Showing papers on "Near and far field published in 1985"

Quantum effects of the interaction of elementary particles with an intense electromagnetic field

Abstract: The characteristic value of the electromagnetic field intensity in quantum electrodynamics is studied and presented. General remarks are given on processes in an intense electromagnetic field and their description is detailed. Photon emission by electron in the field of an intense electromagnetic wave and pair production by a photon in the field of an intense electromagnetic wave are discussed. The authors examine shift and splitting of the atomic level by electromagnetic wave fields. Radiative corrections are presented.

Electromagnetic wave theory

Abstract: Electrostatics and magnetostatics quasi-static current flow in heterogeneous conductors scalar waves in one, two, and three dimensions the electromagnetic field guided cylindrical waves basic radiation fields propagation and radiation in complicated media. Appendix: some essential results from vector analysis.

Microwave holography of large reflector antennas--Simulation algorithms

Abstract: The performance of large reflector antennas can be improved by identifying the location and amount of their surface distortions and then by correcting them. Microwave holography techniques are finding considerable applications as viable tools for performing this task. In these techniques, the complex (amplitude and phase) far-field pattern of the antenna is measured, using a reference antenna. Then, the Fourier transform relationship, which exists between the far field and a function related to the induced current, is invoked to result in the identification of the surface distortions. To critically examine the accuracy of the constructed surface profiles, simulation studies are required to incorporate both the effects of systematic and random distortions, particularly the effects of the displaced surface panels. In this paper, different simulation models are investigated with emphasis given to a model based on the vector diffraction analysis of a curved reflector with displaced panels. The simulated far-field patterns are then used to reconstruct the location and amount of displacement of the surface panels by employing a fast Fourier transform (FFT)/iterative procedure. The sensitivity of the microwave holography technique based on the number of far-field sampled points, level of distortions, polarizations, illumination tapers, etc., is also examined. In addition, the relationships between Az-El and u-v spaces are addressed in the Appendix. Most of the data are tailored to the dimensions of the NASA/JPL Deep Space Network (DSN) 64-m reflector antennas for which the result of a recent measurement is also presented.

Far-field patterns of spaceborne antennas from plane-polar near-field measurements

Abstract: Certain unique features of a recently constructed plane-polar near-field measurement facility for determining the far-field patterns of large and fragile spaceborne antennas are described. In this facility, the horizontally positioned antenna rotates about its axis while the measuring probe is advanced incrementally in a fixed radial direction. The near-field measured data is then processed using a Jacobi-Bessel expansion to obtain the antenna far fields. A summary of the measurement and computational steps is given. Comparisons between the outdoor far-field measurements and the constructed far-field patterns from the near-field measured data are provided for different antenna sizes and frequencies. Application of the substitution method for the absolute gain measurement is discussed. In particular, results are shown for the 4.8-m mesh-deployable high-gain antenna of the Galileo spacecraft which has the mission of orbiting Jupiter in 1988.

Fluorescence quenching in molecules near rough metal surfaces

Abstract: A classical electrodynamic near field calculation for the interaction of a Drude oscillator molecular dipole with a rough metal surface is presented. Effects of electromagnetic coupling between surface bumps (assumed to be prolate hemispheroids) and the bulk metal are included. Fluorescence lifetimes are found to be two to four orders of magnitude smaller than those predicted for a flat plane in calculations on silver, and for sufficiently short distances, an avoided crossing splitting is predicted.

A Numerical Method for Near-Field Array Synthesis

Abstract: A numerical method for near-field array synthesis is developed for arbitrary array geometries. The intended application is for generating a planar field in a test volume for electromagnetic susceptibility testing, but the method is valid for arbitrary field distributions. A uniqueness theorem is utilized to allow the field conditions to be enforced on the surface of the test volume rather than throughout the volume. The synthesis method is a least-squares solution with a constraint on the source norm; the constraint keeps the field small outside the test volume. Numerical results are shown for the case of synthesizing a plane wave in the near field of an array of line sources.

Errata: "Refractive-index profile and modal dispersion prediction for a single-mode optical waveguide from its far-field radiation pattern"

Abstract: Refractive-index profile and modal dispersion in single-mode optical fibers are computed from the measured far-field exit radiation pattern of the fundamental mode. Instead of Fourier transforming the far-field numerically, the near-field is calculated by fitting a series of Gauss-Laguerre functions to the far-field. The coefficients of the series are simply found by matrix inversion and determine directly the refractive-index profile and the modal dispersion within the framework of a scalar theory. The main advantage of the proposed method lies in its inherent insensitivity against random measurement errors. Further possible applications (determination of cutoff wavelength, near-field and refractive-index profile of integrated-optical waveguides with rectangular cross section) are briefly discussed.

Energy deposition in a model of man in the near field.

Abstract: The spatial distribution of the specific absorption rate (SAR) was measured in a full-scale model of man using implantable electric field probes. The model was exposed in the near-field of linear and aperture antennas at 350 MHz. Effects of the wave polarization, antenna position and antenna gain on the SAR distribution and the average SAR in the whole-body and body parts are reported.

Apparatus for scanning and measuring the near-field radiation of an antenna

Abstract: Apparatus for measuring the near-field radiation of an antenna such that its far-field radiation characteristics can be determined. The invention utilizes the principle of the Foucault pendulum. It uses a bob attached to a long pendulum arm that is free to rotate. Rotation of the earth causes an apparent rotation of the swing plane of the pendulum with a period of T=24/sin (latitude). By attaching a field sensing probe to the pendulum bob and mounting the antenna under the Foucault pendulum, the entire antenna aperture can be scanned without moving the antenna. The motion of the probe covers part of an external sphere centered at the pivot point of the pendulum and having a radius equal to the length of the pendulum. Appropriate transformation of the measured near-field data gives the far-field radiation pattern.

Exposure of Human Models in the Near and Far Field - A Comparison

Abstract: The specific absorption rate (SAR) was measured in over 650 locations in a full-scale model of man exposed in the far and near field of antennas at 350 and 915 MHz. The whole-body average, the body-parts average, and the distributions of the SAR's are compared for three wave polarizations for the far and the near-field exposures. Effects on the energy deposition of the antenna type, gain, and location in the near field are discussed.

An Infrared Measurement Technique for the Assessment of Electromagnetic Coupling

Abstract: A non-destructive, non-perturbing infrared measurement technique is under development to observe the effects of electromagnetic coupling of energy to the interior of complicated geometrical structures. The applications, advantages, and disadvantages of this new infrared technology are discussed.

Determination of Depth of Shallowly Buried Objects by Electromagnetic Induction

Abstract: A method of determining the depth of a shallowly buried metallic object by using its electromagnetic response is investigated. The method consists in relating the distance of the object from the sensor to the ratio of voltages induced in two coils by the object when it is subjected to a pulsed magnetic field. While the theory is developed for a spherical object, experimental data for both spheres and spheroids are used to evaluate the technique. It is shown that over the range of depths for which the signal-to-noise ratio is large enough, the error in determining a sphere's depth should not exceed approximately 10 cm.

Application of Boundary-Element Method to Electromagnetic Field Problems (Comments)

Abstract: The above paper has explained a general formulation of the boundary-element method (BEM) for analyzing two-dimensional electromagnetic fields, and has presented numerical examples for some boundary shapes to show that the BEM is a very powerful numerical method for solving electromagnetic field problems. It gives accurate results with far fewer nodes than the finite-element method, and can also treat field problems in unbounded regions without any additional complications.

Maxwellian and cavity electromagnetic fields within continuous sources

Abstract: Maxwellian and cavity electromagnetic fields are derived within a continuous source distribution of volume current or electric and magnetic polarization densities by direct, rigorous differentiation of the vector potential. The derivation, which uses only operations of vector calculus, reveals naturally the contributions from interior and exterior source regions. Results are obtained for static and arbitrarily time‐varying fields as well as for time‐harmonic fields.

Aperture antenna system for measurement of formation parameters

Abstract: An aperture antenna 83 having a slot aperture 81 is used to provide a highly directional inductive electromagnetic field pattern having a strong H component in the near field into the walls of a borehole 12 to evaluate parameters related to the formation. The dimensions of the aperture antenna 81 used are considerably smaller than integral fractions of the wavelength of the electromagnetic field. Vertically and azimuthally shaped electromagnetic energy is directed into the walls of the formation and a receiving antenna 93 having a similar slot aperture 91 is used to evaluate the characteristics of the formation. A single transmitter/receiver combination and a position monitoring device or multiple pairs of receiving and transmitting antennas are circumferentially spaced around a borehole tool and used to evaluate the dip of a formation through which the borehole 12 passes.

Far-field accuracy investigation using modulated scattering technique for fast near-field measurements

Abstract: Near field measurement techniques in conjunction with near-field to far-field transformation algorithms are widely used today. Two of the most important concerns are, firstly, the degree of accuracy achieved, and secondly, the measurement duration. Although high degrees of accuracy can be obtained, the time required to scan completely the near field of an antenna using the classical near-field measurement techniques is rather long. The modulated scattering technique would offer a means to reduce this time by a factor of 10 to 100 while maintaining a reasonable degree of accuracy. Using this technique, however, one introduces further sources of inaccuracy such as the mutual coupling between the elements of the array used to probe the test antenna, and the further limitation of the available measurement dynamic range. In this paper, these two sources of inaccuracy inherent in this technique or other techniques which use a similar set-up, are explored. Multiple reflections between the test antenna and the probe array are ignored. A parabolic reflector is chosen as the test antenna, and an array of dipoles is chosen as the probe antenna in the numerical simulation.

Radiation of Nonsinusoidal Waves by a Large-Current Radiator

Abstract: The calculation of the electric and magnetic field strength produced by a radiator is usually done only for distances large compared with the geometric dimensions of the radiator. This approach makes it Impossible to obtain exact relations between the power fed to the radiator by the driver, the voltage between the radiator terminals, the power radiated to or received from the near zone, and the power radiated to the far zone. This paper develops a solution for the large-current radiator that can be evaluated numerically by computer for any distance, and thus permits the calculation of the power flowing at any time through the surface of the radiator. The knowledge of this power is the basis for the design of efficient radiators.

Anisotropic dielectric properties of media containing aligned nonspherical scatterers

Abstract: Electromagnetic wave propagation in a medium containing a random distribution of aligned, pair-correlated nonspherical scatterers is studied using the T -matrix to characterize the single scatterer response, the quasicrystalline approximation (QCA) and the correlation function. The resulting dispersion equation for the average medium is numerically solved as a function of frequency and the direction of propagation. Numerical results are presented for the attenuation of electromagnetic waves versus frequency, concentration, and direction of propagation.

An optical transmission filter for far field beam correction

Abstract: An optical transmission filter (10) for effecting differential phase delay upon light in a beam polarized in a P dimension, as a function of position within the filter aperture for far field correction. The filter employs two lenses (12) (14) of birefringent material, the crystal optic axes (13) (18) of the respective lens materials being oriented in mutually orthogonal positions and at a 45o angle to the P dimension. The lenses have their adjacent surfaces respectively concave (16) and convex (15) with the same radius of curvature and their non-adjacent surfaces (11) (17) flat. The two lenses may be both spherical or both cylindrical. The correction provides an improvement in beam quality in laser systems operating with larger beam apertures or at higher powers.

New approach for determining non-Gaussian mode fields of single-mode fibres from measurements in far field

Abstract: The far-field RMS width of a single-mode fibre is shown to be related to the first derivative of the power passed through a circular aperture. This relationship allows a direct determination of Petermann's recent definition of mode field radius from data collected using the far-field variable-aperture technique.

Interdisciplinary design analysis of a precision spacecraft antenna

Abstract: The Advanced Communications Technology Satellite (ACTS) will operate in the 20/30 GHz range (Ka Band), and will include a multi-beam antenna (MBA) capable of 0.3 degree scanning spot beams with very high beam-to-beam isolation. The antenna Radio Frequency (RF) performance requirements lead to stringent requirements on the antenna reflector surface shape. A prediction of RF performance of a potential flight model antenna reflector operating under space environmental conditions is made using a radiant heat input model (TRASYS), a thermal analyzer (SINDA), a structural model (NASTRAN), and RF far field pattern simulation. Interfacing software has been written to pass thermal model temperature results to the structural model, and structural model thermal deformation results to the RF far field pattern simulation. A complete analysis can be performed in a single computer run, and potential changes in design can be quickly and easily evaluated using this interdisciplinary design analysis tool.

Improvements in Ultrasonic Measurement Modeling with Applications to Ultrasonic Reliability

Abstract: Over the past several years, work has been reported on the development and implementation of a measurement model relating measured ultrasonic signals obtained through planar or cylindrically curved interfaces to far field scattering amplitudes (1). A number of applications have also been described, including obtaining scattering amplitudes (2) to improve sizing capability via the inverse Born approximation (3), predicting detected signals from cracks in planar (4) and cylindrical (5) geometries, and establishing detection filters (6) for improved inspectability. These applications have used models of diffraction effects for the case of piston source radiation (7) which were developed to account only for the axial pressure fields. A number of desired applications such as treating large flaws and scanning modes will require the ability to model the full radiation field of a probe. As a first step, a model of the radiation of Gaussian profile probes through planar or curved surfaces has been developed (8). With suitable normalization, this model can be used to predict the far-field behavior of piston probes. This allows modeling of full field behavior near the focal region of either focussed probes or focussing part surfaces. This paper will discuss the incorporation of the Gaussian beam theory into the measurement model. Also reported will be several new applications of the measurement model to problems associated with ultrasonic reliability.

Near-field effects and scattered intensities of electromagnetic waves from random rough surfaces

Abstract: We present Monte Carlo numerical calculations for the near-field and scattered diffuse intensities of p-polarized light incident upon a random Gaussian correlated surface. We observe that the near field is dominated by two evanescent waves of momentum parallel to the surface Qs and ∼−Qs that in turn produce oscillations in the near field. This is in agreement with a theoretical diagrammatic expansion in the spatial disorder that included the so-called fan diagrams. Our calculations, within a resolution larger than 4° in the scatter angle, do not show a sharp peak or even an extra contribution in the backward diffuse scattered intensity predicted by that theoretical expansion, however. At present, we cannot make any firm conclusions about this because we have to increase resolution and decrease statistical error to elucidate this point.

Analysis of mode field radius calculated from single-mode fiber far-field radiation patterns

Abstract: We have investigated the angular range and mesh size required to calculate accurate mode field radii (MFR) from single-mode fiber far-field radiation patterns using the inverse Hankel transform. A computer aided fiber modeling program was used to calculate near-field mode distributions for both step-index and triangular index fibers at 1.55 μm. These distributions were then transformed to the far-field and used as input "data" for the inverse transform studies. Experimental far-field data measured on fibers similar to those modeled were also used. We find that the MFR is insensitive to mesh sizes less than 0.6° but the range of far-field data required depends upon the fiber design and the criteria used to determine the MFR. This study provides guidelines for calculating accurate MFR from experimental data when using the Hankel transform.

Secondary pattern computation of an arbitrarily shaped main reflector

Abstract: The secondary pattern of a perfectly conducting offset main reflector being illuminated by a point feed at an arbitrary location is studied. The method of analysis is based upon the application of the Fast Fourier Transform (FFT) to the aperture fields obtained using geometrical optics (GO) and geometrical theory of diffraction (GTD). Key features of the present work are (1) the reflector surface is completely arbitrary, (2) the incident field from the feed is most general with arbitrary polarization and location, and (3) the edge diffraction is calculated by either UAT or by UTD. Comparison of this technique for an offset parabolic reflector with the Jacobi-Bessel and Fourier-Bessel techniques shows good agreement. Near field, far field, and scan data of a large refelctor are presented.

Laser acceleration of particles. AIP conference proceedings No. 130

Abstract: The Second Workshop on Laser Acceleration of Particles was held at UCLA on January 7-17, 1985. Despite the title, the workshop broadened to include studies of all novel acceleration schemes, whether lasers are involved or not. Substantial progress both theoretical and experimental has been made on all three basic types of laser accelerators: near field, far field, and plasma accelerators. Understandings gained from detailed studies of problems on laser excitation and propagation and on particle beam containment and stability have led to concentrations on those specific schemes which appear to be the most promising. (A) near field accelerator; (B) far field accelerator; and (C) plasma accelerator. For non-laser accelerators, the wakefield or two-beam acceleration schemes are the most promising and have received the most attention. In both schemes the electric field generated by a low energy high current beam is used to accelerate a low current second beam to high energies. The two beams can be traveling in separate cavity structures, and the field is transmitted from one cavity to the other through couplers. Or, the two beams can be traveling in the same cavity. In either case the ratio of the electric fields on the two beam trajectories mustmore » be very large. This is equivalent to having a large transformer ratio in a voltage step-up transformer. Experiments are being carried out at DESY (single cavity wakefield accelerator), and at LBL and LLNL (dual cavity two-beam accelerator). In addition, several other non-laser schemes were proposed and discussed at the workshop, but more careful and more extensive analyses are needed for these schemes. Abstracts of individual items from the conference were prepared separately for the data base. (GHT)« less