Showing papers on "Near and far field published in 1979"

Discrete wave number representation of elastic wave fields in three-space dimensions

Abstract: We present the generalization to three dimensions of the discrete wave number representation method of Bouchon and Aki (1977). The method is developed to study the near field of a three-dimensional seismic source embedded in a layered medium. The elastic wave fields are represented by a superposition of plane waves propagating in discrete directions. The discretization is exact and results from a periodic two-dimensional arrangement of sources. The accuracy of the method is checked, in the case of a rectangular dislocation source radiating in an infinite medium, by comparing the results obtained with Madariaga's (1978) exact solution. Examples of the calculation of strong ground motion produced by a thrust fault and a strike slip fault are presented.

Light emission by magnetic and electric dipoles close to a plane dielectric interface. III. Radiation patterns of dipoles with arbitrary orientation

Abstract: We have derived analytic expressions for the angular distribution of light emitted by an atomic system, e.g., a fluorescent molecule, located in a dielectric medium 1 at distance z0 from the interface to a different dielectric medium 2. The theory is rigorously valid for electric and magnetic dipole transitions with arbitrary orientation of the dipole transition moment. (In Paper II [ J. Opt. Soc. Am.57, 1615– 1619 ( 1977)], only the special case of dipoles oriented perpendicular to the interface had been treated.) The radiation patterns of dipoles located on the interface (z0 = 0) and oriented parallel to it, and ensembles of such dipoles radiating incoherently with randomly oriented dipole moments were examined in particular. They differ greatly from the corresponding well-known dipole radiation patterns in an unbounded medium 1, due to the wide-angle interferences of emitted plane waves, and radiation from evanescent waves in the dipole’s near field into medium 2, if this is denser than medium 1.

Elastic scattering of evanescent electromagnetic waves.

Abstract: Analytic and numerical results are given for the elastic scattering of evanescent electromagnetic waves by dielectric spheres. Some polarization and symmetry effects not found in Lorenz-Mie scattering are noted. The possibility of experimental studies is also discussed.

A plane-polar approach for far field reconstruction from near-field measurements

Abstract: It is well-known that the far field of an arbitrary antenna may be calculated from near-field measurements. Among various possible nearfield scan geometries, the planar configuration has attracted considerable attention. In the past the planar configuration has been used with a probe scanning a rectangular geometry in the near field, and computation of the far field has been made with a two-dimensional fast Fourier transform (FFT). The applicability of the planar configuration with a probe scanning a polar geometry is investigated. The measurement process is represented as a convolution derivable from the reciprocity theorem. The concept of probe compensation as a deconvolution is then discussed with numerical results presented to verify the accuracy of the method. The far field is constructed using the Jacobi-Bessel series expansion and its utility relative to the FFT in polar geometry is examined. Finally, the far-field pattern of the Viking high gain antenna is constructed from the plane-polar near-field measured data and compared with the previously measured far-field pattern. Some unique mechanical and electrical advantages of the plane-polar configuration for determining the far-field pattern of large and gravitationally sensitive space antennas are discussed. The time convention exp ( j \omega r ) is used but is suppressed in the formulations.

A uniform GTD solution for the diffraction by strips illuminated at grazing incidence

Abstract: An asymptotic solution for the electromagnetic diffraction by a perfectly conducting strip illuminated at grazing incidence is obtained by an extension of the uniform GTD. Uniform expressions for the scattered field are given for plane, cylindrical, and spherical wave illuminations. Outside the transition regions these essentially reduce to results obtained by an ordinary application of the uniform GTD augmented by slope diffraction. In the case of plane wave illumination a very simple closed form expression is provided for the scattered far field. Numerical results are presented and compared with those calculated from a moment method solution.

Scattering by resistive strips

Abstract: For resistive strips of large electrical width kw illuminated by E- or H-polarized plane waves the geometrical theory of diffraction is used to obtain expressions for the far zone scattered field through second-order terms, valid for directions of incidence and observation away from grazing. The results are then cast as products of functions analogous to those appearing in the known (uniform) expansions for perfectly conducting strips. Each function involves the current on the corresponding half plane, and by invoking this connection, far field expressions are produced which are uniform in angle. In particular, for E polarization the backscattered field at edge-on incidence is shown to consist of two terms, each of which is expressible in terms of the half-plane current, and for all resistivities the resulting values of the field are in excellent agreement with those found by numerical solution of the integral equation, even for kw as small as unity.

Depolarization and scattering of electromagnetic waves by irregular boundaries for arbitrary incident and scatter angles full-wave solutions

Abstract: Explicit expressions are presented for the radiation fields scattered by rough surfaces. Both electric and magnetic dipole sources are assumed, thus excitations of both vertically and horizontally polarized waves are considered. The solutions are based on a full-wave approach which employs complete field expansions and exact boundary conditions at the irregular boundary. The scattering and depolarization coefficients axe derived for arbitrary incident and scatter angles. When the observation point is at the source these scattering coefficients are related to the backscatter cross section per unit area. Solutions based on the approximate impedance boundary condition are also given, and the suitability of these approximations are examined. The solutions are presented in a form that is suitable for use by engineers who may not be familiar with the analytical techniques and they may be readily compared with earlier solutions to the problem. The full-wave solutions are shown to satisfy the reciprocity relationships in electromagnetic theory, and they can be applied directly to problems of scattering and depolarization by periodic and random rough surfaces.

Electromagnetic induction effects at an ocean coast

Abstract: This paper reviews recent progress in model calculations towards understanding electromagnetic induction effects at ocean coasts. Early models consisted of two adjacent quarter-spaces of different conductivity, whereas the newer models simulate the ocean with a very thin sheet of a perfect conductor placed on top of a uniform Earth medium. The inducing field is assumed to arise from a monochromatic plane wave incident vertically from above. With any of these models one succeeds at once in explaining the occurrence of large vertical magnetic fields when the inducing electric field is polarized parallel to the coast (E-polarization), thereby also confirming the highly directional character of the coast effects as discovered a few years before by Parkinson. Another important step was made when, first numerically, then analytically, the behavior of the horizontal component of the magnetic field at the surface was rigorously calculated. For H-polarization (inducing magnetic field parallel to shore) this horizontal surface field is uniform, but is not so for E-polarization. Indeed, it has now been shown that the surface field can vary appreciably close to the coast, particularly on the ocean side of shore. With E-polarization, very large currents flow in the ocean, parallel to shore, with the result that the effect of the coast is felt at very large distances over both land and sea. Under H-polarization induction the range of the coast effect is very much shorter, in fact almost an order of magnitude shorter over the land and even reducing to zero at the surface of the perfectly conducting model ocean. The magnetic fields at the ocean floor have also been calculated, which should be of interest in the rapidly expanding field of marine survey and prospecting.

Electrodeless fluorescent light source having reduced far field electromagnetic radiation levels

Abstract: An electrodeless fluorescent light source includes an electrodeless fluorescent lamp (70) having a phosphor coating on its inner surface (74) and an induction coil (72) wherein the magnitude of the far field electromagnetic radiation, produced directly by the induction coil (72), is minimized. The induction coil (72) is driven by a high frequency power source (78). The induction coil (72) includes current loops which are configured so that the magnetic dipole moment of each current loop is offset by the magnetic dipole moment of other current loops in order to minimize the net magnetic dipole moment of the induction coil (72). One embodiment of the induction coil (72) includes a conductor wound in the shape of a square prism. The current on adjacent side edges of the prism is in opposite directions, thus resulting in two pairs of mutually opposing magnetic dipole moments.

Bounds on the optimum performance of planar antennas for pulse radiation

Abstract: The general problem of optimizing the design of planar electromagnetic pulse radiators is discussed. It is shown that bounds on the performance of such radiators can be determined by formulating field quantifies as inner products and solving a variational problem. Results of a simple example are given where the bound on the peak electric field is round for a finite-sized radiator having a current distribution which is frequency band-limited. The bound on the peak electric field component along an arbitrary orientation of direction in the radiator's far field is also presented. These results provide insight into the synthesis of electromagnetic pulse radiators, but further work is necessary if the technique discussed here is to lead to the design of improved electromagnetic pulse antennas.

An underwater towed electromagnetic source for geophysical exploration

Abstract: Low-frequency electromagnetic methods are used in geophysical exploration to detect the magnetic field distortion between a transmitter and receiver produced by locally conductive bodies. Both ground and airborne systems are in current use. It is possible to similarly conduct underwater geophysical exploration by using an underwater towed source of electromagnetic radiation and a receiving magnetic or electric field detector. The receiver can be towed on an auxiliary cable, mounted on a boom on the towing platform, or land based. An underwater towed electromagnetic source suitable for ocean-bottom exploration has been constructed, and its underwater propagation characteristics at low frequency have been studied. This underwater calibrated source (UCS) is 4 m long, weighs 383 kg in air, and can produce vertical and horizontal magnetic dipoles and a horizontal electric dipole. Powered by a current-feedback-controlled, high-power, modified sonar amplifier, the UCS can produce 9710 ampereturn.m2 of magnetic dipole or 200 A.m of electric dipole at 50 A at frequencies up to 200 Hz without significant attenuation from coil inductance. This paper concentrates on the mechanical, hydrodynamic, and magnetic design details of the UCS and the electrical system, consisting of the high-current drive power system and the shipboard monitoring system for attitude and depth detectors.

Note on the far field of a Gaussian beam

Abstract: It is shown that contrary to some recent claims the far field of a Gaussian beam does not contain contributions from high spatial frequency components of the source. Consequently no evanescent waves take part in the formation of the far field. Corresponding results for other types of wave fields are also briefly discussed.

The Propagation and Encoding of Information in the Scattered Field by Complex Zeros

Abstract: The analytic nature of the scattered field in the Fresnel region is explored. It is found that in the near field, as well as in the far field, the zeros encode the information about the object wave. The scattered field is essentially an entire function of exponential type, although in certain circumstances a multiplicative factor may impose a higher-order envelope.

Determination of the probability density function of height on a rough surface from far-field coherent light scattering

Abstract: We show that if the first order probability density function of height on a rough surface is assumed to be symmetrical, it can be calculated from measurements of scattered coherent light made in and near the centre of the far-field. We derive the theory of the method and present some experimental results obtained for ground glass surfaces.

Near fields of a vector electric line source near the edge of a wedge

Abstract: Simple closed form expressions are obtained for the fields of an electric line source with transverse or longitudinal current flow parallel to and near the edge of a perfectly conducting wedge. The effect of an edge which is not perfectly sharp is investigated by considering a cylinder-tipped half plane.

Analysis of a terminated parallel-plate waveguide with a slot in its upper plate

Abstract: A slotted parallel-plate waveguide, terminated in a specified load, is analyzed in this paper. The guide is excited by an interiortemwave and illumination from an exterior source. An integral equation is developed for the unknown electric field in the slot and it is solved numerically for several cases of interest. Values of slot electric field, scattering parameters, and the far field radiated through the slot are presented graphically.

Antenna radiation pattern of cyclotron harmonic waves in a hot magnetoplasma

Abstract: The near and far field radiation pattern of a very short antenna is studied in the plasma regime. ωc < ω < 2ωc, ωp ≫ ωc (where ωp and ωc are the electron plasma and cyclotron frequencies, respectively). The experiment is performed in a large laboratory magnetoplasma. Cyclotron harmonic waves are detected in all directions, not only those corresponding to the purely perpendicular direction of detection as was observed previously. This new result is theoretically explained and studied in detail. Constant phase curves deduced from the dispersion relation for a hot Maxwellian electron magnetoplasma are considered. They are found to be described by the low damped part of the polar dispersion curves. The different shapes of the wave surfaces (convex and concave with respect to the excitation source) are in good agreement with experimental results. The effects of plasma and magnetic field inhomogeneities on the dispersion relation are discussed. Experimental isoamplitude curves are presented in the whole space around the transmitter. The theoretical dispersion curves show that the energy of the cyclotron harmonic waves tend to propagate principally outside a cone whose apex is the transmitter and whose axis is parallel to the direction of the magnetic field. The theoretical opening angle of this cone is given by the direction of the group velocity at the inflexion point of the polar dispersion curve where the Landau damping increases suddenly. This theoretical result is in reasonable agreement with the spatial distribution of energy observed experimentally. From this study it is shown that the simple dispersion relation for perpendicular propagation can be used to predict qualitatively the distribution of cyclotron harmonic waves in all directions for dense magnetoplasma conditions.

Near field and damping effects on the radiation of ion acoustic waves from antennas

Abstract: Radiation patterns of a longitudinal wave transmitted in a warm plasma from monopole antennas are numerically calculated taking wave damping and near field effects into consideration. Both a wire and a circular grid are examined. The patterns are measured experimentally for ion acoustic waves. Experimental results agree with the theoretical prediction. Both effects broaden the radiation patterns.

Electrostatic and Electromagnetic Field Computation for the H.V. Resistive Divider Design

Abstract: In this paper, the behaviour of the elec trostatic and electromagnetic field of a resistive divi der for H.V. measurements was studied using the finite element technique.

Numerical Electromagnetic Code (NEC)-Reflector Antenna Code: Part II. Code Manual.

Abstract: : The code manual documents a detailed explanation of the Numerical Electromagnetic code - Reflector Antenna Code by which the near field and far field of a typical Navy reflector antenna can be calculated. One important feature of the code is the capability for a general reflector rim shape. Another important feature is the capability to input a practically arbitrary volumetric feed pattern. Only the class of parabolic surfaces was implemented in the computer code. The theoretical approach for computing the fields of the general reflector is based on a combination of the Geometrical Theory of Diffraction (GTD) and Aperture Integration (AI) techniques. Typically, AI is used to compute the main beam and near sidelobes; GTD is used to compute the wide-angle sidelobes and the backlobes. The theoretical background on which the computer algorithms are based is described along with descriptions of the main program and the various subroutines. For each subsection of the main program and subroutine, the purpose and method are included, accompanied by a flow diagram, a key variable list and a listing of the code. (Author)

A Uniform GTD Analysis of the Scattering of Electromagnetic Waves by a Smooth Convex Surface.

Abstract: : An approximate asymptotic high frequency solution is obtained for the field exterior to a smooth, perfectly conducting convex surface when it is excited by a ray optical electromagnetic field. This asymptotic solution is uniform in the sense that it is valid within the transition regions adjacent to the shadow boundaries where the pure ray optical solution based on the geometrical theory of diffraction (GTD) fails, and it reduces to the GTD solution in terms of the incident, reflected, and surface diffracted ray.