Showing papers on "Near and far field published in 1987"

Spatial distribution of the internal and near-field intensities of large cylindrical and spherical scatterers

Abstract: Spatial distributions of the near-field and internal electromagnetic intensities have been calculated and experimentally observed for dielectric cylinders and spheres which are large relative to the incident wavelength. Two prominent features of the calculated results are the high intensity peaks which exist in both the internal and near fields of these objects, even for nonresonant conditions, and the well-defined shadow behind the objects. Such intensity distributions were confirmed by using the fluorescence from iodine vapor to image the near-field intensity distribution and the fluorescence from ethanol droplets impregnated with rhodamine 590 to image the internal-intensity distribution.

Absorption of electromagnetic radiation by superconducting YBa2Cu3O7: an oxygen-induced phenomenon

Abstract: In the superconducting state, YBa2Cu3O7 absorbs electromagnetic radiation over a wide range of frequencies (8 MHz-9 GHz). The absorption is extremely sensitive to temperature, particle size and the magnetic field and depends crucially on the presence of oxygen. A possible explanation for the phenomenon based on the formation of Josephson junctions is suggested.

FFT applications to plane-polar near-field antenna measurements

Abstract: The four-point bivariate Lagrange interpolation algorithm was applied to near-field antenna data measured in a plane-polar facility. The results were sufficiently accurate to permit the use of the FFT (fast Fourier transform) algorithm to calculate the far-field patterns of the antenna. Good agreement was obtained between the far-field patterns as calculated by the Jacobi-Bessel and the FFT algorithms. The significant advantage in using the FFT is in the calculation of the principal plane cuts, which may be made very quickly. Also, the application of the FFT algorithm directly to the near-field data was used to perform surface holographic diagnosis of a reflector antenna. The effects due to the focusing of the emergent beam from the reflector, as well as the effects of the information in the wide-angle regions, are shown. The use of the plane-polar near-field antenna test range has therefore been expanded to include these useful FFT applications. >

Apparatus for circularly polarized radiation from surface wave transmission line

Abstract: Disclosed is an arrangement of two conical radiators that electromagnetically interact with one another to produce circularly polarized radiation in a surface wave transmission system. One of the radiators includes an annular conductive region that coaxially surrounds the surface wave transmission line with a pair of spiral antenna arms extending outwardly along the conical surface of the radiator from oppositely disposed positions on the outer boundary of the annular conductive region. The second radiator, which is spaced apart from the first radiator, includes a circular conductive region to which the end of the surface wave transmission line is joined and further includes a pair of spiral antenna arms that extend outwardly along the surface of the second conical radiator. The annular opening in the first radiator is dimensioned so that one-half of the surface wave energy incident on the first radiator is radiated and the remaining one-half of the electromagnetic energy propagates through the circular opening of the annular conductive region and is radiated by the second radiator. The orientation between the first and second radiators is established both with respect to axial distance between the radiators and the spatial position of the inner ends of the spiral antenna arms to cause the individual signals radiated by the two radiators to combine in a manner that results in far field circular polarization.

Two-dimensional beam diffraction by a half-plane and wide slit

Abstract: The far field of a two-dimensional beam resulting from an electric line source at a complex position is described, its half-power beamwidth determined, and its validity as an antenna beam indicated. Farfield diffraction by a half-plane is then determined from an exact uniform solution for an isotropic line source by making the source position complex. The same basic solution and technique are used for beam diffraction by a wide slit, with first-order interaction between the slit edges included. Numerical results for normal incidence illustrate the evolution of the diffraction patterns from those for an omnidirectional source to those for a highly directive beam. Results for plane wave incidence by a slit also come out of this solution. The remarkable simplicity and convenience of this method relative to alternative asymptotic procedures is discussed.

Scattering of TM excitation by coupled and partially buried cylinders at the interface between two media

Abstract: An analysis of scattering from coupled conducting cylinders near the planar interface between two semi-infinite, homogeneous halfspaces of different electromagnetic properties and from partially buried conducting cylinders is presented. The perfectly conducting cylinders of general cross sections are of infinite extent and the excitation is transverse magnetic to the cylinder axes. Coupled integral equations for the unknown current induced on the cylinders are derived and a numerical method for solving them is described. In addition, a simple technique is employed to determine the far-zone scattered field from knowledge of the cylinder current. Data displaying the distribution of the induced current and the scattered field patterns for cylinders of interest are presented and discussed.

Diffraction free arrangement

Abstract: Arrangements for generating a well defined traveling wave beam substantially unaffected by diffractive spreading. In different embodiments, the beam can be an electromagnetic wave, a longitudinal beam such as an acoustic beam, or any type of beam to which the Helmhotz generalized wave equation is applicable. Pursuant to the teachings herein, a beam is generated having a transverse dependence of a Bessel function, and a longitudinal dependence which is entirely in phaser form, which results in a beam having a substantial depth of field which is substantially unaffected by diffractive spreading. In first and second disclosed embodiments, respectively, an optical and an acoustical beam are generated by placing an annular beam source (30, 40) in a plane conjugate to the focal plane of a focusing means (32, 42), which results in the generation of a well defined beam (38', 44') because the far field intensity pattern of an object is the Fourier transform thereof, and the Fourier transform of such an annular shape is a circular function.

Far-zone behavior of electromagnetic fields generated by fluctuating current distributions

Abstract: General expressions are derived for various quantities that characterize the far field generated by any fluctuating localized current distribution that is statistically stationary, at least in the wide sense. In particular, expressions are derived for the cross-spectral density tensors of the electromagnetic field, for the coherence matrix and the Stokes parameters, for the degree of polarization, and for the degree of coherence of the far field in terms of the cross-spectral density of the transverse part of the source current. The analysis is illustrated by considering radiation from a fluctuating linear current source.

Measurement of mode field radius in axially nonsymmetrical single-mode fibers with arbitrary power distribution

Abstract: A new method for the measurement of mode field radius in axially symmetrical and nonsymmetrical single-mode fibers with non-Gaussian power distribution is proposed. It is based on monitoring the power passed through a variable-width slit in the far field. A few results for different types of single-mode optical fiber are presented.

Polarizability tensors in low‐frequency inverse scattering

Abstract: The problem treated is that of determining the geometric configuration and constitutive parameters of an object from a knowledge of the low-frequency scattered far field produced by an incident plane wave. This problem is recast as a constrained optimization problem for a functional defined in terms of the polarizability tensor elements associated with the object. In particular, a method is described for finding the dimensions and orientation of that ellipsoid, the scattering properties of which best approximate the measured far field.

Etude du champ proche d'un laser diaphragmé.

Abstract: A circular aperture is generally used inside a metrologic gas laser to suppress transverse modes. The resonant field inside the laser or the near field observable at the output display longitudinal or transverse oscillations due to diffraction. These fields are different from the Gaussian modes usually obtained from nondiaphragmed cavities. This article studies the near field of a diaphragmed laser in both experimental and theoretical aspects. For this purpose we use a field expansion on the basis of geometrical Laguerre Gauss modes and we calculate the field diffracted by an aperture, inside a resonant cavity. This allows calculation of the measurable field at the laser output. Numerical applications and corresponding experiments are done with a single frequency laser working on the 3.39-μm Ne line. Excellent agreement is obtained between experimental and theoretical curves.

The application of finite-element calculations to the remote-field inspection technique

Abstract: Calcul par elements finis applique a la technique de controle par courants de Foucault a champ lointain

Multiple scattering of EM waves by dielectric spheres located in the near field of a source of radiation

Abstract: An analysis of multiple scattering of electromagnetic (EM) waves by two loss-free dielectric spheres with radii greater than a wavelength and located in the bear field of a source of radiation is presented. The incident field is expressed in terms of spherical vector wave functions (SVWF). Translational and rotational addition theorems are employed to express the SVWF of the incident field in the coordinate system associated with the dielectric scatterer. Numerical computations are performed for obtaining the amplitude and phase patterns of fields multiply scattered by two loss-free dielectric spheres, whose centers are located on the boresight axis and in the nearfield of an open-ended circular cylindrical waveguide excited in its dominant mode. Numerically computed results show good agreement with measured results obtained from a systematic experimental study on forward scatter performed in the X -band.

The new MININEC (version 3): A mini-numerical electromagnetic code

Abstract: : MININEC is a method of moments computer program for the analysis of thin wire antenna problems. MININEC uses a Galerkin procedure to solve for the wire currents using an integral equation formulation that relates the electric field and the vector and scalar potentials. This approach results in a compact computer code suitable for use on a microcomputer. MININEC solves for the impedance and currents on arbitrarily oriented wires including configurations with multiple wire junctions. Optics include lumped impedance loading, near electric and near magnetic fields, and far field patterns for free space or perfectly conducting ground. MININEC is written in the BASIC language compatible with IBM PC DOS. Keywords: Method of moments; Computer programs; Galerkin procedure; Thin wire antennas; Numerical Electromagnetics Code.

CO2 laser scintillation interferometer for the measurement of density fluctuations in plasma confinement devices

Abstract: Density fluctuations in magnetically confined plasmas have been measured from the phase scintillations impressed onto a 10.6‐μm wavelength laser beam probe, using a Mach–Zender interferometer operating in the optical near field. By careful design, the effects of vibration and air‐borne sound waves, which limit the instrument’s sensitivity, have been reduced to a level such that the minimum detectable signal is 10−6 rad in the frequency range 10 kHz–1 MHz, corresponding to a plasma density fluctuation level (δN/N) of 10−4.

Vertical currents in microstrip antennas

Abstract: An analysis of the radiation characteristics of a vertical current inside a dielectric-layer over ground is carried out. The general solution for the fields is given for the case of a vertical current which is independent of the vertical ( z ) coordinate. An "equivalence-principle" for the replacement of the vertical current by an appropriate surface current is presented. Expressions for the complex input power and for the power which goes into the lowest order transverse magnetic (TM)-type surface-wave mode are derived. In addition, graphs for the case of a short vertical dipole inside a dielectric-layer over ground for the radiation pattern, radiation resistance, and surface-wave resistance are presented. The calculations described above can be applied to the analysis of vertical feeds and short circuits in microstrip antennas.

Hybrid Y-junction laser array

Abstract: A semiconductor laser array having a plurality of waveguides at least some of which are directly joined at Y-junctions. The region near the Y-junctions provides a phase boundary condition in which lightwaves propagating in adjacent waveguides are in phase. A combination of strong and weak waveguiding is provided, with strong waveguides that eliminate evanescent coupling from occuring at least in the Y-junction regions, and with weak guides near one or both end facets permitting evanescent coupling. The evanescent coupling between adjacent weak waveguides preserves the in phase relationship that was established in the Y-junction regions, resulting in a diffraction limited single lobe far field output. Alternatively, even without evanescent coupling, the modes can adjust their phases in the weak waveguides, where the propagation constant is less tightly specified by the geometry.

The SNFGTD method and its accuracy

Abstract: The spherical near-field geometrical theory of diffraction (SNFGTD) method is an extended aperture method by which the near field from an antenna is computed on a spherical surface enclosing the antenna using the geometrical theory of diffraction. The far field is subsequently found by means of a spherical near-field to far-field transformation based on a spherical wave expansion of the near field. Due to the properties of the SNF-transformation, the total far field may be obtained as a sum of transformed contributions which facilitates analysis of collimated beams. It is demonstrated that the method possesses some advantages Over traditional methods of pattern prediction, but also that the accuracy of the method is determined by the quasioptical methods used to calculate the near field.

Spurious electromagnetic energy discriminator for electro-optical inspection systems

Abstract: An electro-optical system for inspecting an object includes a source producing a beam of electromagnetic energy, for example, a laser. It also includes a means for discriminating between electromagnetic energy of the beam scattered from objects within a predetermined distance of a plane intersected by the beam of electromagnetic energy and electromagnetic energy not so scattered. Thus, spurious electromagnetic energy scattered by other objects near the inspected object are ignored.

Analysis and performance of channeled-substrate-planar double-heterojunction lasers with geometrical asymmetries

Abstract: The effects of geometrical asymmetries on the optoelectronic properties of CSP-DH lasers using self-consistent calculations of the optical field and the electron-hole distribution in the active layer are analyzed and compared with device measurements. Laser properties modeled include gain profile, threshold, slope efficiency, near field, and far field. This analysis shows that small geometrical asymmetries due to device fabrication can produce significant changes in the optical and electrical properties of CSP-DH lasers, especially at high-output power levels. For example, a 0.5 μm misalignment of the Zn diffusion with respect to the substrate channel can produce lateral near-field and far-field shifts of 0.6 μm and 2.5°, respectively, and limit single spatial mode operation to about 30 mW.

Intensity fields of continuous‐wave axisymmetric transducers

Abstract: The intensity fields of disk, bowl, and conical transducers have been evaluated theoretically throughout their nearfield regions at a single frequency. The results have been compared to the fields resulting from the square of the pressure (P2). It is shown that the two types of fields differ most markedly on axis and close to the transducer face. The direction of energy flow has also been analyzed, and it is demonstrated that parallel flow lines occur in regions where P2 is the closest to intensity.

The annular aperture antenna with a hemispherical center conductor extension

Abstract: An annular aperture antenna mounted on an infinite ground plane and containing a hemispherical center conductor extension above the ground plane is investigated. A Green's function for the region above the ground plane is derived so as to be compatible with numerical solution techniques. A magnetic field integral equation is obtained in terms of the unknown tangential aperture electric field and is solved by the method of moments. A comparison between flush mounted and hemispherically extended annular aperture antennas is presented for the tangential aperture electric field, the coaxial line apparent input admittance, and the far radiated field.

Capacitive Arrays for Robotic Sensing

Abstract: Electromagnetic arrays have been used effectively for many years in optimizing the responses of antenna systems. The basic principles that make arrayed antennas work and make them easy to control can also be applied to near field electromagnetic array sensors. The array factor allows for flexibility in sensor geometry. Firstly, by exciting only a portion of an array in a sequential fashion one can physically scan and interrogate a region of a sample without having to move the sample or the probe head itself. Secondly, the field configurations can be altered by selectively exciting electrodes of an array. Also, the information received can be selected by combining electrodes to form different effective receiver geometries. Thirdly, array configurations allow for real-time analog signal processing. For instance, one can perform pattern matching by choosing the spatial resolution of the probe to match the spatial resolution of the desired feature.

Far-field behavior of injection-locked semiconductor laser arrays

Abstract: We have developed a computer model of injection-locked gain-guided semiconductor arrays to explain some experimental observations: the creation of a single-lobed far field from a normally double-lobed far field; the increase in the divergence angle of the far-field lobe(s); the shifting of power from one off-axis lobe to the other; and the regression back to a double-lobed far field when the ratio between array power and injection power becomes too large. The model is then extrapolated to look at properties of the index-guided case: inefficient end-element injection but an on-axis single-lobed center-element injection.

Model calculations of fields related to near-field optical scanning of a silver strip on a glass substrate

Abstract: The vertical energy flux density for the electromagnetic fields near the surface of a silver strip on a glass substrate is computed for an incident plane wave to aid in the measurement of the width of the strip. The dimensions of the strip cross section, e.g., 300 nm by 100 nm, are a fraction of the wavelength of the incident light , 632.8 nm. The flux 1 nm above the surface shows sharp spikes at the edges of the strip. The features of the fields near the surface could be used for accurate determination of the width of the strip by measurements up to about 30 nm above the strip. The effects of other variables are also shown in the figures.

Method for antenna measurements by means of the complex reflection factors of the object being measured

Abstract: In making antenna measurements for determining the direction of pattern of the occupancy of the near and far field and the gain and efficiency, transmission-type measuring methods have hitherto been used. According to the invention, reflection measuring methods are proposed for this. The object being measured, the antenna to be measured, is not connected to transmitter or receiver but, with defined passive terminations, is measured geometrically three-dimensionally as reflection object via the complex scattering matrix of the reflection factor. To determine the gain by measurements and for calibration, reflection standards such as, for example, spheres or corner reflectors or planar reflectors are used.

Ponderomotive stabilization of flute modes in mirrors: Feedback control and numerical results.

Abstract: Ponderomotive stabilization of rigid plasma flute modes is numerically investigated by use of a variational principle, for a simple geometry, without eikonal approximation. While the near field of the studied antenna can be stabilizing, the far field has a small contribution only, because of large cancellation by quasi mode-coupling terms. The field energy for stabilization is evaluated and is a nonnegligible fraction of the plasma thermal energy. A new antenna design is proposed, and feedback stabilization is investigated. Their use drastically reduces power requirements.

Analysis of Thermal Radiation from an Inhomogeneous Cylindrical Human Body Model

Abstract: The thermal radiation from a cylindrical human body model at microwave frequencies is treated analytically. The human body model is taken to be a homogeneous cylinder at temperature T having a localized internal thermal inhomogeneity at temperature T + Delta T. The mean energy density for the near field outside the cylinder is determined by employing the dyadic Green's function of the homogeneous cylinder and the fluctuation-dissipation theorem. Analytical results are derived for the contributions of the homogeneous cylinder and the inhomogeneity region. Numerical results are presented for several geometries at low microwave frequencies where a reasonable transparency of tissues is expected. The possibility of using microwave radiometry techniques to measure temperature distributions in depth is discussed in relation to hyperthermia and the development of noninvasive diagnostic techniques. It is shown that the emission from surrounding tissues limits the detectability of thermal inhomogeneities inside the body and that by using low microwave frequencies (~1 GHz), temperature measurement at depths up to 2 cm can be performed.

Effects of Electromagnetic Source Type and Orientation on Signal Falloff with Distance

Abstract: In EMC testing it is usually assumed tha t the field mea­ sured by a vertical dipole antenna is produced by a vertical electric dipole source in the equipment under test. This pa­ per calls attention to the fact tha t vertical electric fields are also produced off the end of a horizontal electric dipole, and broadside to a horizontal magnetic dipole. Similar effects hold true for horizontal field measurements. Failure to identify the type and orientation of the source can lead to errors in mak­ ing corrections for ground-reflected signals when extrapolating measurements to larger distances.