Showing papers on "Near and far field published in 1984"

Approximate formulas for the far field and gain of open-ended rectangular waveguide

Abstract: Approximate formulas are derived for the far field and gain of standard, open-ended, rectangular waveguide probes operating within their recommended usable bandwidth. (Such probes are commonly used in making near-field antenna measurements.) The derivation assumes first-order azimuthal dependence for the fields, and an E -plan pattern given by the traditional Stratton-Chu integration of the transverse electric ( TE_{10} ) mode. The H -plane pattern is estimated by two different methods. The first method uses a purely E -field integration across the end of the waveguide. The second, more accurate method approximates the fringe currents at the shorter edges of the guide by isotropically radiating line sources. The amplitude of the line sources is determined by equating the total power radiated into free space to the net input power to the waveguide. Comparisons with measurements indicate that for X -band and larger waveguide probes, both methods predict on-axis gain to about 0.2 dB accuracy. The second method predicts far-field power patterns to about 2 dB accuracy in the region 90\deg off boresight and with rapidly increasing accuracy toward boresight.

An Electromagnetic Near-Field Sensor for Simultaneous Electric and Magnetic-Field Measurements

Abstract: This paper describes the theory of a single sensor to perform simultaneous electric and magnetic near-field measurements. The theory indicates that it is possible to measure the magnetic-loop and electricdipole currents using a loop antenna terminated with identical loads at two diametrically opposite points. The theory also indicates that it is possible to choose an ideal load impedance for achieving equal electric and magnetic-field responses of the loop. Preliminary experiments have been performed using a plane-wave field to verify these results.

An integral equation for electromagnetic scattering from homogeneous dielectric bodies

Abstract: A single surface integral equation for problems involving electromagnetic scattering from homogeneous dielectric bodies illuminated by time-harmonic sources is developed via the equivalence principle. The equation is formulated in terms of an equivalent electric current defined at the body surface. When allowed to radiate in a homogeneous medium having the material parameters of the exterior medium of the original problem, the electric current solution to the integral equation produces the correct scattered electric and magnetic fields external to the body.

Nonlinear propagation of electromagnetic waves in magnetized plasmas

Abstract: The nonlinear propagation of an electromagnetic wave along an external magnetic field in a uniform plasma is reconsidered in order to include the combined effects of the relativistic-mass and ponderomotive-force nonlinearities. The results of previous works on this subject are improved significantly.

Surface diagnosis of large reflector antennas using microwave holographic metrology: An iterative approach

Abstract: The radiation pattern of an antenna can be significantly affected by reflector surface errors A minimization of losses due to surface inaccuracies is, therefore, required A suitable procedure for the determination of the deviations of the surface from its ideal geometry is based on the microwave holographic technique In this procedure, the complex pattern of the antenna is measured, and the Fourier transform relationship, existing between the radiation pattern and the function related to the current distribution on the reflector surface, is applied It is naturally desirable to limit the amount of measured data The present investigation is concerned with the concept of the Fourier transform relationship between the induced current and the far field It is demonstrated that the relationship, in general, can be interpreted as a summation of many Fourier transforms An iterative scheme is employed to continue the far-field pattern outside the measurement window

The signature of an air gun array; computation from near-field measurements including interactions; practical considerations

Abstract: Iterative technique to calculate a notional array of noninteracting sources from the near-field hydrophone measurements. The notional signatures form the basis for calculating the array signature in any direction. Prudent positioning of the hydrophones, one close to each air gun. In normal operation the forward motion of the hydrophones and upward motion of the air gun bubbles are important effects which must be included in the equations. Array signatures obtained in very bad weather conditions (force 8) are presented. In this extreme test the signatures are very stable from shot to shot.--Modified journal abstract.

Propagation of γ-radiation in strong magnetic fields of pulsars

Abstract: The propagation, decay and absorption of electromagnetic waves in magnetospheres of pulsars are considered with emphasis on the resonant effects in the vacuum polarization.

Microstrip disk antennas, Part II: The problem of surface wave radiation by dielectric truncation

Abstract: The far zone fields radiated from a microstrip disk antenna with a truncated dielectric layer were obtained using the cavity model with magnetic side walls, the dyadic Green's functions are stratified media, and the integral representation analysis of the fields in the complex plane. It is shown that the truncation of the dielectric is responsible for the ondulations observed in the radiation patterns of these antennas and that they result from the interference of the space wave fields with those radiated as a consequence of the incidence of the surface wave into the truncation region. It is suggested that this interference problem may be alleviated by truncating the dielectric as far as possible from the antenna elements and by using lower values of dielectric thickness to disk radius ratios d/a , and of dielectric relative permittivities \epsilon_{r} .

Optical beam characteristics of Schottky barrier confined arrays of phase-coupled multiquantum well GaAs lasers

Abstract: The characteristics of phase-coupled laser arrays fabricated using Schottky barriers on the p-contact to confine the current flow to 5 or 10 stripes on 9 μm centers are described. The gain guided laser spots are very equally spaced, and in some cases do not coincide exactly with the position or number of conducting stripes. The self adjusting nature of the active region, together with the phase shift of π between the neighboring N laser spots results in extremely smooth far-field patterns consisting of 2 narrow, intense outer lobes, and N-2 much weaker interior lobes. The mode structure closely approximates the higher order modes observed in wide stripe laser. The separation and width of the outer lobes is defined by diffraction theory, and determined by the separation and number of lasing spots. Deviations from diffraction theory are found in some cases. The contributions to the far field were studied by examining the magnified image of the exit facet as a narrow slit was passed across the far field. The phase contributions present in the far field were studied by introducing a relative phase shift between the two symmetric halves of the far field. The relative phase change modifies the interference pattern of the magnified near-field image, causing a spatial shift of the imaged spots in the plane of the junction.

A Local Field Study of a Water-Immersed Microwave Antenna Array for Medical Imagery and Therapy

Abstract: A water-immersed microwave array system for medical imaging is described, and a theoretical analysis of its Iocal field pattern is presented. The theoretical study also applies to similar systems for medical therapy purposes. It is shown that, using a technique of phase and amplitude conjugations, a satisfactory three-dimensional focusing for targets located in the neighborhood of the array may be achieved. The focusing resolutions for transverse and Iongitudinal directions are approximately lambda/2 and lambda , respectively, where lambda is the wavelength in the dielectric. By increasing the element spacing of the array, the resolutions can be as good as 5.3 and 11.7 mm, respectively, at the operating frequency of 3 GHz.

Boresight chamber assembly apparatus

Abstract: A boresight chamber apparatus employs a near field measurement technique for measuring the amplitude and phase distributions of an antenna under test and then deriving the boresight error from such measurements. The antenna under test is mounted within an anechoic test chamber on a wall of the chamber. Located adjacent the antenna is a near field probe which is moved along a line parallel to the plane of the antenna. The probe, as moved, scans parallel to the surface of the antenna. At prescribed equally spaced points along the line, the amplitude and phase difference between reference signal and the amplitude and phase developed by the antenna is measured. These measured data allow one to calculate the boresight error of the antenna. The probe position is controlled by a computer which also samples the output of the phase comparator at each of the points that the probe is positioned in to thereby derive an accurate representation of the amplitude and phase distributions of the antenna. The boresight error is subsequently calculated by the computer from the measured amplitude and phase data or using only the phase data.

New Formulas That Extend Norton's Farfield Elementary Dipole Equations to the Quasi-Nearfield Range

Abstract: : New formulas for the electric and magnetic fields produced by the four elementary dipole antennas have been derived for the air-to-air, subsurface-to-air, air-to-subsurface, and surface-to-surface propagation cases These formulas are of rather simple form and reduce to previously derived results when either (1) the measurement distance is much less than a free-space wavelength, (2) the Sommerfeld numerical distance is small, or (3) the measurement distance is much greater than a free-space wavelength These are valid at any frequency and at any range beyond a certain minimum distance for the flat-earth case

Beam collimation and focusing of multi-emitter or broad emitter lasers

Abstract: An optical system for collimating the radiation from a semiconductor laser, the laser near field having a single beam waist position in a first direction and having an elongated but phase coherent near field emitting pattern in a second direction which forms a far field low divergence beam pattern, the optical system characterized by a lens system for imaging said near field of said single beam waist position in the first spatial direction while imaging the elongated, phase coherent far field radiation pattern in the second direction. The laser is a multi-emitter or broad emitter semiconductor laser capable of producing one or more output beams. The lens system may include focusing optics to focus radiation in the near field pattern emitted from the laser in the first direction and in the far field pattern of radiation emitted from the laser in the second direction.

Penetrating electromagnetic wave applicators

Abstract: It is demonstrated that in the near field of suitably designed applicators, the penetration depth of the electromagnetic wave can greatly exceed the usual skin depth. The application of this principle in therapy is suggested.

Complex ray analysis of radiation from large apertures with tapered illumination

Abstract: Near and far zone calculation of the radiation characteristics of large apertures with tapered illumination generally requires time-consuming numerical integraton over the aperture plane because geometrical theory of diffraction (GTD) asymptotic techniques are difficult to apply to focused beam systems. Asymptotic methods do, however, become feasible for complex ray systems since ray fields originating in a complex coordinate space yield beam type fields in physical space. In particular, rays emanating from a complex source point are known to generate a real beam type field without sidelobes, very similar to that produced by a Gaussian amplitude taper. These considerations are generalized here to one-dimensional taper profiles of the form exp [ -kI ], where k is the wavenumber and I has a polynomial dependence on the aperture coordinate. Profile shapes ranging from Gaussian to rectangular are accommodated thereby. The analysis is performed by complex ray tracing from the analytically extended complex aperture plane to the observer. Criteria are developed for inclusion of all relevant rays, and only these, and for their domains of validity. These criteria are based on saddle point analysis of the exact field integral, whose direct numerical evaluation furnishes the reference solution. Saddle points and steepest descent paths corresponding to various envelope polynomials are traced in detail for near and far zone fields. Numerical comparisons between saddle point asymptotics (completely equivalent to complex ray tracing) and the reference solution establish that simple complex ray theory is useful and accurate for Gaussian-like profiles with moderate envelope gradients but that the strong gradients in almost square profiles require an excessively large number of contributing complex rays. For the latter profiles, where edge-diffraction-like phenomena predominate, modification of conventional GTD would provide a better alternative. Thus, this study establishes the complementary utility of complex ray tracing for smoothly tapered, and of GTD for almost rectangular, aperture distributions.

Scattering by right angle dielectric wedge

Abstract: An asymptotic solution of electromagnetic waves scattered by a right-angled dielectric wedge for plane wave incidence is obtained. Scattered far fields are constructed by waves reflected and refracted from dielectric interfaces (geometric-optical fields) and a cylindrical wave diffracted from the edge. The asymptotic edge diffracted field is obtained by adding a correction to the edge diffraction of physical optics approximation, where the correction field in the far-field zone is calculated by solving a dual series equation amenable to simple numerical calculation. The validity of this result is assured by two limits of relative dielectric constant \varepsilon of the wedge. The total asymptotic field calculated agrees with Rawlins' Neumann series solution for small \varepsilon , and the edge diffraction pattern is shown to approach that of a perfectly conducting wedge for large \varepsilon . Calculated far-field patterns are presented and the accuracy of physical optics approximation is discussed.

Plane-wave diffraction by a wedge--A spectral domain approach

Abstract: The canonical problem of plane wave diffraction by a wedge in the context of the spectral domain approach which exploits the relationship between the induced current on a scatterer and its far field is investigated. It is shown how the exact solution to the wedge diffraction problem can be manipulated in a form which enables one to interpret the far scattered field as the Fourier transform of the physical optics (PO) current on the two faces of the wedge augmented by the fringe current near the tip of the wedge. A uniform asymptotic expansion for the total field which slightly modifies the Ansatz in the uniform asymptotic theory of electromagnetic edge diffraction is constructed.

Optical study of the EH11 mode in a hollow circular oversized waveguide and Gaussian approximation of the far-field pattern.

Abstract: This paper gives the main characteristics and an optical description of the fundamental EH11 mode propagating in hollow circular oversized waveguides. The quasi-Gaussian beam radiated by this mode in the far field is derived from a recently developed optical model to calculate the Gaussian beam transmission through hollow circular oversized waveguides. A simple relation is found between optical characteristics of the EH11 mode in the waveguide and the quasi-Gaussian beam in the far field. The results agree well with previous theoretical and experimental works.

Microscopic generalization of the Kliewer-Fuchs theory of surface electromagnetic fields

Abstract: Maxwell's equations for a metallic half space are formulated in a mixed Fourier representation, in which microscopic surface models lead to a coupling of field modes of different polarization and wave number This formulation is very useful for numerical calculations of surface fields, since it allows one to decouple the slowly varying transverse from the surface bound longitudinal electric field To demonstrate this, the formalism is applied to the infinite-barrier jellium model with a random-phase-approximation dielectric response, the simplest microscopic model of a metallic half space Some known results are reproduced, and interesting new results on the power-absorption mechanism at a jellium surface are obtained and discussed in relation to other model calculations

Dipole ring array antenna for circularly polarized pattern

Abstract: A NAVSTAR satellite has a navigation antenna array beamed toward the earth. A communications antenna array for communicating with other satellites requires a pattern null near the axis and high gain to the sides with minimum losses. This is achieved with a dipole ring array comprising eight elements surrounding the navigation array. The ring has a diameter of 1.1 wavelength, and is fed with equal amplitudes and a third mode phase progression, which produces good circular polarization in the far field. For a different sized dipole ring, there will still be an optimum phase distribution which will give good circularly polarized patterns.

A Hybrid Method of Moments-GTD Technique for Computing Electromagnetic Coupling Between Two Monopole Antennas on a Large Cylindrical Surface

Abstract: The problem of analyzing the electromagnetic interference (EMI) between two electronic systems when their antennas are located on or near large scattering objects is a difficult one. In this paper, a hybrid technique for combining the method of moments with the geometrical theory of diffraction (GTD) is extended to account for the mutual coupling between two monopole antennas on a large circular cylinder by means of curved surface-wave diffraction. The results of this technique are compared to those of an analysis program used by the United States Air Force which predicts electromagnetic coupling between aircraft antennas. The close agreement between the results of these two different analysis techniques serves to validate both computer programs.

Efficient low cost transducer system

Abstract: A low cost, efficient, ultrasonic transducer and method comprises a narrow beam electrostatic transducer (15') and a beam transformer (60) operated in air for expanding the emitted beam of the electrostatic transducer relative to a single axis. The beam transformer is a sonic reflecting surface (61, 62) spaced from the ultrasonic beam generator and within the near field thereof and having a geometric surface in which all points of said surface are generated by the revolution of a line about a fixed axis which is normal to the plane of the electrostatic transducer. A partial surface of a cone, the axis preferably being offset from the axis of the narrow beam electrostatic transducer and placed in the near field thereof transform the narrow beam to a broad beam with the beam transformer being an effective coupling element for the emitted beam and the return echo for ranging systems which are both efficient and easy to produce and do not have significant alignment problems. Acoustic energy absorbing material (30, 31) can be used to bound the perimeter of the reflecting surface to better define and control the beam in cross section. The invention is particularly useful for ranging, guidance, and surveillance systems wherein an ultrasonic beam, narrow in one direction of propagation, is projected in air over a surface to detect range to an object, presence or absence of an object and its location and entrance of an object to a specified area under surveillance.

In-line Fraunhofer holography at a few far fields.

Abstract: The process of in-line Fraunhofer holography of particles is studied by an analysis of the four terms in the reconstructed field. Specific results are discussed for objects with circular cross section. The image shape and contrast are described in detail. The study is particularly useful when the recording is performed at a few far fields. It has been found that the recording at a higher far field is better due to a higher image-to-background irradiance ratio at the edges of the reconstructed image.

Study of the radiation from an undulator: Near field formulation

Abstract: The spectrum of undulator radiation has been formulated analytically for the near field case and compared with that of the far field case. For an undulator built for the free electron laser experiment in the vacuum ultraviolet storage ring at the National Synchrotron Light Source, it has been calculated that prominent near field effects of the fundamental component could be detected by setting a narrow slit at a specific position on the observation plane off the undulator axis, and the near field effects might disappear within an error of 1% on the undulator axis.

A Nonlinear Analysis of the Effects of Transient Electromagnetic Fields on Excitable Membranes

Abstract: The transmembrane voltage produced by a transient electromagnetic field has been determined using a nonlinear model of the cellular membrane The influence on the membrane voltage of the various parameters characterizing the incident field, such as wave-shape, time-width, and amplitude, has been analyzed In particular, the amplitude of the incident field for which the cell's behavior can be assumed as linear and the threshold level for exciting action potentials on the membrane have been determined Potential hazards for humans exposed to transient fields are examined in light of this interaction mechanism