Modal analysis is presented for the realized element gain E - and H -plane patterns for a two-dimensional circular cylindrical arrays with concentric dielectric sleeve. Numerical results indicate presence of significa...
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Modal analysis is presented for the realized element gain E - and H -plane patterns for a two-dimensional circular cylindrical arrays with concentric dielectric sleeve. Numerical results indicate presence of significant surface wave effects. A design method of dielectric windows to reduce the guided wave effects is proposed. The method, validated by direct evaluation of the element pattern for the optimized window geometries, shows significant reduction of surface wave effects.
The evolution of surface and leaky waves as a function of frequency is examined for perpendicular polarization [transverse-electric (TE N ) modes] in configurations consisting of a dielectric slab bounded by media hav...
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The evolution of surface and leaky waves as a function of frequency is examined for perpendicular polarization [transverse-electric (TE N ) modes] in configurations consisting of a dielectric slab bounded by media having different refractive indices. We find that, at any frequency, these waves occur in sets of four types for all higher-order ( N > 1) mode numbers. The characteristics and the field patterns of these wave types are discussed, and the special behavior of the first two ( N = 0, 1) modes is described in detail.
A recursive technique for multiple threshold selection on digital images is described. Pixels are first classified as edge pixels or nonedge pixels. Edge pixels are then classified, on the basis of their neighborhoods...
A recursive technique for multiple threshold selection on digital images is described. Pixels are first classified as edge pixels or nonedge pixels. Edge pixels are then classified, on the basis of their neighborhoods, as being relatively dark or relatively light. A histogram of the graytone intensities is obtained for those pixels which are edge pixels and relatively dark and another histogram is obtained for those pixels which are edge pixels and relatively light. A threshold is selected corresponding to the graytone intensity value corresponding to one of the highest peaks from the two histograms. To get multiple thresholds, the procedure may be recursively applied first using only those pixels whose intensities are smaller than the threshold and then only those pixels whose intensities are larger than the threshold.
The lateral shift of both the reflected and the transmitted beams are examined for incidence on layered configurations. In particular, beam displacements are evaluated at a layer placed between two different semi-infi...
The lateral shift of both the reflected and the transmitted beams are examined for incidence on layered configurations. In particular, beam displacements are evaluated at a layer placed between two different semi-infinite media. We find that the reflected beam may undergo a large displacement, which occurs in either a forward or a backward direction. The transmitted beam is also substantially displaced, but its shift occurs in a forward direction only.
In adjustable-speed drives the constraints imposed on the motor at its electrical and mechanical port are not fixed but subject to change. It is found that the counterelectromotive force developed by the motor is uniq...
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In adjustable-speed drives the constraints imposed on the motor at its electrical and mechanical port are not fixed but subject to change. It is found that the counterelectromotive force developed by the motor is uniquely defined by the specified load power, efficiency of the mechanical transmission, source voltage, and resistance of the energized circuit. In this respect ac motors fed by inverters behave like dc motors, because the dc source does not "see" the motor reactances. Relations are derived for the various machines and inverter types. With the EMF determined, the main design problem becomes the selection of its constitutive factors. The involved trade-offs are examined for the extreme cases of drives requring maximum torque at top speed and at stalling.
A recursive algorithm is derived which easily permits the determination of the terms in a partial fraction expansion associated with multiple poles. The algorithm is readily programmed on a digital computer.
A recursive algorithm is derived which easily permits the determination of the terms in a partial fraction expansion associated with multiple poles. The algorithm is readily programmed on a digital computer.
The Gaussian beam method has recently been introduced into synthetic seismology to overcome shortcomings of the ray method, especially in transition regions due to focusing or diffraction where ray theory fails. One p...
The Gaussian beam method has recently been introduced into synthetic seismology to overcome shortcomings of the ray method, especially in transition regions due to focusing or diffraction where ray theory fails. One proceeds by discretizing the initial data as a superposition of paraxial Gaussian beams, each of which is then traced through the seismic environment. Since Gaussian beam fields do not diverge in ray transition regions, they are ‘uniformly regular’ although the quality of this regularity depends on the beam parameters and on the ‘numerical distance’ which defines the extent of the transitional domain. However, when Gaussian beam patches are used to simulate non-Gaussian initial data, there arise ambiguities due to choice of patch size and location, beam width, etc., which are at the user's disposal. The effects of this arbitrariness have customarily been explored by trial and error numerical experiment but no quantitative recommendations have emerged as yet. As a step towarda prioripredictive capability, it is proposed here to perform a systematic study on analytically tractable prototype models of how the parameters and location of a single beam affect the quality of the observed seismic field, especially in ray transition regions. The conversion of ordinary ray fields into beam fields in canonical configurations can be accomplished conveniently by displacing a real source point into a complex coordinate space. Thus, the desired beam solutions can be obtained directly from available ray, and even paraxial ray, fields. Complex ray theory and its implications are reviewed here, with an emphasis on improvements of beam tracking schemes employed at present.
Progressing and oscillatory waves provide alternative descriptions of propagation in layered media. The former characterizes the wave motion in terms of direct and multiple wavefronts or rays, and the latter in terms ...
Progressing and oscillatory waves provide alternative descriptions of propagation in layered media. The former characterizes the wave motion in terms of direct and multiple wavefronts or rays, and the latter in terms of resonances or modes. Each description is convenient and physically incisive when it requires few elements but inconvenient and physically less transparent when it requires many elements. In the latter event, it is desirable to express many inconvenient elements collectively as fewer convenient ones. For a variety of propagation environments, this can be done by expressing rays collectively in terms of modes, or modes collectively in terms of rays. When performed selectively, there emerges a hybrid representation combining ray fields and modal fields in uniquely defined proportions. Rigorous for longitudinally invariant layers, the raya-mode equivalence can be extended approximately also to layered media with weak longitudinal heterogeneity. The theory, based on Poisson summation and on alternative treatments of wave spectra, is presented here and is illustrated for applications in electromagnetics, underwater acoustics andSHelastic motion.
Progressing and oscillatory waves provide alternative building blocks for constructing source-excited time-harmonic or transient fields in various propagation and scattering environments. Progressing waves describe th...
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Progressing and oscillatory waves provide alternative building blocks for constructing source-excited time-harmonic or transient fields in various propagation and scattering environments. Progressing waves describe the field in terms of direct and multiple reflected-diffracted wavefront or ray arrivals, while oscillatory waves describe the field in terms of body resonances and (or) guided modes. Each description is convenient and physically incisive when it requires few constituent elements but inconvenient and physically more obscure when it requires many elements. Collective summation of many inconvenient elements into fewer convenient ones, when possible, provides a means of switching from a poorly to a more rapidly convergent field representation. Rays (wavefronts) and modes (resonances) fall into the category of such bilaterally convertible wave fields. They not only have complementary convergence properties but furnish, respectively, local and global environmental discriminants. When combined in self-consistent hybrid form, where the number retained of the one uniquely determines the grouping required of the other, there emerges a rigorous theory of propagation and diffraction with high versatility. Groupings can be chosen so as to eliminate "difficult" elements (for example, caustic forming or transitional ray fields) and replace these by "well behaved" ones (modes) of the complementary species, or to model multiple interference in the one as simple interference in the other. The theory, based on Poisson summation or on alternative treatment of wave spectral representations, is presented and applied to numerous coordinate separable, but also weakly nonseparable, environments in electromagnetics, underwater acoustics and elastic motion, with numerical comparisons to highlight the salient features of the hybrid appraoch.
When viewing materials having Rayleigh velocity greater than the velocity in the coupling fluid, the output voltage of the ren flection acoustic microscope varies with defocus distance as a result of coupling to the R...
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