discussed. We describe the essential features of the Neumann Series, overrelaxation methods, Krylov subspace methods, and the conjugate gradient technique. Proofs of convergence of the conjugate gradient method are directly available when the underlying integral operator is self‐adjoint, and in this case the method is equivalent to the Krylov method. However, for non‐self‐adjoint operators the conjugate gradient method requires an implicit symmetrization which results in poorer convergence than that obtained using the Krylov method. Some convergence results are also available for overrelaxation methods for both self‐adjoint and non‐self‐adjoint operators. Relations between all of the methods will be described and numerical performance will be contrasted using a uniform square error criterion. All the methods are treated in the continuous operator form which is especially useful in using the physical setting to arrive at effective prec

ISSN:0048-6604    

DOI:10.1029/90RS00934

年代:1991

数据来源: WILEY

摘要:

In deriving the electric and magnetic fields in a continuous source region by differentiating the vector potential, Yaghjian (1985) explains that the central obstacle is the dependence of the integration limits on the differentiation variable. Since it is not mathematically rigorous to assume the curl and integral signs are interchangeable, he uses an integration variable substitution to circumvent this problematic dependence. Here, we present an alternative derivation, which evaluates the curl of the vector potential volume integral directly, retaining the dependence of the limits of integration on the differentiation variable. It involves deriving a three‐dimensional version of Leibniz' rule for differentiating an integral with variable limits of integration, and using the generalized rule to find the Maxwellian and cavity fields in the source regio

ISSN:0048-6604    

DOI:10.1029/89RS03057

年代:1991

数据来源: WILEY

摘要:

Electromagnetic scattering by multiple objects in the resonance region is investigated numerically with particular emphasis on direct application to object characterization. The influence of coupling between objects on scattering properties is demonstrated. The average scattering cross section is suggested as a tool to extract useful information about an unknown object adjacent to other known objects from total scattering data measured from multiple scatterers.

ISSN:0048-6604    

DOI:10.1029/90RS00906

年代:1991

数据来源: WILEY