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Showing papers on "Near and far field published in 1973"



Journal ArticleDOI
TL;DR: In this article, an analytical model for the response of a solid to periodic surface stresses is applied to cases where the stresses arise from Lorentz forces on currents flowing in a static magpetic field.
Abstract: Calculations and measurements of the efficiencies of electromagnetic transducers for Rayleigh and Lamb waves are presented. Existing solutions for the response of a solid to periodic surface stresses are applied to cases where the stresses arise from Lorentz forces on currents flowing in a static magpetic field. Results are presented for both electri- cally insulating and conducting solids. In the former case, the current must be passed through wires bonded directly to the surface, while in the latter case they may be induced in the metal surface by a nearby coil with no physical contact. Included are expressions for the efficiencies of transmitting and receiving transducers along with a brief discussion of ap- propriate reciprocity relations and equivalent circuits. The theoretical predictions are compared to experimental results for the transduction of 130 kHz flexural waves on 3/8-in aluminum and steel plates. With no adjustable parameters, the theory closely predicts the absolute level of performance on the aluminum plate. Data on steel is qualitatively the same but suggests that magnetostrictive coupling is of comparable magnitude to the Lorentz force coupling mechanism. The dependence of transducer performance upon separation from the plate is also found to be accurately described by the model.

134 citations


Patent
16 Jul 1973
TL;DR: In this article, a Cassegrainian antenna system has a planar array as the feed and an intermediate reflector is positioned in the near field of the array for substantially collimated illumination with all array elements operating in phase.
Abstract: A Cassegrainian antenna system has a planar array as the feed. An intermediate reflector is positioned in the near field of the array for substantially collimated illumination with all array elements operating in phase. Accordingly, an on-axis main beam is radiated from the main reflector upon illumination by energy from the intermediate reflector. By impressing a linear phase gradient across the array, the main beam is controllably tilted off-axis.

119 citations


Journal ArticleDOI
TL;DR: In this paper, experiments were described for determining the distributions of current, the driving point impedance, and the electromagnetic fields characteristic of dipole antennas immersed in the sea near the surface.
Abstract: Experiments are described for determining the distributions of current, the driving-point impedance, and the electromagnetic fields characteristic of dipole antennas immersed in the sea near the surface. Measured and theoretical results are compared at two frequencies with good agreement.

87 citations


Journal ArticleDOI
TL;DR: In this article, the authors presented energy diagrams indicative of the electromagnetic activity in the sea and made estimates of the resolution required in electromagnetic research at sea can be made, and they showed that absolute minima of 1 γ and 0.05 μV/m are necessary for magnetic and electric fields, respectively.

87 citations


Journal ArticleDOI
TL;DR: In this article, a review of the status of problems involving the interaction of matter and optical-band electromagnetic radiation of intensity such that any medium becomes a fully ionized plasma is presented.
Abstract: A review is presented of the status of problems involving the interaction of matter and optical-band electromagnetic radiation of intensity such that any medium becomes a fully ionized plasma. The following are considered within the framework of the single-electron approximation in the case of nonrelativistic energies: stimulated bremsstrahlung and absorption of an electron in the field of a strong electromagnetic wave, stimulated two-photon Compton scattering, and scattering of electrons in the field of an intense standing electromagnetic wave (the Kapitza-Dirac effect). The role played by these processes in the heating of plasma by laser radiation is analyzed, as is the question of the possibility of obtaining amplification (negative absorption) of light in transitions in a continuous spectrum. The bibliography is brought up to date to the middle of 1971.

81 citations


Journal ArticleDOI
TL;DR: The difference in notations used by researchers in the dielectric waveguide field and those primarily interested in waveguide lasers is discussed, and the equations for the field components of the various modes in large radius hollow dielectrics waveguides are rederived in terms of the more widely used notation.
Abstract: The difference in notations used by researchers in the dielectric waveguide field and those primarily interested in waveguide lasers is discussed, and the equations for the field components of the various modes in large radius hollow dielectric waveguides are rederived in terms of the more widely used notation. Certain linear combinations of these modes that give linearly polarized field distributions are then considered to be launched into free-space at a waveguide termination. The resultant Fresnel and Fraunhofer field distributions are useful in identifying the modes of oscillation and in choosing mirror apertures that will restrict oscillation on the fundamental waveguide mode.

74 citations


Journal ArticleDOI
TL;DR: In this paper, the mean square angular divergence of the beam, the beam cross-section, and the power received by a planar aperture in the medium and the mutual coherence function were investigated.
Abstract: Transport methods have been employed to study the propagation of a narrow-angle electromagnetic beam through a turbulent plasma. Results are presented for the mean-square angular divergence of the beam, the beam cross section, and the power received by a planar aperture in the medium and the mutual coherence function. The problem of backscattered radiation is also considered and results are obtained which reduce, in the limit of zero true absorption, to previous results.

68 citations



Journal ArticleDOI
TL;DR: In this article, the diffraction pattern due to a plane H-polarized electromagnetic wave is investigated, when this wave is incident upon an infinitely long slit of finite width in an opaque screen of non-vanishing thickness.
Abstract: The diffraction pattern due to a plane H-polarized electromagnetic wave is investigated, when this wave is incident upon an infinitely long slit of finite width in an opaque screen of non-vanishing thickness. The screen is located between the plane boundaries of two media with different electromagnetic properties. A Green's function formulation of the problem is employed, leading to a system of four coupled integral equations in which the field distributions in the slit occur as unknowns. Numerical results are presented for the field just below the screen as well as for the far field pattern.

55 citations


20 Aug 1973
TL;DR: The results of an experimental study of induced Cherenkov excitationn of electromagnetic radiation in the centimeter band by a beam of relativistic electrons from a strong current accelerator are given in this article.
Abstract: The results of an experimental study of induced Cherenkov excitationn of electromagnetic radiation in the centimeter band by a beam of relativistic electrons from a strong-current accelerator are given. The conversion coefficient of electron energy into electromagnetic radiation is 12 to 15%. (auth)

Journal ArticleDOI
TL;DR: In this paper, the governing equations for the induction of electromagnetic fields in the ocean by ionospheric and oceanic sources are presented and a uniformly conducting layered model and a non-uniformly conducting thin-sheet model are discussed with reference to the interpretation of fields observed in the oceans.

Patent
01 Oct 1973
TL;DR: In this paper, a cost-effective and practical technique for obtaining far-field patterns of complex radiating or receiving structures such as ships, aircraft, or shore site antennas is proposed.
Abstract: A cost-effective and practical technique for obtaining far-field patterns of complex radiating or receiving structures such as ships, aircraft, or shore site antennas Actual phase and amplitude measurements are obtained in the near-field and then they are processed in a digital computer using theoretical Zmatrix numerical techniques to calculate the far-field patterns by solving the matrix for the current distribution on the structure By using near-field measurements, the method is significantly less expensive than either modeling or complete computer solutions, yet the results achieved are highly accurate

Journal ArticleDOI
TL;DR: In this paper, the authors investigated the basic wave mechanism responsible for both the lateral (Goos-Hanchen) shift of a gaussian light beam incident from a denser medium upon the interface to a rarer medium, and the weak radiance that accompanies the reflected beam.
Abstract: By utilizing a rigorous plane-wave representation, we investigate the basic wave mechanism that is responsible for both the lateral (Goos-Hanchen) shift of a gaussian light beam incident from a denser medium upon the interface to a rarer medium, and the weak radiance that accompanies the reflected beam. We thus find that the complete reflected field contains a geometric-optics component, whose domain is given by the mirror reflection of the incident beam, as well as a lateral-wave component which occupies a much larger domain. Our results show that the near field of the lateral-wave component interferes with the geometric-optics component and thereby produces the Goos-Hanchen shift of the reflected beam. The intermediate and far fields of the lateral-wave, on the other hand, are responsible for the weak radiance that trails the reflected beam. By demonstrating that both the beam shift and the trailing radiance represent two facets of the same wave process, the lateral-wave field is shown to provide a unified interpretation of two effects that have previously been regarded as separate phenomena. The dependence of these effects on beam width, angle of incidence, and polarization are also considered.

Journal ArticleDOI
TL;DR: In this article, the moments of electric and magnetic dipoles whose far field is equivalent to the Rayleigh term in a given radiating electromagnetic field are expressed as integrals over an arbitrary closed surface of the near electromagnetic fields.
Abstract: Expressions are derived for the moments of electric and magnetic dipoles whose far field is equivalent to the Rayleigh term in the far field of a given radiating electromagnetic field. The equivalent dipole moments are expressed as integrals over an arbitrary closed surface of the near electromagnetic fields and simplification is achieved by rewriting these expressions in terms of the static potentials from which the near fields are derived. The results are valid regardless of the complexity of the field and the medium within the integration surface as long as the exterior is homogeneous, isotropic and source-free.

Journal ArticleDOI
TL;DR: In this article, the authors considered the radiation of an arbitrary source in a stratified system including anisotropic slabs, and evaluated the transverse field and the far field through saddle-point integration.
Abstract: The radiation of an arbitrary source in a stratified system including anisotropic slabs is considered. Starting from an abstract-operator formulation given by Bresler and Marcuvitz [1956, 1957], the transverse field is evaluated by using a Fourier-integral representation. The Green's function is expressed through the evaluation of the source-free solutions. The scattered field and the plane-wave response are examined. An example is given concerning the field radiated in the presence of an anisotropic ferrite slab. The far field is evaluated through a saddle-point integration.


Journal ArticleDOI
TL;DR: In this article, the electromagnetic field due to a localized charge-current distribution is expanded in terms of a set of multiple moments defined with respect to a given time-like world line, leading to the identification of the radiation field.


Journal ArticleDOI
TL;DR: A possible mechanism for microwave-neuron interaction, when the nerve is irradiated by a thermally insignificant electromagnetic field, is described and attention is given to both homogeneous and inhomogeneous broadening effects.

Journal ArticleDOI
TL;DR: In this paper, a spherical wave expansion of the far-field pattern data is used to calculate the antenna gain correction factor directly from measured far field pattern data, without any assumptions regarding antenna aperture field distributions.
Abstract: A new method is presented for calculating near-field antenna gain correction factors directly from measured far-field pattern data by using a spherical wave expansion of the pattern. This eliminates the need for any assumptions regarding antenna aperture field distributions. The only significant assumption in the new method is to neglect multiple scattering between the antennas. The method is applied to the case of a horn antenna. Calculated results are compared to direct measured results, demonstrating agreement to within 0.03 dB. The method is also compared to the method of Chu and Semplak, with similar agreement. The sensitivity of the results to truncation error and noise in the data is also investigated and contrasted to sensitivity of prior methods to errors in the assumed field distribution.

Journal ArticleDOI
TL;DR: It is shown that when underwater inhomogeneities dominate the small-angle scattering, the limiting angular resolution is independent of wavelength and the general procedure for estimating beam spot size is presented.
Abstract: An expression is derived for the mean irradiance distribution of a finite cross-section laser beam propa: gating in sea water. The effects of large-scale refractive variations on beam quality due to temperature and salinity fluctuations and the incoherent scattering of suspended biological particles with indices of refraction close to that of sea water are included. The analysis is general; it is valid in both the near and the far field of the laser transmitting aperture and also for an arbitrary complex electromagnetic optical wavefunction in the exit pupil of the transmitting aperture. Analytic results are presented for the beam pattern and spot size of a Gaussian laser beam. In particular, the manner in which image resolution, beam spread, and the parameters that describe the inhomogeneities in the water are quantitatively related is discussed. Furthermore, we present a general procedure for estimating beam spot size. Finally some comments regarding focused-beam propagation and limiting angular resolution are given. It is shown that when underwater inhomogeneities dominate the small-angle scattering, the limiting angular resolution is independent of wavelength.

Journal ArticleDOI
TL;DR: In this paper, the authors employed the Wiener-Hopf technique to derive the near-field solution for scattering by a semi-infinite conducting tube due to an axially incident plane wave.
Abstract: The Wiener-Hopf technique is employed to derive the near-field solution for scattering by a semi-infinite conducting tube due to an axially incident plane wave. Numerical results are included and emphasis is given to the interpretation of the results in terms of the error in the measurement of the magnetic field due to the presence of the sensor boom. It is found that at interior resonance frequencies the measurement error can be as large as 100 percent in the immediate neighborhood of the end of the boom. Guidelines for reducing such errors are suggested.

Journal ArticleDOI
TL;DR: In this article, a non-uniform source was considered for the induction of electric currents in a semi-infinite conductor with a plane boundary, and the results indicated that at longer periods the source configuration considerably influences the apparent resistivity calculations.
Abstract: Summary Electromagnetic induction in the Earth by a non-uniform current source is considered. Apparent resistivity calculations from the Cagniard formula are made for models in which the Earth has lateral as well as horizontal discontinuities and the source is a gaussian electrojet. These results are compared with those for a uniform inducing field. The results indicate that at longer periods the source configuration considerably influences the apparent resistivity calculations. Many authors have studied the problem of electromagnetic induction in twodimensional Earth models by a uniform source. Solving the problem of induction by a non-uniform source is more difficult, but is receiving increasing attention. The general theory of the induction of electric currents in a semi-infinite conductor with a plane boundary was first considered in a classic paper by Price (1950). In this work, Price made detailed calculations of the induced field and currents produced by a periodic or aperiodic line current parallel to the face of the conductor, and indicated a method for the calculation for any other inducing fields. Also, Price (1962) has considered the theory of magnetotelluric methods and has extended the theory to take account of the distribution of the ionospheric inducing field. Hermance & Peltier (1970) have employed the method of Price (1950) to study the magnetotelluric fields of a line current and Peltier & Hermance (1971) of a gaussian electrojet over a horizontally stratified Earth. Also, Dosso & Jacobs (1968) have made analogue model measurements of electromagnetic variations in the near field of an oscillating line current. Furthermore, Schmucker (1971) has recently given results of some work in which he considers a non-uniform source over a laterally inhomogeneous Earth in which the lateral changes in conductivity are confined to a limited depth range. Peltier & Hermance (1971) solved the two-dimensional diffusion equation (for the E-polarization case in which the electric field is parallel to the strike of any discontinuities) by a separation of variables and related it to the current source by a Fourier integral which synthesizes the actual fields from elementary fields in the manner described by Price (1950). When this method alone is used, a solution can be obtained only for a layered Earth. However, this method may be used to obtain boundary values, and when these are used in conjunction with the Jones & Price (1970) finite difference method a more general class of conductivity configurations for the Earth may be considered.

Patent
26 Feb 1973
TL;DR: In this paper, a Schwarzschild antenna with feed horns mounted on a turnstile waveguide switch is described, where microwave energy is emitted from the feed horn onto an adjacent mirror that reflects the energy to the antenna reflectors.
Abstract: A Schwarzschild Antenna includes feed horns respectively mounted on a turnstile waveguide switch. During transmission, as each horn passes a particular arc of rotation, microwave energy is emitted from the horn onto an adjacent mirror that reflects the energy to the antenna reflectors. The result is a unidirectional scan in the far field. A tracking mode of operation is also provided. Either of the two modes of operation may be selected by the operator.

Journal ArticleDOI
TL;DR: In this paper, the wave equations for the electromagnetic radiation are given in the Newman-Penrose formalism, and the radiation for a simple physical system is calculated and discussed, and a simple point charge is considered.
Abstract: Gravitational perturbations in the neighborhood of a point charge can induce electromagnetic radiation even if the symmetry of a configuration indicates that the charge is not moving. Wave equations for the electromagnetic radiation are given in the Newman-Penrose formalism, and the radiation for a simple physical system is calculated and discussed.


Journal ArticleDOI
TL;DR: In this paper, the self-focussing of an electromagnetic electric-type wave beam propagating in an inhomogeneous plasma across a strong magnetic field has been analyzed and conditions indispensable for the formation of a self-sustained waveguide channel were determined.

Patent
26 Feb 1973
TL;DR: In this paper, a quasi-spherical multi-reflector radar antenna has a subreflector and a main reflector with an aperture formed centrally therein, and a mirror wheel is positioned behind the aperture while a fixed microwave energy beam is directed axially or radially against the wheel for subsequent reflection to the far field.
Abstract: A quasi-spherical multi-reflector radar antenna has a subreflector and a main reflector with an aperture formed centrally therein. A mirror wheel is positioned behind the aperture while a fixed microwave energy beam is directed axially or radially against the wheel for subsequent reflection to the far field. As the mirror wheel rotates, the microwave energy reflected from the mirror wheel reflects at varying angles to effect unidirectional sector scanning of a high gain pencil or oval beam in the far field.

Journal ArticleDOI
TL;DR: In this paper, a modification to the halfplane diffraction coefficients is given for a plane wave incident on a curved screen, which is used to calculate the near field of a parabolic reflector under plane-wave illumination.
Abstract: A modification to the halfplane diffraction coefficients is given for a plane wave incident on a curved screen. The result is used to calculate the near field of a parabolic reflector under plane-wave illumination. Comparison with the physical-optics method shows excellent agreement.