Showing papers on "Near and far field published in 1988"

Spherical near-field antenna measurements

Abstract: * Chapter 1: Introduction * Chapter 2: Scattering matrix description of an antenna * Chapter 3: Scattering matrix description of antenna coupling * Chapter 4: Data reduction in spherical near-field measurements * Chapter 5: Measurements * Chapter 6: Error analysis of spherical near-field measurements * Chapter 7: Plane-wave synthesis * Appendix 1: Spherical wave functions, notation and properties * Appendix 2: Rotation of spherical waves * Appendix 3: Translation of spherical waves * Appendix 4: Data processing in antenna measurements * Appendix 5: List of principal symbols and uses

Near-field probe used as a diagnostic tool to locate defective elements in an array antenna

Abstract: Results of an experimental study are presented in which the near-field probe was used as a diagnostic tool to locate the defective elements in a planar array. The near-field data were processed not only to obtain the far-field patterns of the array under the test, but also to reconstruct the aperture field for diagnostic purposes. The backward transform enables the near-field probe to identify accurately aperture faults at a distance, free of interactions and couplings with the array elements. In practice, to recover the aperture field properly from the near-field distribution, the evanescent components in the computed far-field spectrum must be excluded from the inverse process with fast-Fourier-transform (FFT) techniques. For low-gain array antennas, a correction on the far-field spectrum is required to remove the contribution of the probe and the element factor before the inverse transform, strongly enhancing the resolution. >

Techniques for measuring the electromagnetic shielding effectiveness of materials. II. Near-field source simulation

Abstract: Shielding effectiveness relates to the ability of a material to reduce the transmission of propagating fields in order to electromagnetically isolate one region from another. Because the shielding capability of a complex material is difficult to predict, it often must be measured. A number of approaches to simulating far-field source are studied, including the use of coaxial transmission-line holders and a time-domain system. In each case, the system frequency range, test sample requirements, test field type, dynamic range, measurement time required, and analytical background are considered. Data taken on a common set of materials are presented. >

Near‐field optical scanning microscopy in reflection

Abstract: The resolution of near‐field optical scanning microscopy (NFOS) is determined by the dimensions of the microscopic light source rather than the diffraction limit. To demonstrate NFOS in reflection, intensity changes in the (backward) scattering from a 70–100 nm diam hole in a metal film were recorded while the sample was scanned in close proximity to this aperture. Raster‐scan images of a planar metal test pattern yield a resolution comparable to the size of the aperture.

Shielded electromagnetic transducer

Abstract: An electrical to electromagnetic transducer for applying electromagnetic energy to damaged potions of the living body, which provides high efficiency generation of electromagnetic fields for electromagnetic therapy by directing electromagnetic radiation to the damaged body part. Electromagnetic radiation is initially generated by a dipole consisting of a bar of high permeability material wrapped with an electrically conductive coil. The dipole is placed between a conductive shield and the damaged body part. An electrical signal passes through the coil which causes a magnetic field to be generated through and around the high permeability material. The field radiation pattern of the dipole is directed toward the damaged body part by a conductive shield. Magnetic fields which are generated away from the damaged body part intersect the conductive shield and establish eddy currents. These eddy currents in turn generate magnetic fields opposite and nearly equal to the magnetic fields generated by the electromagnetic source. These resultant redirected electromagnetic fields then reinforce the electromagnetic field directed towards the damaged body part and diminish the electromagnetic field directed away from the damaged body part.

Canonical sources and duality in chiral media (antenna arrays)

Abstract: The authors examine the characteristics of antenna arrays embedded in unbounded chiral media using the Green's dyadic for electric sources and the Green's vector for magnetic sources. The purpose is to bring to light the new characteristics of sources, both point and extended, which interact with this medium and to examine general characteristics of sources located in a medium with handedness. Very simple quality relations are presented that are characteristic of chiral media when the results are written in terms of the circular eigenmodes. Appropriate measures of chirality such as the chirality admittance and impedance and a dimensionless chirality factor are introduced as needed. It is shown that, in the far field, both point and extended sources, whether electric or magnetic, radiate two electromagnetic eigenmodes which are of opposing handedness. Sources that access only one of the eigenmodes of the medium are demonstrated. Several applications of the results and array performance in chiral media are noted. >

Extrapolation of near-field RCS measurements to the far zone

Abstract: An algorithmic procedure for extrapolating near-field radar cross-section (RCS) measurements to the far zone has been derived, coded, and experimentally validated. The deviation of the extrapolation algorithm uses an optical model to estimate the surface currents induced on the scattering body by the incident field, and a specially weighted version of the Fourier transform to calculate the near-field scattering amplitudes associated with such surface currents. The extrapolation entails three steps. First, near-field measurements of the scattered electric and/or magnetic field are used to infer the monostatic vector potential. Next, the inverse Fourier transform of the inferred vector potential is multiplied by a special weighting function to estimate an equivalent obliquity factor. Finally, the far-field scattering pattern is estimated by taking the Fourier transform of the reweighted obliquity factor. This extrapolation procedure has been validated using anechoic-chamber data taken on a right-circular aluminium cylinder 25 lambda high and 2.5 lambda in radius at near-field range of 19% of 2D/sup 2// lambda where D is the nominal target diameter and lambda the radiation wavelength. The extrapolated RCS pattern for this target was compared with an analytical estimate of its far-zone pattern and good amplitude and phase agreement was observed over a 20 degrees cone of scattering angles. >

Coupling of saturated electromagnetic systems to non-linear power electronic devices

Abstract: 2-D saturated electromagnetic systems associated with nonlinear electric circuits are analyzed. The nonlinear electromagnetic equations are modeled with the help of finite elements. The nonlinear electric equations are directly coupled with the electromagnetic ones, and the Newton-Raphson method is used to solve the resulting matrix system. Numerical results for a saturated magnetic core coil fed by an electric source through a diode are reported. The results illustrate the importance of coupling the two systems of equations. >

Stochastic optics: A reaffirmation of the wave nature of light

Abstract: Quantum optics does not give a local explanation of the coincidence counts in spatially separated photodetectors. This is the case for a wide variety of phenomena, including the anticorrelated counting rates in the two channels of a beam splitter, the coincident counting rates of the two “photons” in an atomic cascade, and the “antibunching” observed in resonance fluorescence. We propose a local realist theory that explains all of these data in a consistent manner. The theory uses a completely classical description of the electromagnetic field, but with boundary conditions of the far field that are equivalent to assuming a real fluctuating, zero-point field. It is related to stochastic electrodynamics similarly to the way classical optics is related to classical electromagnetic theory. The quantitative aspects of the theory are developed sufficiently to show that there is agreement with all experiments performed till now.

Use of sampling expansions in near-field-far-field transformation: the cylindrical case

Abstract: A modified sampling technique is applied to the reconstruction of the antenna far-field pattern from near-field measurements on a cylinder. It is shown that, if a proper phase factor is singled out from the tangential components of the measured electric field, the samples spacing along the axis of the measurement cylinder increases linearly with the cylinder radius. Thus, when this radius is substantially larger than the antenna's dimension, the sample's spacing can be much greater than the half-wavelength spacing commonly accepted as the maximum possible one, without losing the computational efficiency of the standard approach. >

Coherent, monolithic two‐dimensional (10×10) laser arrays using grating surface emission

Abstract: Two‐dimensional, coherent AlGaAs laser arrays consisting of 100 (10×10) active elements have been fabricated using single quantum well laser structures. A surface relief grating is used both for feedback and outcoupling. The elements of the array are index‐guided ridge lasers. In one array design, the elements are coupled laterally by evanescent field overlap while in the second design, the coupling is by Y branches. Longitudinal coherence is achieved by injection coupling. The far field, emanating from a 60 μm by 5 mm aperture, measures 0.01°×1°. Both types of arrays emit more than 1 W peak power. The mode spectrum of the emitted power is contained in a ∼2–3 A wavelength interval at ∼1 W.

Electromagnetic scattering by buried objects of low contrast

Abstract: The Born approximation is used to derive the plane-wave scattering matrix for objects of low dielectric contrast. For general shapes a numerical integration over the volume of the scatterer is required, but analytical expressions are derived for a sphere, a circular cylinder and a rectangular box (parallelepiped). The plane-wave scattering-matrix theory is used to account for the air-Earth interface. Numerical results are presented for the scattered field and far field for plane-wave excitation. The scattered field are weak for low-contrast objects, but the near-field results have application to electromagnetic detection of buried objects. >

Application of spherical near-field measurements to microwave holographic diagnosis of antennas

Abstract: Microwave diagnosis of antennas is considered as a viable tool for the determination of reflector surface distortions and location of defective radiating elements of array antennas. A hybrid technique based on the combination of the spherical near-field measurements and holographic metrology reconstruction is presented. The measured spherical near-field data are first used to construct the far-field amplitude and phase patterns of the antenna on specified regularized u-v coordinates. These data are then utilized in the surface profile reconstruction of the holographic technique using a fast-Fourier-transform (FFT)/iterative approach. Results of an experiment using a 156-cm reflector antenna measured at 11.3 GHz are presented for both the original antenna and the antenna with four attached bumps. Several contour and gray-scaled plots are presented for the reconstructed surface profiles of the measured antennas. The recovery effectiveness of the attached bumps has been demonstrated. The hybrid procedure presented is used to assess the achieved accuracy of the holographic reconstruction technique because of its ability to determine very accurate far-field amplitude and phase data from the spherical near-field measurements. >

The determination of the electromagnetic field and SAR pattern of an interstitial applicator in a dissipative dielectric medium

Abstract: The spatial distribution of microwave energy absorbed per unit mass (the specific absorption rate or SAR) in biological tissue is calculated for a class of interstitial antennas. The insulated interstitial applicator is simulated as an asymmetrically drive antenna. An expression for the electric field intensity near the antenna is derived and calculated by direct numerical evaluation of a surface integral over the insulation. The predicted SAR patterns obtained using the calculated electric field intensity and the tissue conductivity agree very well with the measured SAR distributions around three different applicators in muscle-equivalent phantoms. >

A brief history of near-field measurements of antennas at the National Bureau of Standards

Abstract: The US National Bureau of Standards (NBS) played a pioneering role in the development of practical planar near-field antenna measurement techniques. A brief history is presented of that role, which began with theoretical studies to determine corrections for diffraction in a microwave measurement of the speed of light. NBS contributions to the development of nonplanar near-field measurement theory and practice are also described. >

Scattering of transverse-electric electromagnetic waves with a finite nonlinear film

Abstract: Exact analytical results for the scattering of a plane transverse-electric electromagnetic wave with a nonlinear thin film is derived. Optical bistable and multistable behaviors in the reflectivity as a function of the input intensity are found.

An optically linked electric and magnetic field sensor for Poynting vector measurements in the near fields of radiating sources

Abstract: A unique, single-element antenna measurement scheme that can simultaneously measure the electric, magnetic, and time-dependent Poynting vectors of electromagnetic (EM) fields is described. The electric and magnetic responses of the antenna sensor are separated by a O degrees /180 degrees hybrid junction. The resulting two RF voltages, along with relative phase and frequency information, are transmitted to a remotely located vector analyzer by a pair of well-matched fiber optic downlinks. The remote receiver measures and displays the electric dipole response, the magnetic loop response, and the time phase difference between the two. This information is sufficient to determine the time-dependent Poynting vector. Both a theoretical analysis and a discussion of experimental measurements performed, which describe the capabilities and performance of a working prototype of the antenna measurement scheme, are presented. The results demonstrate that a three-axis (isotropic) version of this system could be used to measure the near fields of EM sources, as well as to completely describe the resultant flow of energy. >

Near field effects in off-axis undulator radiation

Abstract: It is shown that near field effects can be important when off-axis radiation is used from an undulator, even when the source to observer distance is relatively large compared to the undulator length. The effects on the spectral and angular distributions are examined in detail, both by direct integration of the radiation formula and by use of an analytic approximation, and it is shown that the main features of the results can be described in terms of a single new parameter.

Accurate determination of planar near-field correction parameters for linearly polarized probes

Abstract: A procedure used by the US National Bureau of Standards (NBS) for accurately determining the plane-wave receiving parameters of both single- and dual-port linearly polarized probes is described. Examples are presented, and the effect of these probe receiving characteristics in the calculation of the parameters for the antenna under test is demonstrated using the required planar near-field theory. The planar near-field theory necessary to accomplish probe correction and to formulate probe parameter errors is presented in a concise and meaningful way to help understand when probe correction is or is not needed. >

FM laser transmitter

Abstract: A laser transmitter utilizes optical far field combination of two independent lasers (80,90) and variation of RF exciting power (86) to one (80) to produce sufficient frequency modulation. Two bores (122,124) within the same dielectric body (120) are provided with two independent sets of electrodes (134,135, 138,139) and two independent RF power sources (150,152). The lasers are operated at different frequencies and the beams transmitted in parallel adjacent paths to provide a combined far field optical beam component at the beat frequency of the two lasers. Variation of the frequency of one of the lasers provides frequency modulation of the far field beat frequency, enabling reception and demodulation of the modulated beam without use of a local oscillator laser at the receiver. The transmitter may also be used in a laser radar.

First‐order Wiener‐Hermite expansion in the electromagnetic scattering by conducting rough surfaces

Abstract: The application of the Wiener-Hermite expansion to the scattering of electromagnetic waves by rough conducting surfaces is examined. In this work the expansion is carried out only up to terms of first order in the Wiener-Hermite functionals for the surface current, the average far field, and the coherent and incoherent responses. Nevertheless, the expressions obtained for these quantities contain contributions from all orders of multiple scattering and can be viewed as partial sums of selected terms in the perturbation theory expansion. The results following from this approach are compared to those calculated by using second-order perturbation theory and also by recasting the latter in Pade approximant form.

JUNCTION code user's manual. Electromagnetic scattering and radiation by arbitrary configurations of conducting bodies and wires

Abstract: : The report gives a brief description of the computer program JUNCTION which results from the application of numerical procedures. The program invokes the method of moments to solve a coupled electric field integral equation for the currents induced on an arbitrary configuration of perfectly conducting bodies and wires. An important feature of the code is its ability to treat wire-to-wire, surface-to-surface, and wire-to-surface junctions. Wires may be connected to surfaces at essentially arbitrary angles and may be attached to surface edges or vertices. Results obtained using this algorithm are in the form of electric current and charge densities and far field patterns. This report gives a brief description of the computer program JUNCTION which results from application of numerical procedures. Keywords: Subroutines; Computer program documentation; Electromagnetic environment effects; Electromagnetic interference. (jhd)

Acoustic radiation force on a rigid sphere in the near field of a circular piston vibrator

Abstract: The acoustic radiation force exerted on a rigid sphere whose time‐averaged center is at any specified point on the axis of a circular piston vibrator was investigated theoretically including points in the near zone of the radiation with a view to applying it to the absolute measurement of ultrasonic intensity. The velocity potential of incident waves in the near field was expressed in the form of an infinite series of spherical surface harmonics, thereby allowing the use of the conventional scattering theory to calculate the radiation force. The theoretical framework has the advantages that it includes neither approximations nor numerical integrations, and that it is applicable to elastic or compressible sphere cases with slight modifications of the boundary conditions.

The Diffraction Near Fields and Lau Effect of a Square-wave Modulated Phase Grating

Abstract: The intensity distribution in the near field of a square-wave modulated phase grating is calculated. It is found that the Talbot images as well as the Fresnel images of this phase grating are similar to the object structure. The Lau effect for the case of square-wave modulated phase grating is studied with reference to diffraction theory.

Ambient electromagnetic field compensating magnetic pick-up circuit for integrated drive generators

Abstract: This invention relates to a magnetic pick-up circuit for use in integrated drive generators of the type employed on board aircraft for power generation. Prior art magnetic pick-up circuits failed to compensate for interference due to operation within an ambient electromagnetic field or failed to preserve the ambient electromagnetic field. The magnetic pick-up circuit includes in combination a first coil responsive to an ambient electromagnetic field and a modulated flux field and a second coil responsive to the ambient electromagnetic field connected to the first coil in opposition to thereby insure that a signal induced by the ambient electromagnetic in the first coil is cancelled by a signal induced by the ambient electromagnetic field in the second coil. Means for providing an output signal representative of modulation of the modulated flux field are coupled to the first coil and the second coil. This output signal is free from distortion due to the ambient electromagnetic field.

Theory and practice for a spherical-scan near-field antenna range

Abstract: A spherical-scan near-field antenna range is described. The procedure for real-time data processing is outlined. Relations for the number of data points, largest aspect angle measured, and data acquisition and processing times as a function of the antenna diameter-to-wavelength ratio and the measurement distance are given. Experimental results are presented for an antenna enclosed by a minimum sphere having a diameter of 240 wavelengths. All the equations for the spherical wave analysis, probe correction, and radial transformation are provided in appendices in numerically convenient form without the use of any special functions. >

Generalized analysis of the spherical lens as a launcher of electromagnetic missiles

Abstract: It is found that, like the directly transmitted radiation field previously investigated, an internally multiply reflected radiation field can also become an electromagnetic missile under certain different electromagnetic‐missile conditions. The analysis is carried out in a general way, i.e., for the point source located either inside or outside the dielectric sphere. The electromagnetic‐missile conditions are obtained for general multiply reflected radiation fields.

A method of modifying a surface of a body using electromagnetic radiation

Abstract: A method of modifying a surface of a body such as a semiconductor body is described in which a beam of electromagnetic radiation is directed towards the surface so that the electromagnetic radiation is incident on the surface at or near the Brewster angle, the electromagnetic radiation incident on the surface preferably being so polarised that the electric vector of the electromagnetic radiation lies in the plane of incidence of the electromagnetic radiation at the surface.

Phase retrieval radio holography in the Fresnel region: tests on the 30 m telescope at 86 GHz

Abstract: The application of phase retrieval radio holography in the Fresnel region has been studied using a 86 GHz transmitter as signal source. Maps of the aperture-plane field distribution were made with a surface resolution of about 100 independent pixels across the telescope diameter. Over the 2.7 km propagation path used (0.005 of the farfield distance), the measurement accuracy was found to be determined by atmospheric scintillation. The root mean square measurement errors in the derived aperture-plane phase distribution were about 65 microns when expressed as axially resolved surface errors. The signal to noise ratio and dynamic range requirements are easy to fulfill in the Fresnel zone, while the necessary near field corrections can be applied with high accuracy. There are thus definite advantages to phase retrieval radio holography in this near field zone.