Showing papers on "Near and far field published in 1990"

Spherical reflector as an electromagnetic‐missile launcher

Abstract: A spherical reflector illuminated by a point source is found to be capable of launching an electromagnetic missile. Since the surfaces of the reflector are not complete spherical surfaces, the problem cannot be solved by simple boundary matching. The electromagnetic fields have to be solved in three regions separately. The general solutions are matched on the boundaries to obtain a set of coefficient equations. Under the conditions for an electromagnetic missile, the coefficient equations are solved asymptotically. The evaluation of the Poynting vector shows that the energy radiated from the reflector has the same slow rate of decay as for the spherical lens.

Bragg Scattering of Electromagnetic Waves from the Air/Sea Interface

Abstract: The advent of radar during World War II had consequences for research in air/sea interactions which were far from obvious in the face of the immediate wartime need to detect enemy aircraft. When the newly-discovered tool was put to use to detect targets on or near the surface of the sea, these targets were often obscured by strong echos from the ocean itself. This “sea echo” constituted a considerable nuisance to those engaged in locating enemy vessels so investigations into the nature of this unwanted return were launched. Early theoretical work concentrated on explaining sea echo in terms of either return from the sea surface itself or from the spray and bubbles above the surface. Methods of modelling electromagnetic scattering which had proved useful to the characterization of many types of targets were adapted for use modelling sea return. Specular reflection seemed to explain the characteristics of sea echo for small incidence angles but not for larger ones. The standard method of carrying out such calculations utilized the “Kirchoff principle,” also known as physical optics or the tangent plane method, which assumes that the surface is smooth in the sense that the radius of curvature is large compared to the electromagnetic wavelength. In addition to yielding disturbingly low return at large incidence angles, the method was unable to account for the observation that vertically polarized return was generally stronger than horizontal at such angles. Since interference between radiation directly incident on bubbles above the surface and radiation incident on the bubbles following reflection from the surface could in principal yield such polarization differences, theories based on Rayleigh scattering by the bubbles were also proposed.

Radiation from cylindrical leaky waves

Abstract: Formulas are derived for the far-infrared radiation pattern of cylindrical leaky waves propagating on a planar surface. The formulas can be used to predict the radiation pattern of a general class of leaky-wave antennas, consisting of a finite-size source which excites a radially propagating leaky wave on some planar surface. Leaky-wave antennas consisting of antenna elements embedded in dielectric layers (microstrip elements) fall into this category. Using the equivalence principle, formulas are derived for both transverse electric (TE) and transverse magnetic (TM) leaky waves with arbitrary propagation constants. The formulas allow for radiation from cylindrical apertures of arbitrary size, so that the effect of truncating the supporting planar surface with an absorbing material can be determined. Particular attention is devoted to the case of a leaky wave for which the real and imaginary parts of the complex propagation constant are equal, since this type of wave has been shown to be responsible for broadside radiation in certain leaky-wave antennas comprised of dielectric layers. >

The power radiated by a vibrating body in an acoustic fluid and its determination from boundary measurements

Abstract: The power radiated into a specified angular sector by a vibrating object immersed in a fluid is expressed as a quadratic functional of the boundary normal velocity field. The diagonalization of the functional, obtained through the singular value decomposition of its discretized version, identifies a set of orthonormal boundary velocity patterns, each corresponding to a far‐field pattern belonging to a set of functions orthonormal in the angular sector of interest. Any boundary normal velocity field can be represented as a linear superposition of the orthonormal patterns. The velocity patterns having high radiation efficiencies form a subset, whose dimension depends upon the object boundary shape and size in wavelengths. The other velocity patterns do not radiate efficiently and contribute mainly to the evanescent field in the neighborhood of the object. Assuming that some noise is present, only the radiation patterns associated with the efficiently radiating velocity patterns are observable in the far field. Therefore, the dimension of their set defines the number of degrees of freedom of the far field. The efficiently radiating velocity patterns constitute a set of spatial filtering functions, separating the radiating from the essentially nonradiating components of an arbitrary boundary normal velocity field.

Radiation loss from planar waveguides with random wall imperfections

Abstract: Presents a new method of analysing radiation loss from optical waveguides with random wall imperfections, in which the imperfections are treated as an equivalent current source. The method allows calculation of both the far field pattern and the loss, and gives a result which is simpler than that obtained by other methods.

Focusing of diode laser beams: a simple mathematical model.

Abstract: A simplified mathematical model for the far field of a monomode diode laser is employed for easy but fairly accurate computations of the optical field in the focal region. The present treatment is concerned with laser junctions significantly narrower than the wavelength. The field distribution in the plane perpendicular to the diode junction is considered in detail. The results of computations are shown to agree well with the measurements. Hence, the computational code is valuable for the designing of optical devices, such as diode-fiber couplings and laser Doppler anemometers. The present work is not concerned with design calculations for specific applications. Instead, it is intended to illustrate the general features of the proposed mathematical model of monomode diode laser beams.

Far-field pattern determination from the near-field amplitude on two surfaces

Abstract: The possibility of determining the far field of radiating systems by measuring only the near-field amplitude is investigated. The main difficulties of the problem are examined in some detail and a new near-field/far-field transformation technique is developed, based on the measurement of the near-field amplitude over two surfaces surrounding the antenna under test. The accuracy of the far-field reconstruction results are related both to the distance between such surfaces and to some a priori information concerning the near-field phase and/or the radiating system. The information on the radiating system allows relaxation of the need for any information on the near-field phase provided that the distance between the measurement surfaces is high enough. Conversely, the knowledge of a more or less corrupted near-field phase allows reduction of such distances without affecting the accuracy of the far-field reconstruction. Numerical examples validating the effectiveness of the developed algorithm are provided for the planar scanning case. >

Antenna radiation in the presence of a chiral sphere

Abstract: The radiation emitted by electromagnetic sources placed both inside and outside of a homogeneous sphere of chiral media is studied using an exact formulation. For both cases, dyadic Green’s functions are found in terms of spherical vector wave functions. The radiated fields and radiation resistance are examined for a dipole located at the center of the chiral sphere. For this case, it is shown that by choosing the sphere’s size and material parameters properly, purely circular polarization can be achieved in the far field. It is also demonstrated that the radiation resistance of the dipole can be increased.

Electromagnetic field of a vertical dipole over an imperfectly conducting half‐space

Abstract: The electromagnetic field generated by a vertical electric dipole on or near the surface of the Earth is studied for continuous-wave excitation. The analytical foundation established by Sommerfeld and developed by many others is reviewed, and simple explicit new formulas are derived for the three components of the electromagnetic field and represented graphically. Their range of validity is defined. They show that the surface waves constitute the entire far field along the air-earth boundary.

The principles of electromagnetic theory

Abstract: 1. Electric charges and currents 2. The electromagnetic field and the aether relations 3. Polarization and magnetization 4. The fusion of electromagnetism with mechanics and thermodynamics 5. Electromagnetic materials 6. Electrostatics 7. Dielectrics 8. Magnetism 9. Conductors 10. Radiation 11. Electromagnetic wave propagation.

Recent Advances in Electromagnetic Theory

Abstract: 1 Electromagnetic Waves in Chiral Media.- 2 Norms of Time-Domain Functions and Convolution Operators.- 3 An Overview of the Theory of the Near-Zone Doppler Effect.- 4 Analysis of Channeled-Substrate-Planar Double-Heterostructure Lasers Using the Effective Index Technique.- 5 Correlation Theory of Electromagnetic Radiation Using Multipole Expansions.- 6 On Fractal Electrodynamics.- 7 Fringing Field Effects in VLSI Structures.- 8 Some Methods of Reducing the Sidelobe Radiation of Existing Reflector and Horn Antennas.- 9 Group Symmetries of Antenna Arrays.- 10 Many-Body Problems in Electromagnetic Theory.- 11 Electromagnetic Shielding.- 12 Coherence Effects in Guided Wave Optical Systems.- 13 How Scattering Increases as an Edge Is Blunted: The Case of an Electric Field Parallel to the Edge.- 14 Electromagnetic Fields Determined from Two-Dimensional Infrared Thermal Patterns.- 15 Dielectric Waveguide Theory.- Author Index.

Scattering by an Impedance Sphere Coated with a Chiral Layer

Abstract: The scattering of a plane, linearly polarized electromagnetic wave by a sphere on whose surface an impedance boundary condition holds, and that is covered with a concentric layer of chiral material, is considered. Exact, explicit expressions are derived for the scattered field coefficients. The co-polarized and cross-polarized components of the far backscattered field are determined and discussed. The value of this canonical problem as a benchmark for computer codes is pointed out.

Model for reflection near field optical microscopy

Abstract: We discuss a model that describes the optical interactions between a dielectric tip and a surface exhibiting roughness of subwavelength size (infinite tracks). Such a model gives new insight into the resolution achievable by scanning near field optical microscopy. The dielectric tip is schematized as a cone whose extremity reduces to a small sphere acting as a dipolar scattering center, allowing separation of the contributions from the near field lying at the air-sample interface of other long range terms associated with the progressive waves coming from the surface. It is shown that because of its fast spatial dependence, the near field detected by the tip contains subwavelength features of the object. Relationships with preliminary experiments are discussed.

The propagation of short pulses of ultrasound from a circular source coupled to an isotropic solid

Abstract: New finite‐difference formulations in cylindrical coordinates are given allowing calculations to be made of the field radiated by a circular source coupled to an isotropic solid. Theoretically predicted results are compared with earlier theoretical and experimental results for the case of a pointlike source. Calculations of the spacial distribution of the field radiated by a finite‐sized circular source are compared with stroboscopic visualizations of the field radiated by a transducer typical of those used in nondestructive evaluation using ultrasound. Calculated field‐point particle‐velocity waveforms are compared with experimentally measured waveforms received using a miniature piezoelectric receiver directly coupled to the solid surface as well as with calculated results obtained using an impulse‐response model. Good agreement between theoretical and experimental results is observed. The field radiated into solids is described in terms of plane and edge waves. The results are of particular significance for explaining the limitations of nondestructive evaluation using ultrasonic pulse‐echo techniques.

Efficient parallel receive beam forming for phased array imaging using phase rotation

Abstract: Far field imaging systems have long exploited the principle of spatial Fourier inversion of array data to obtain multiple beams simultaneously. In medical ultrasound imaging, a simple Fourier inversion cannot be used to produce simultaneous receive beams because of the near field nature of the beam formation process. In this paper, a method for multiple beam formation in the near field using phase rotation (i.e., Fourier inversion) is presented. This method, if used in conjunction with a digital beam former described in a companion paper, can produce multiple simultaneous beams of high precision with little increase in the complexity of the beam former.

Dense sets and the projection theorem for acoustic harmonic waves in a homogeneous finite depth ocean

Abstract: The scattering of a ‘plane wave’ off a submerged body situated in an ocean of finite depth is investigated. The index of refraction is considered to be depth-independent. It is shown that the far field is not unique; hence, the problem of determining the shape of an object from its far field is not well-posed. If solutions are sought among a restricted class of problems the ‘dense set’ property implies that the problem can be made well-posed.

Nonlinear electromagnetic propulsion system and method

Abstract: The nonlinear electromagnetic propulsion system (NEMPS) and method prescribes a electromagnetic propulsion system based on an extremely low frequency (ELF) radiating antenna structure driven by a matched high-current pulsed power supply. The ELF antenna structure resembles a modified three-dimensional multiple-turn loop antenna whose geometry is optimized for the production of reaction thrust rather than the radiation of electromagnetic energy into space, and it is current-driven rather than voltage-driven. The rigid three-dimensional geometric asymmetry, made up of flat electrical conductors that form a partially closed volume in the loop antenna structure, traps magnetic flux, thereby causing a magnetic field density gradient along a single axis. This gradient then causes an imbalance in the magneto-mechanical forces that normally result from the interactions of the loop antenna's internal magnetic field with the current in the conductors of the loop antenna structure. The pulsed power supply is designed to provide the proper waveform to the antenna structure at an impedance matching the load impedance of the antenna. The rise time and shape of the input current waveform are crucial to maximizing the production of reaction thrust. Input voltage is at a nominal value sufficient to allow the desired high input current. >

Reduced‐confinement antennas for GaAlAs integrated optical waveguides

Abstract: Monolithically integrated reduced‐confinement antennas are shown to produce reductions of >35% in the far‐field beam divergence for radiation emitted from single‐mode GaAlAs slab waveguides, yielding far‐field beams as narrow as 8.2° FWHM along the direction perpendicular to the wafer surface. Reduced confinement of the guided mode near the output endface is achieved using a novel molecular beam epitaxy growth technique to produce a longitudinal variation in the refractive index and thickness of the waveguide film. Unlike present horn antennas, the reduced‐confinement geometry has the distinct advantage of being compatible with two‐dimensional antenna development.

Electromagnetic field singularities at the tip of an elliptic cone

Abstract: Electromagnetic field singularities at the tip of an elliptic cone

Measured underwater near‐field e‐patterns of a pulsed, horizontal dipole antenna in air: comparison with the theory of the continuous wave, infinitesimal electric dipole1

Abstract: At Delft Geotechnics the technique of ground-penetrating radar is in use for the detection of buried objects such as pipes. To enable us to give our ‘measurements in the field’ a more quantitative interpretation than can be deduced from these alone, a series of experiments has been started under well-defined conditions. A cylindrical vessel containing water simulates wet soil. Mounted horizontally above the water surface is a pulsed triangular half-wave dipole which is used as a transmitting antenna (TA). It has a carrier-frequency of about 160 MHz and a pulse repetition-frequency of about 50 kHz. A movable receiving dipole (‘probe’) in the water measures the transverse, mutually orthogonal Eφ,- and Eθ-components of the pulses as a function of probe-position (r, θ, φ) and of the height h of the TA above the water surface. When these pulses are Fourier-transformed, the transverse electric fields Eφ and Eθ at 200 MHz are obtained. The resulting field patterns are compared with computational results on the basis of the theory of the continuous wave, infinitesimal electric dipole (‘point dipole’). It can be concluded that: 1 Far-field conditions have not fully developed at a depth of about 2.50 m, the largest value of the radius r at which field patterns were measured, although it represents a distance of about 15 wavelengths. 2 The attenuation constant of the tapwater used, as deduced from E-field measurements for θ= 0, 2.50 m < r < 2.75 m, is slightly less than the value measured using a network analyser and air line combination, in agreement with (1). 3 Eφ field patterns calculated using the value of the conductivity σ corresponding to the former value of the attenuation constant agree reasonably well with the measured patterns for r≤ 2.50 m and for θ < 20° at all antenna heights considered. Calculated Eφ patterns do not agree so well with the measured patterns when h is close to zero. With increasing height the agreement inproves. 4 In accordance with the theory of the point-dipole, the angular distribution of the radiation patterns of the TA becomes wider as the frequency decreases. 5 The normalized underwater pulse-spectra shift to lower frequencies with increasing r. This can be explained since the attenuation constant of the water rises with rising frequency.

Near‐field acoustic holography for an axisymmetric geometry: A new formulation

Abstract: Near‐field acoustic holography has recently been extended to nonseparable coordinate systems by applying the boundary elements method. The formulation currently implemented applies singular value decomposition to treat the presence of evanescent waves in the near field of the structure. A new method is introduced for an axisymmetric geometry that is better suited for broadband data than the singular value decomposition algorithm, enabling reduced computation time and providing a new expansion for the surface field with a set of functions that can be expressed as real functions, independent of frequency and independent of the choice of a measurement surface. The method is tested on numerically generated data.

Spherical near-field scanning: determining the incident field near a rotatable probe

Abstract: A spherical near-field scanning algorithm is developed for determining incident fields inside a probe's minimum sphere. This differs from the well-known spherical near-field scanning formulation which determines fields outside the source's minimum sphere. The scanner size depends on the extent of the region of interest and not on the extent of the (possibly much larger) source. The data can be collected using a standard roll-over-azimuth positioner. The practical implementation of this technique is discussed. >

A fast method for computing radiation from printed circuit boards

Abstract: An alternative approach for computing radiation from a multilayer circuit board is presented. Compared to a full-wave electromagnetic approach, this method is much faster and requires less storage space for data. Computational efficiency is the key attribute of the approach. It allows solutions, previously unobtainable, to complex and realistic structures, such as those arising from typical product designs. The radiation is calculated using a transmission line model to find the amplitude and spatial distribution of currents in printed circuit traces. The far field is computed using the Green's function of a current element just above, or inside, a thin dielectric sheet. Two examples are considered: (1) a comparison of results in the frequency domain obtained by using the method described here with those of a full-wave electromagnetic approach based on the method of moments; and (2) a comparison of simulation results with real semianechoic chamber measurements. >

Process and apparatus for optical near field microlithography

Abstract: Direct scanning microlithography process of a substrate such as a wafer, by means of an optical and/or electronic beam, for obtaining photomechanical or electromechanical lithography of submicrometric structures at the surface of the substrate, wherein the source of the optical and/or electronic beam used for lithography is kept at an appropriate distance from the substrate by means of a waveguide proximity probe, such as a fiber optic proximity probe capable of measuring rapid variation, depending on the distance, of the intensity of an electromagnetic wave reflected by the substrate within the near field area located at the end of the probe. The invention also concerns microlithography devices using this process.

Transverse pattern modifications in a stable apertured laser resonator.

Abstract: The effects of hard apertures on the transversal mode distribution and the output near and far field distributions are investigated both theoretically and experimentally. Wavefronts propagating in different directions inside a CO2 laser cavity were measured at the same plane and found to have different transversal amplitude profiles. The influence of intracavity aperture’s location and diameter on the near field distribution and far field beam quality were measured and calculated. Quantitative figures of merit are introduced to evaluate the departure of the transversal mode patterns from ideal Gaussians.

Magneto-optical means for reading disks by comparing signals at leading and trailing edges of laser beam

Abstract: Recording medium on which information is magneto-optically recorded is irradiated with a linearly polarized light beam. A light beam reflected from or passed through the recording medium is changed in phase by a wave plate. Through an analyzer the differently polarized components of the light beam from the wave plate are combined by aligning the polarizing directions of the light beam. The light intensities of the forward and backward parts of the moving direction of the recording medium in the far field of the light beam from the analyzer are detected by a photo detector. A difference between the detected signals of the forward and backward parts is generated to reproduce the information recorded on the recording medium. Furthermore, the light beam from the wave plate is splitted into two light beams linearly polarized in mutually orthogonal directions. Two light intensities of the two light beams for the forward and backward parts of the moving direction of the recording medium in the far field of the respective light beams are detected by two photo detectors. Two differences between the outputs from the two photo detectors are determined by two subtractors. A difference or sum between the two subtractors is determined and the information is reproduced on the basis of thus determined difference or sum.

A technique of synthesizing the excitation currents of planar arrays or apertures

Abstract: A technique of synthesizing or reconstructing the excitation currents of a planar array of aperture-type antennas from the known near-field patterns of the radiating source is presented. This technique uses an exact solution to the fields radiated by the aperture antenna without disregarding the source currents. Typical numerical computations have been carried out to validate the analytical technique developed. Sensitivity and stability of the numerical computations performed have been studied. The available iterative bandlimited signal extrapolation technique is used to reconstruct the aperture excitation currents only if the far-field patterns of the radiating source are known. Far-field patterns of aperture antennas measured in the laboratory were also used to reconstruct the aperture electric field distribution in the principal plane. >