Near-field analysis by the plane-wave spectrum approach

Historically, most near-field analyses have relied upon aperture-integration techniques. The purpose of this paper is to point out the advantage of the plane-wave spectrum (PWS) approach for analysis of antenna near fields. The advantage of the PWS approach over aperture-integration techniques is quite substantial throughout the entire near-field region for circular apertures. Furthermore, the advantage increases in proportion to closeness to the aperture. The PWS approach also appears to offer advantage for noncircular apertures, e.g., rectangular, at least for large aperture sizes.     

The transmission coefficient of elliptical and rectangular apertures for electromagnetic waves

A simple scalar method using Kirchhoff's boundary values is applied to the diffraction problems of circular, elliptical, and rectangular apertures for normally incident electromagnetic waves. As far as circular apertures are concerned, a simple formula can be derived not only for the diffraction pattern but also for the transmission coefficient. This formula yields good results for apertures greater than0.8lambda. Even in the ease of elliptical apertures a simple formula can he derived for the diffraction pattern. For the elliptical aperture, as well as the rectangular one, the transmission coefficient was found in the form of an integral. Relief models and diagrams are given for the transmission coefficients of the elliptical and the rectangular apertures as a function of the two aperture parameters. Diagrams are given which explain the dependence of the transmission coefficient on the aperture parameters. A comparison with other more complicated methods of approximation and with measurements shows both good agreement and the great practical value of the simple method of approximation used.     

Comments on "New plane wave spectrum formulations for the near-fields of circular and strip apertures"

The above paper (see ibid., vol.24, p.438-449, July (1976)) applies a plane wave spectrum (PWS) formulation to diffraction problems involving circular and strip apertures and gives new results in terms of Fresnel integrals for the electric field near the aperture. In this note, a discussion of those new results is presented; conclusions are: As a technique for solving electromagnetic aperture diffraction problems, the particular PWS described gives inadequate results, especially for near-fields, and by using the standard Keller's formula, a geometrical theory of diffraction (GTD) solution for the diffracted field from a circular aperture is obtained, but the solution does not in general agree with the one given     

Time Domain Analysis of the Near-Field Radiation of Shaped Electrically Large Apertures

The near-field radiation of several electrically large apertures has been computed based on the aperture field convolution method, including a square aperture, a circular aperture, and a serrated edge aperture. The frequency range is 8.0–12.0 GHz. The aperture diameters are limited to 5.00 m and the corresponding electrical size is 133–200 wavelengths. The time domain spectrum has been obtained by chirp z-transform with a Hamming window and the mechanism for the generation of the aperture near-field radiation in time domain is analyzed. The calculation results demonstrate that the near-field radiation of the aperture can be approximately seen as the synthesis of a plane wave from the aperture, cylindrical waves from the edges, and spherical waves from the corners. By this method, the direction, position, and magnitude of incoming waves in the near-field region can be estimated, and the aperture design can be modified to meet the requirements of the near-field radiation. The near field of the three apertures mentioned above has been compared, and it is shown that the serrated edge aperture has more uniform direct wave and lower diffraction waves, making it a candidate for compact range (CR) aperture design.      

Closed-form low frequency solutions for electromagnetic waves through a frequency selective surface

A simple closed-form approximate solution is given to the problem of transmission of a low-frequency electromagnetic wave through a frequency selective surface (FSS). FSS are periodic metal plates (or their complimentary apertures) sandwiched between dielectric slabs. At low frequencies, the induced currents on the metal plates may be approximated by a known function with a constant coefficient to be determined by the boundary conditions. Based on such a "one-mode" approximation, we derive a closed-form solution for the scattered field for FSS with multiple narrow rectangular slots, with a single wide rectangular aperture, and a circular aperture. When compared with the available exact solutions, we find that the one-mode approximation is valid when the periodaof the FSS is such thata/lambdais small enough that no grating lobe appears, e.g.,a < lambdafor normal incidence.     

在A-SNOM中L-V形纳米小孔的优化设计

采用时域有限差分法对L形和V形纳米小孔进行优化设计。结果表明,两种小孔的出射峰值光强均比具有相同开孔面积的方孔提高了两个数量级。发现在具有相近峰值光强的情况下,且在距离出射面小于60nm的范围内,V形孔光斑尺寸在x方向和y方向的宽度可以比L形孔的光斑小10～15nm。本文对优化设计L形和V形小孔在纳米显微、光存储、光刻、光学操作等近场光学应用领域方面,具有一定的指导意义。     

Aperture-sampling requirements [and reply]

Comments on the paper “Aperture-sampling requirements in planar near-field and pattern calculations” by T. Milligan (IEEE Antennas. Propag. Mag., vol.38, no.5, p.60-3, 1996). The comment discusses the case when aliasing errors are significant. The reply shows that the errors are predictable and depend on the edge condition of the aperture. It is possible to extend the sampling of apertures to the errors of numerical integration of an aperture, using either the trapezoidal or Simpson's rule. The errors have a predictable distribution, which depends on the aperture edge condition, because numerical integration is a weighted sampling. A further comment is finally given     

The efficiency of near-field Cassegrainian antennas

The aperture efficiency of a near-field Cassegrainian antenna is approximated with a Fresnel field analysis. Curves are presented which give the efficiency of the system in terms of the amplitude distribution on the feed aperture and the Raleigh distance to the subreflector. It is found that separation between feed aperture and subreflector can be as great asfrac{1}{2}d^{2}/lambdawithout excessive loss.     

Electromagnetic coupling through small apertures in a conducting screen

The electromagnetic field coupling through small apertures illuminated by an arbitrary incident plane wave is discussed for general aperture shapes. A set of new integral equations in a form highly amenable to numerical solution techniques is derived. Based on the application of the Rayleigh series method, an analytical solution is obtained for the first few terms of the expansion of the apertureE- field of a circular aperture. Numerical results are also constructed for the aperture field and the diffracted field of small rectangular apertures and compared with those of the circular apertures.     

On the aperture and pattern space factors for rectangular and circular apertures

A consequence of a recently discovered edge condition for planar apertures is that all planar aperture distributions are separable physically into a product of an edge factor and an aperture space factor, analogous to the way in which the radiation pattern separates into a product of an element factor and a pattern space factor. An exact relationship between these aperture and pattern space factors for physically realizable vector fields is derived here for rectangular and for circular apertures. For rectangular apertures it leads to a two-dimensional set of doubly orthogonal functions that are characteristic of the aperture geometry. Characteristic functions for circular apertures, however, are shown to exist only if the vector fields are circularly symmetric, although for scalar fields they exist for completely arbitrary aperture distributions with arbitrary edge taper. For rectangular apertures the characteristic functions consist of products of spheroidal functions and for circular apertures they are obtained from a generalization of the spheroidal functions. Some of the properties of these generalized spheroidal functions are developed here.     

Input Admittance of a Circular Waveguide Aperture Covered by a Dielectric Slab

The input admittance of a circular waveguide opening onto an infinite flange with a homogeneous lossy dielectric coating is derived using integral transforms and assuming the dominant TE11mode as the trial field at the aperture. Admittance calculations for low-loss dielectric sheets of various thicknesses are given and compared with measurements obtained using ground planes of varying sizes. Excellent agreement with theory is obtained for the circular waveguide terminated into a square ground plane only1lambdato2lambda. on a side. A quantitative comparison of the surface wave contribution to the total admittance is also given.     

The "very-near-field" region of equiphase radiating apertures

The purpose of this article is to investigate the so-called "near-field reactive" or "very-near-field" region of radiating apertures with different shapes and aperture illuminations. In this unusual and rather unknown region, we underline the specific behavior of the radiated fields, the wave impedance, and the power density. Thanks to these properties, we succeed in. determining, in the first part, the outer boundary of the "very-near-field" region of a circular aperture with a uniform illumination law. Then, we review the influence of the illumination law and of the aperture shape on the extent of this region, through the cases of parabolic or cosine laws, and the case of square-shaped aperture. This study can find various applications in the wireless communications and radar antenna domains and, more recently, in electromagnetic compatibility problems.     

Coaxial feeds for high aperture efficiency and low spillover of paraboloidal reflector antennas

Coaxial feeds produce an approximate sector-shaped pattern, an almost optimum pattern of a feed for high aperture efficiency and low spillover of paraboloid antennas. Such a coaxial feed consists of a central circular waveguide which is surrounded by one or more conductors with circular cross sections. Theoretical and experimental investigations on coaxial feeds excited by H11modes have shown that the first ring yields the highest increase in the aperture efficiency of paraboloid antennas illuminated by them. Measurements performed on paraboloid antennas illuminated by a coaxial feed with only one ring yielded aperture efficiencies of 68 to 75 percent for angular apertures of the paraboloidal reflector of100degto160deg. Circularly symmetric patterns in conjunction with almost linearly polarized aperture fields can be achieved by multimode coaxial feeds. The values for the aperture efficiency, which are calculated for paraboloid antennas illuminated by multimode coaxial feeds, nearly reach the theoretical optimum. The measured values are 68 to 80 percent. In addition, the multimode feeds produce very little cross polarization.     

Diffraction loss of stable optical resonators with internal limiting apertures

In many stable laser resonators, the diffraction loss is principally or entirely due to internal occluding elements with two or more limiting apertures, rather than to any mirror size limitations. Such resonators, with any number of arbitrarily located circular or rectangular limiting apertures, can be decomposed into a set of equivalent empty symmetric cavities with finite size mirrors. When the end mirrors are convex, with radii of curvature greater than the distance to the nearest aperture, the equivalent symmetric cavities are stable, and their diffraction losses are given by graphs published by Fox and Li [6]-[8] and reproduced here for the reader's convenience. The method is illustrated by examples with circular and rectangular limiting apertures.     

Decreasing antenna beamwidth with increasing scan angle

All antennas, including phased arrays, exhibit beam broadening in the near field and also beam broadening in the far field at scan angles off broadside. For scan angles in the near field, the effective aperture is reduced, thereby producing less beam broadening due to the near field. At any scan angle, there is a near-field distance at which the broadside far-field beamwidth plus broadening equals the scan angle far-field beamwidth plus broadening. For larger scan angles, the beamwidth is less than at broadside. Calculations of this phenomenon are made for uniform linear and for tapered circular apertures.     

A bound on EMP coupling

A bound on the energy coupled to a load by means of an antenna or aperture subjected to a step function electromagnetic pulse (EMP) is given by the polarizabilities of the antenna or aperture. Examples of coupling bounds for a slender electric dipole antenna, a circular aperture, and a narrow slot aperture with depth are given. It is demonstrated, by use of the bound on the integral of the effective area over all frequencies, that the maximum energy absorbed by an object subjected to a step function EMP is bounded in terms of the static polarizabilities of the object. (The energy spectrum of the step function EMP bounds the spectrum of EMP described by the double exponential over all frequencies.) It is demonstrated that this bound can be used on apertures in infinite ground planes for which Babinet's principle fails to apply (apertures having depth and cavity backed apertures), where the requirement is simply to use the polarizabilities on the incident side of the aperture.<>     

Coarray synthesis with circular and elliptical boundary arrays

An elliptical boundary aperture is a collection of points lying on an ellipse from which energy is transmitted and/or received. An important special case is the circular boundary aperture. When these apertures are used with beamforming to produce a narrowband image of a far-field source, the corresponding point spread function (PSF) is characterized by high sidelobes. The concept of the coarray of an imaging system is used here to develop techniques which synthesize the effect of a more desirable PSF with an elliptical boundary aperture. Techniques are given for use in active imaging of spatially coherent sources, as well as passive imaging of spatially incoherent sources. Discrete arrays and continuous apertures are considered separately. The approach shows that the PSF synthesis problem can be solved in many more ways than previously recognized, and this fact is exploited to develop procedures which have a least-squares optimality property.     

金属舱近场孔缝耦合特性数值分析

为分析和解决车载天线对舱内电子设备电磁干扰的问题,建立了全尺寸金属舱模型,运用基于时域有限积分法的商业软件CST,重点分析了孔口形状、开孔与辐射源的间距及相对位置对孔缝近场耦合特性的影响,并对比了典型孔缝结构所造成的近场电磁泄漏,系统建立了孔口形状、开孔与辐射源的间距、相对位置以及孔缝结构类型等4个因素与孔缝近场耦合强度之间的数值关系。所得结论有利于指导车载天线及舱内设备的布局。     

基于菲涅尔衍射的圆孔直径测量

本文提出一种新的圆孔直径测量的方法。平行光照射下,圆孔直径和菲涅尔衍射光强分布中峰值轮廓直径存在一定的关系,根据该关系可以实现圆孔直径的测量。仿真和实验数据证明,该方法对于直径5~10 mm的圆孔可以达到亚微米级的测量精度。     

A comparison among 1-, 3-, and 7-horn feeds for a 37-beam MBA

A very common multiple beam antenna (MBA) configuration consists of a collimating device illuminated by an array of feeds. The collimating device is usually a reflector or a lens. The feeds are usually horn antennas with a circular aperture. The reflector is usually offset-fed to eliminate aperture blockage; the lens is center fed. The antenna's feeds are excited to produce a finite number of beams, so as to provide contiguous coverage of the field of view. The designer is forced to minimize the angular spacing between adjacent beams in order to maximize the minimum gain over the antenna's field of view. On the other hand, the feed horn's aperture gain is maximized when the feed horn spacing and its aperture diameter are equal. This results in antenna efficiency of the order of 30% when a single feed horn is excited to produce a beam. When a cluster of 3 or 7 adjacent feed horns are excited to produce a single beam, antenna efficiency can be increased to 50%. When it is tolerable, several identical antenna apertures can be used to replace a single aperture configuration. In this case, each of M apertures produces approximately N/M beams of an MBA that produces N beams. Horns producing adjacent beams do not illuminate the same aperture. This permits the use of a much larger horn aperture for a given beam spacing. This results in reduced spillover, higher gain of each beam, and increased antenna efficiency of each aperture. This paper investigates the maximization of gain for several lens antennas. It shows that antenna gain is increased as its focal length is decreased. That is, a focal length-to-diameter ratio (F/D) less than 1 is preferred