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1.
Abstract

We show that the evanescent part of the plane wave representation of the free-space dyadic Green function contributes to the far field only along a distinguished axis. The claim of contribution of evanescence in all directions is incorrect as it arises from a flawed procedure, but a limited version of that claim may have merit.  相似文献   

2.
Abstract

In a series of papers, Xiao has described a decomposition of the electric field of an electric dipole into homogeneous and evanescent components. Although the decomposition may be formally correct, the components do not correspond with the usual homogeneous and evanescent components, and the conclusions made about resolution of near-field and conventional microscopy are misleading.  相似文献   

3.
The magnetic field of radiation emitted by an electric dipole contains travelling and evanescent waves when represented as an angular spectrum. The evanescent waves decay exponentially away from the xy-plane, and will therefore not contribute to the detectable radiation in the far field, in general. It is well known, however, that in a small region around the z-axis the evanescent waves of the electric field do end up in the far zone. We have studied the corresponding magnetic evanescent waves, and we have found that the evanescent waves of the magnetic field do not contribute to the far zone in the neighbourhood of the z-axis. When considering the neighbourhood of the xy-plane, it appears that both the electric and magnetic evanescent waves end up in the far field, and the travelling and evanescent waves contribute equally to the radiation in the far zone. Close to the dipole the radiation field diverges, and we have shown that this is entirely due to the evanescent waves.  相似文献   

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This paper numerically investigates the effect of an externally evanescent magnetic field on total entropy generation in a fluid enclosed in a square cavity by using a control volume finite element method to solve the conservation equations at Prandtl number of 0·71. The values of relaxation time of the magnetic field are chosen, so that the Lorentz force acts only in the transient state of entropy generation in natural convection. The total entropy generation was calculated for, fixed value of irreversibility distribution ratio, different relaxation time varying from 0 to 1/5 and Grashof number varying from 104 to 105. The effects of the Hartman number and the magnetic field inclination angle on the evolution of total entropy generation throughout the transient regime were investigated. Results show that the application of evanescent magnetic field not only suppresses the fluctuation of the total entropy generation in the transient state, but also reduces the gap for magnetic field relaxation time less than 1/10.  相似文献   

6.
Terahertz pulse propagation in the near field and the far field   总被引:1,自引:0,他引:1  
We present a detailed investigation of the propagation properties of beams of ultrashort terahertz (THz) pulses emitted from large-aperture (LA) antennas. The large area of the emitter is demonstrated to have substantial influence on the temporal pulse profile in both the near field and the far field. We perform a numerical analysis based on scalar and vectorial broadband diffraction theory and are able to distinguish between near-field and far-field contributions to the total THz signal. We find that the THz beam from a LA antenna propagates like a Gaussian beam and that the temporal profile of the THz pulse, measured in the near field, contains information about the temporal and spatial field distribution on the emitter surface, which is intrinsically connected to the carrier dynamics of the antenna substrate. As a result of pulse reshaping, focusing of the THz beam leads to a reduced relative pulse momentum, with implications in THz field-ionization experiments.  相似文献   

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Xu YL 《Applied optics》1997,36(36):9496-9508
In electromagnetic multisphere-scattering calculations the reexpansion method for seeking a single-field representation of the total scattered field is found impracticable because of severe numerical problems. We present a simple single-field expansion of the total scattered far field based on an asymptotic form of vector translational addition theorems. With this asymptotic expansion of the far field, we derive analytical expressions for the scattering properties of an arbitrary aggregate of spheres. Resulting formulas are free from numerical problems in practical applications. Theoretical predictions from this far-field solution for various aggregates of spheres that we tested agree favorably with laboratory microwave scattering measurements. Some numerical results are presented and compared with experimental data.  相似文献   

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Hartley  James  Pennebaker  James W.  Fox  Claire 《Scientometrics》2003,57(3):389-398
Two computer-based style programs were used to analyse the Abstracts, Introductions and Discussions of 80 educational psychology journal articles. Measures were made of the overall readability of the texts as well as of sentence lengths, difficult and unique words, articles, prepositions and pronouns. The results showed that the Abstracts scored worst on most of these measures of readability, the Introductions came next, and the Discussions did best of all. However, although the mean scores between the sections differed, the authors wrote in stylistically consistent ways across the sections. Thus readability was variable across the sections but consistent within the authors. This revised version was published online in August 2006 with corrections to the Cover Date.  相似文献   

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A tunable asymmetrically-embedded-aperture interferometer configuration is proposed to enhance the measurement sensitivity of subwavelength variation. With this configuration, an aperture of reference was posited asymmetrically, relative to the test aperture, that exhibited subwavelength variation. By a shift in the relative position of the reference aperture, the detection sensitivity of measuring the subwavelength variation in the far field can be enhanced to a desired value at any specific detection width. We discuss the underlying mechanism of optimization and address the difference between the embedded-aperture interferometry and tunable asymmetrically-embedded-aperture interferometry with respect to tolerance.  相似文献   

15.
A semiempirical method for calculating the far acoustic field of the lift rotor of a helicopter operating in the regime of oblique flow over it is described. The basic parametric relations for the acoustic radiation power of rotor noise components have been obtained on the basis of the Lamb idea that vortex-free motion arises under the action of a periodic force on an infinitely small volume of the medium. All sources of lift rotor noise are subdivided into two groups pertaining, respectively, to the inductive and profile parts of the total power supplied to the rotor. A comparison has been made between the results of calculation of the harmonic components of lift rotor noise made on the basis of the power method and the experimental data for the Mi-28 helicopter.  相似文献   

16.
Karpenko Physicomechanics Institute, Ukrainian Academy of Sciences, L'viv. Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 29, No. 2, pp. 12–19, March–April, 1993.  相似文献   

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Summary Higher order approximations to the laminar boundary-layer flow of an incompressible fluid in the wake of a symmetrical disturbance are given in the present paper. The two-dimensional case, documented elsewhere in great detail, [1], [2], [3] and [4] is reconsidered. The Euler transformation is introduced and higher-order expansion terms are derived. The asymptotic expansions given in the paper are, of course, valid only to the extent that the boundary layer approximations apply, i.e. (for the rotationally symmetrical case) within a space of revolution with the centre of the wake as axis of symmetry. The terms neglected in the complete equations of motion become of order unity for very smallx, where the expansion is not applicable in any case (as becomes large), and at very large (respectively),x being given.The axisymmetrical case is expanded in a like manner, but in both cases the inner and outer coördinate expansion problem of matching with the near wake, considered by Meksyn [16] and Berger [11] is not treated: this, mainly, because its detailed form would depend on the particular upstream conditions obtaining, a subject which is outside the scope of the present work.  相似文献   

20.
A limiting approach inquiry into far field effects on local field equations for mixed mode surface flaws is investigated by computer analysis. Curves showing the diminishing effect of the far field stress towards the cracks tip are plotted using fringe radii ratios vs fringe number. The zone used in the experimental determination of K1 and K2 is superimposed on these curves to investigate the extent to which K1 and K2 determined by the two parameter method are influenced by the far field stress.  相似文献   

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