Showing papers on "Near and far field published in 1980"

Transient response of multiconductor transmission lines excited by a nonuniform electromagnetic field

Abstract: The time-domain transmission-line equations for uniform multiconductor transmission lines in a conductive, homogeneous medium excited by a transient, nonuniform electromagnetic (EM) field, are derived from Maxwell's equations. Depending on how the line voltage is defined, two formulations are possible. One of these formulations is considerably more convenient to apply than the other. The assumptions made in the derivation of the transmission-line equations and the boundary conditions at the terminations are discussed. For numerical calculations, the transmission -line equations are represented by finite-difference techniques, and numerical examples are included.

Long-wavelength electromagnetic propagation in heterogeneous media

Abstract: Existing effective-medium-type theories for the propagation of long-wavelength electromagnetic radiation in heterogeneous media are examined, and structural effects, neglected by such theories, are introduced by a multiple-scattering approach that yields an effective propagation wave vector. Results are presented for propagation through an infinite periodic array of small spheres immersed in a host of different permittivity (or permeability). The procedure is generalized to aperiodic systems to include the lowest-order corrections for small-sphere volume fill fraction $\ensuremath{\eta}$ (for arbitrary scattering strength) and for weak scattering (for arbitrary $\ensuremath{\eta}$). In all cases significant effects due to structure-induced multipole fields are seen to occur. A simple parametrization of deviations from the lowest-order result, the Maxwell-Garnett expression, is proposed in order to extract information on structural multipoles or clustering effects from experimental data. We present the results of calculations for mixtures of real dielectrics and for small metal spheres embedded quasirandomly in a dielectric host, and describe generalizations to include the effects of particle coating and size distributions on optical properties.

SPASYN-an electromagnetic relative position and orientation tracking system

Abstract: Two relatively remote independent body coordinate frames are related in both position and orientation (six degrees of freedom) using precise electromagnetic field measurements. Antenna triads are fixed in each body frame. Variously polarized excitations in one body are correlated with signals detected in the remote body. Near-field and far-field processing strategies are presented with applications.

Attenuation and Radiation Characteristics of the HE /Sub 11/ - Mode

Abstract: The asymptotic properties of the fundamental mode HE /sub 11/ inside a large waveguide of finite surface impedances are discussed. The analysis applies to corrugated waveguides, certain optical fibers and wave-guides with metal walls coated by a dielectric layer. It is shown that for k-->/spl infin/ the HE /sub 11/ mode has the following two properties: it is polarized in one direction and the field vanishes at the boundary. Because of these properties, it is useful in the design of microwave feeds, since it minimizes cross-polarization and edge illumination at the aperture. It is also useful for long distance communication because of its low attenuation constant. Both the far field of a feed and the attenuation constant are discussed. It is shown that rectangular apertures have negligible cross-polarization over wider bandwidth than circular apertures. Furthermore, if the medium inside a waveguide is Iossless, so that power is lost only at the boundary, then the attenuation constant is very small, it is asymptotic to (ka)/sup -2/ for large ka, where k= 2 pi/lambda and a is a characteristic dimension of the waveguide. A rectangular waveguide consisting of four metal plates coated with thin dielectric layers is shown to be attractive for long distance communication, because of its simplicity of fabrication and its low attenuation.

Inverse scattering method in electromagnetic optics: Application to diffraction gratings

Abstract: We present a theoretical and numerical method for solving problems of inverse scattering in optics given data on the far field, find the scattering object. This method is applied to perfectly conducting diffraction gratings. From the efficiency curve in a Littrow mounting and in the TE case, we derive the shape of the grating surface. Two different cases must be distinguished. The first problem, which we call “reconstruction,” is to compute the profile when the efficiency is experimentally known. In the second one, called “synthesis,” we give a priori an efficiency curve and look for the corresponding grating(s), if it actually exists. We show several theoretical reconstructions for various gratings, and present our first results in the very difficult field of synthesis. The relevance of this method in the domain of electromagnetic optics is then outlined by its application to two other problems.

Irradiation of Prolate Spheroidal Models of Humans in the Near Field of a Short Electric Dipole

Abstract: Analysis of the near-field irradiation of prolate spheroidal models of humans and animals by a short electrical dipole is described. The method of solution involves an integral equation formulation of the problem in terms of the transverse dyadic Green's function and expanding the fields irradiated by a short dipole in terms of the vector spherical harmonics. The extended boundary condition method (EBCM) is employed to solve the integral equations. The power distribution and the average specific absorption rate (SAR) are calculated and plotted as a function of the separation distance. It is shown that for a dipole placed along the major axis of the spheroidal (k-polarization), and for a very short separation distance, d= 0.15 lambda, the relative power values at both ends of the spheroid are about 40 compared with the ratio of 15 in the planewave exposure case. Furthermore, the calculated average SAR values as a function of the separation distance were found to oscillate around the constant value obtained from the planewave irradiation case. Differences between the near-and far-field exposure cases occurred only at separation distances shorter than 0.5 lambda where the magnitudes of the electric and magnetic energy densities are higher than the time-average radiation power density.

Annular (HSURIA) resonators: some experimental studies including polarization effects

Abstract: A repetitively pulsed CO2 laser facility was developed for testing annular resonators. The large-aperture device exhibits generally uniform gain over an annular region of 18-cm o.d. and 10-cm i.d. The half-symmetric unstable resonator with internal axicon (HSURIA) was tested at equivalent Fresnel numbers up to 4.5. This resonator design incorporates a W-axicon mirror beam compactor that transforms a cylindrical-mode region into an annular-mode region. Two HSURIA configurations were evaluated: (a) with a conical end mirror and (b) with a flat end mirror in the annular leg. With the conical end mirror, the aligned resonator produced a predominantly higher-order azimuthal mode with an on-axis null in the far field. The output was strongly linearly polarized with the electric-field vector tangential to the optic axis in both the near and far fields. The higher-order tangentially polarized mode appears to be the result of a geometric polarization scrambling effect caused by the conical end mirror. The boundary conditions for the conical or W-axicon mirrors imply that the radial electric field has a 180° phase shift on reflection, whereas the tangential component is unchanged. Thus, a tangentially polarized mode is self-reproducing, but a linearly polarized mode is not. To eliminate the polarization scrambling effect in the HSURIA, the conical end mirror was replaced with a flat end mirror. The HSURIA with a flat end mirror produced a central spot in the far field that indicated an l = 0 mode with no spatial variations in polarization. Beam quality was measured in terms of the ratio n2 of the theoretical (geometric-mode) power transmitted through an aperture of the central lobe diameter to the observed power; n2 values as low as 1.2 were obtained. The variation of beam quality with tilt of the flat end mirror indicated a factor of 2 degradation in n2 for a 20-μrad tilt, which is in good agreement with theory.

Near-field pattern analysis of airborne antennas

Abstract: Results of a newly formulated analysis for computing patterns of an aperture or monopole antenna mounted on the fuselage of an aircraft are presented. Approximate models of the aircraft structure are employed in conjunction with the geometrical theory of diffraction (GTD) to obtain the computed fields. A major feature of the analysis is that it can accommodate receiver range specifications varying from as close as a wavelength to the aircraft surface to the true far field. This feature is especially useful in that computed on-aircraft pattern performance can be compared with measurements taken at any convenient range, including the near field. Further, after such crucial checks between computations and measurements have been made, the numerical solution can be employed to predict accurately the far-field performance of the on-aircraft antenna system. The accuracy of the numerical solutions obtainable with the analysis is demonstrated by comparison with model measurements.

Representation of vector electromagnetic beams

Abstract: The propagation of a vector electromagnetic beam in a linear homogeneous dielectric half-space is considered using the Whittaker potentials for the electromagnetic fields. A relation between the Whittaker potentials and the vector and scalar potentials of the electromagnetic theory is obtained. The polarization properties of the beam are discussed in the paraxial approximation and beyond.

Electromagnetic absorption in a multilayered slab model of tissue under near-field exposure conditions

Abstract: The electromagnetic energy deposited in a semi-infinite slab model consisting of skin, fat, and muscle layers is calculated for both plane-wave and near-field exposures. The plane-wave spectrum (PWS) approach is used to calculate the energy deposited in the model by fields present due to leakage from equipment using electromagnetic energy. This analysis applies to near-field exposures where coupling of the target to the leakage source can be neglected. Calculations were made for 2,450 MHz, at which frequency the layered slab adequately models flat regions of the human body. Resonant absorption due to layering is examined as a function of the skin and fat thicknesses for plane-wave exposure and as a function of the physical extent of the near-field distribution. Calculations show that for fields that are nearly constant over at least a free-space wavelength, the energy deposition (for skin, fat, and muscle combination that gives resonant absorption) is equal to or less than that resulting from plane-wave exposure, but is appreciably greater than that obtained for a homogeneous muscle slab model.

Quantization of evanescent electromagnetic waves: Momentum of the electromagnetic field very close to a dielectric medium

Abstract: The problem of the momentum of the electromagnetic field near a homogeneous dielectric plane surface is studied. In the particular case of evanescent waves, our calculations are consistent with the experimental result of S. Huard and C. Imbert which points out that, during an interaction between a moving atom and a surface wave of pulsation $\ensuremath{\omega}$, the exchanged momentum is greater than $\frac{\ensuremath{\hbar}\ensuremath{\omega}}{c}$. This work leads at first to a straightforward procedure for quantizing the momentum parallel to the diopter of the field. The authors proceed to investigate the form of the component perpendicular to the interface which is transmitted by the field to an atom during an interaction. (In the case of an evanescent wave, this problem involves the interpretation of the imaginary part of the wave vector.) A general theory shows that the presence of a medium widens and shifts the $\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}}$ levels of the field. This result agrees with the uncertainty principle.

On computation of electromagnetic fields on planar surfaces from fields specified on arbitrary surfaces

Abstract: A technique is presented for solution of the inverse problem of calculating the electric field on a planar surface from the electric field specified on a nearby surface. An integral equation is derived that relates two orthogonal components of the electric field on the nearby surface to the respective components of the plane wave spectrum of the planar electric field. The integral equation is solved by an iterative technique, and the planar near field is calculated by an inverse Fourier transform of the plane wave spectrum.

Numerical Study of Optical Ray Retracing in Laser-Plasma Backscatter.

Abstract: : Optical ray retracing in stimulated Brillouin backscatter from laser-produced plasmas is studied numerically, using a 2-D propagation code (BOUNCE). This code treats steady state behavior in the strong damping limit, and includes self focusing effects. Ray retracing phenomena are grouped into two limiting cases. In the 'whole beam' limit, the pump field at the lens has a broad spatial profile that can be focused to a long narrow waist within the plasma. This geometry selectively amplifies only those initial noise components that are propagating back along the axis, where the net gain is highest. The simulations show that such effects exhibit a pronounced threshold due to self focusing, and disappear when the focal width becomes comparable to the length of the gain medium. In the opposite limit, where the pump radiation at the lens is dominated by small scale transverse structure, the backscatter can reproduce this structure in detail (i.e., exhibit wavefront reversal) when the far field of the pump produces an interference pattern in the plasma.

Plane-wave spectrum approach for the calculation of electromagnetic absorption under near-field exposure conditions.

Abstract: The exposure of humans to electromagnetic near fields has not been sufficiently emphasized by researcher. We have used the plane-wave-spectrum approach to evaluate the electromagnetic field and determine the energy deposited in a lossy, homogeneous, semi-infinite slab placed in the near field of a source leaking radiation. Values of the fields and absorbed energy in the target are obtained by vector summation of the contributions of all the plane waves into which the prescribed field is decomposed. Use of a fast Fourier transform algorithm contributes to the high efficiency of the computations. The numerical results show that, for field distributions that are nearly constant over a physical extent of at least a free-space wavelength, the energy coupled into the target is approximately equal to the resulting from plane-wave exposed.

Electromagnetic surface impedance for a layered earth for general excitation

Abstract: An expository analysis of electromagnetic induction in an N-layered half space is presented. The objective is to develop general expressions for the surface impedance matrix at the air/earth boundary that exhibit the dependence on the source field configuration. The justification for ignoring this dependence is given.

Numerical study of optical ray retracing in laser-plasma backscatter

Abstract: Optical ray retracing in stimulated Brillouin backscatter from laser-produced plasmas is studied numerically, using a 2-D propagation code (bounce). This code treats steady-state behavior in the strong damping limit, and includes self-focusing effects. Ray retracing phenomena are grouped into two limiting cases. In the ''whole-beam'' limit, the pump field at the lens has a broad spatial profile that can be focused to a long narrow waist within the plasma. This geometry selectively amplifies only those initial noise components that are propagating back along the axis, where the net gain is highest. The simulations show that such effects exhibit a pronounced threshold due to self-focusing, and disappear when the focal width becomes comparable to the length of the gain medium. In the opposite limit, where the pump radiation at the lens is dominated by small-scale transverse structure, the backscatter can reproduce this structure in detail (i.e., exhibit wavefront reversal) when the far field of the pump produces an interference pattern in the plasma. The plasma then behaves as an active volume hologram as it amplifies the backward-propagating random noise fields.

A Comprehensive Study of Subsurface Propagation from Horizontal Electric Dipoles

Abstract: The general exact integrals for the electromagnetic field of an electric dipole in the earth near its boundary with the air are evaluated numerically over the full range of conductivities and permittivities that characterize different parts of the earth's surface. The following ranges are included: permittivities from 2 to 80, conductivities from 4 × 10-6 to 4 S/m, frequencies from 10 to 109 Hz, radial distances from 0.1 to 100 km, and depths of the source and point of observation from 0.15 to 15 m. The generally useful component of the electric field E? is represented in graphs and contour diagrams that display the characteristics of propagation in their dependence on the frequency, the radial distance, the depth, and the electrical parameters of the earth. They are interpreted in terms of approximate formulas of Banos which are evidences of the lateral-wave nature of the field.

Automatic phase alignment system for a tracking antenna

Abstract: The tracking signals of a monopulse satellite tracking system are optimized by aligning the phases of the azimuth and elevation channels (the "difference" channels) with the reference (or "sum") channel using an autophase controller and a simulated far field source in the near field. The autophase controller monitors the output of the tracking receiver and adjusts phase shifters along the transmission paths of the azimuth and elevation channels, in a manner to maximize the outputs of the receiver. The simulated far field source provides an alignment signal for the autophase controller. The simulated far field source consists of a signal generator, a near field probe, and near field phase compensators. The probe is positioned to avoid any nulls in the simulated tracking signal for the frequencies of interest. The probe is also positioned to limit the range of the phase shifts introduced by near field placement, for the frequencies of interest. Near field phase shift compensators are used to offset the phase shifts introduced by the use of a near field source instead of a far field source.

Source array scaling for wavelet deconvolution

Abstract: A seismic source array is normally composed of elements spaced at distances less than a wavelength while the overall dimensions of the array are normally of the order of a wavelength. Consequently, unpredictable interaction effects occur between element and the shape of the far field wavelet, which is azimuth-dependent, can only be determined by measurements in the far field. Since such measurements are very often impossible to make, the shape of the wavelet—particularly its phase spectrum—is unknown. A theoretical design method for overcoming this problem is presented using two scaled arrays. The far field source wavelets from the source arrays have the same azimuth dependence at scaled frequencies, and the far field wavelets along any azimuth are related by a simple scaling law. Two independent seismograms are generated by the two scaled arrays for each pair of source-receiver locations, the source wavelets being related by the scaling law. The technique thus permits the far field waveform of an array to be determined in situations where it is impossible to measure it. Furthermore it permits the array design criteria to be changed: instead of sacrificing useful signal energy for the sake of the phase spectrum, the array may be designed to produce a wavelet with desired amplitude characteristics, without much regard for phase.

On cylindrical near-field scanning techniques

Abstract: The agreement between the coupling equations obtained in the literature by using the reciprocity theorem and the scattering matrix formulation is demonstrated. The field is expanded in cylindrical vector wave functions and the addition theorem for these functions is used. The communication may serve as a tutorial introduction to the cylindrical scanning techniques.

Prediction of Far Field Sound Radiation from Transformers

Abstract: A method has been presented for predicting from surface vibration data the sound radiated by a power transformer. The technique, which is well-suited to numerical computations, is based on modeling the transformer tank as a concatenation of rigid right-angled wedges. The vibration data may be the results of measurements or of auxiliary calculations. The variation of the radiation pattern with respect to distance and angle can be investigated with an accuracy that is limited primarily by the spacing of surface measurement points. The method has been verified in a field test using controlled excitation and in a check against previously obtained analytical and experimental results. The calculation of sound radiation patterns from surface acceleration measurements made on a production line transformer operating under simulated load conditions indicate the usefulness of this technique in predicting the noise performance characteristics of power transformers.

Electromagnetic Scattering by a Slender Body of Revolution: Axially Incident Plane Wave

Abstract: Scattering of an axially incident plane electromagnetic wave by a perfectly conducting slender body of revolution is studied. The scattered field is represented as the field due to a distribution of magnetic and electric dipoles distributed along a segment of the axis of the body and lying entirely inside the body. The boundary conditions on the body lead to a pair of coupled linear integral equations for the (unknown) dipole densities. The first few terms in the asymptotic expansion of the dipole densities, as well as the extent of the distributions, are found in terms of the slenderness ratio of the body. The far field behavior of the scattered fields, the induced dipole moments, and the scattering cross section are computed. The special case of a prolate spheroid is treated to illustrate the general theory, and the general results are found to agree with the exact solution in this case.

Near‐field and beam‐waist position of the semiconductor laser with a channeled‐substrate planar structure

Abstract: We demonstrated astigmatism in the output beam from the double‐current‐confinement channeled‐substrate planar laser. A gradual transition from gain guiding toward index guiding is observed as the thickness of the cladding layer t is reduced. The near field is stable, and both the beam width and the beam‐waist position are independent of the pumping levels over the range of current tested (up to 3 Ith).

Properties of electromagnetic radiation from a partially correlated current distribution

Abstract: A free-space solution is found for the field correlation tensors as a Green’s function integral over the correlation tensor for the source currents. This solution is used to formulate a vector theory of partial coherence in an angular spectrum representation. The theory is used to analyze the angular distribution of radiation into the far field for several special cases of source polarization and coherence. It is found that the radiation pattern from unpolarized, quasihomogeneous sources depends on the coherence properties of the source in the same manner as described in scalar theory. However, for sources that are not unpolarized this is not generally the case.