弱散射条件下半空间有耗媒质的近似重建公式

对于半空间弱有耗媒质,本文导出了用反射系数重建媒质相对介电常数和电导率剖面的近似反演公式,它们以解析闭式给出。     

弱散射条件下半空间有耗媒质的近似重建公式

对于半空间弱有耗媒质，本文导出了用反射系数重建媒质相对介电常数和电导率剖面的近摊似反演公式，它们以解析闭式给出。     

随机不均匀媒质中的几何光学近似方法

根据惠更斯－菲涅尔原理指出几何光学适应性判据是相邻两个相位面之间任意两条射线互不相交。在此基础上得到了电波斜入射平面分层随机不均匀散射媒质下，几何光学近似方法的适用条件，首次指出随机不均匀媒质中的几何光学近似方法适用范围显著地依赖于媒质介电常数的涨落。     

任意连续媒质层介电常数剖面的分域重建

本文应用网络原理导出了渐变媒质层中反射系数的微分方程，进而获得了重建媒质介电常数的近似解析闭式。通过给出任意媒质层的等效网络ｓ参数，可以得到任意连续媒质层介电常数剖面的一种分域重建方法。重建实例证明了本方法的有效性。     

电磁波在负二次分布介质中反射的解析理论

本文研究电磁波在一类特殊非均匀媒质中的反射问题，首先，根据矩阵分析理论导出了反射系数具有解析形式的一类特殊非均匀媒质剖面－－负二次剖面，进行获得了反射系数的具体表达式，它由简单的初等函数给出，最后文中验证了本文公式的正确性。     

任意连续媒质层介电常数剖面的分域重建

本文应用网络原理导出了渐变媒质层中反射系数的微分方程,进而获得了重建媒质介电常数的近似解析闭式.通过给出任意媒质层的等效网络S参数,可以得到任意连续媒质层介电常数剖面的一种分域重建方法.重建实例证明了本文方法的有效性.     

纯导电媒质中反射系数的非线性方程及其逆散射解

对于纯导电媒质，在高频及小入射角度情况下，本文应用微波网络方法导出了反射系数所遵循的非线性微分方程，并通过一非线性变换，给出了重建电导率剖面的新的逆散射解。     

频域Mur条件差分近似的构造和应用

本文构造了Ｍｕｒ条件的频域有限差分近似。结果推导的非均匀、各向异性媒质中有限差分方程，可以有效、方便地计算电大尺寸复杂质体的电磁散射问题。     

修正T矩阵公式及半空间随机球粒媒质的电磁波多散射

本文将“两外空间”概念与普通Ｔ矩阵公式相结合,提出了修正Ｔ矩阵公式,作为应用实例,本文用修正Ｔ矩阵理论研究了半空间随机球粒媒质的电磁波多散射,在Ｆｏｌｄｙ近似下得到了确定媒质有效波散的一个超越方程,低频时的数值结果表明：修正Ｔ矩阵方法当颗粒稀疏分时是可行的,当颗粒致密分布时,尚需采用引入颗粒对分布函数的准晶近似。     

《微波学报》1994年总目录

第1期(总第36期)非连续媒质介电常数剖面的近似重建公式 崔铁军梁昌洪介质加载圆柱腔的场分析 刘宏宋文淼调谐滤波器的腔间耦合结构研究 姚毅黄尚锐GaAsMESFET小讯号等效电路参数提取 陈显 聂春雷孙玲玲改进的FD—TD法分析二维理想导体TM近区散射问题 张玉胜汪文秉基于FD—TD的矩形波导中多层有耗介质柱数值分析 张峰魏昭志傅君眉背腔式槽阵的谱域法 顾长青遮研式低回波金属支架的设计 薛明华徐永斌微波炉炉腔阻抗的计算沈秀英张军短毫米波不1l亚毫米波接受机的发展现状 胡驰杨鸿生第二期(总第37期)连续波RCS自动测试系统 王兵阮成…     

Novel applications of an approximate profile inversion forone-dimensional medium

In one-dimensional inverse scattering, the approximate formulas to reconstruct the permittivity profiles are very interesting because of their closed forms. Nevertheless, as the result of Born approximation, they are invalid under the strong-scattering conditions which restrict their applications. In this paper, a novel approximation for inverting the permittivity profile of a one-dimensional inhomogeneous medium is derived by using an equivalent microwave networking technique. Compared with the published formulas of Ladoucer and Jordan (1985), the novel approximation is not only more general in the form, but suitable for both weak-scattering and strong-scattering conditions. Reconstruction examples show that when the relative permittivity reaches 80, the novel formula is effective, which show the validity of this scheme     

Inverse scattering method for one-dimensional inhomogeneous lossymedium by using a microwave networking technique

The formulation of reflection coefficients from an inhomogeneous lossy medium illuminated by TE and TM waves is approximately derived, in closed form, by using a microwave network method. From the formulation, a novel inverse scattering scheme to reconstruct simultaneously the permittivity and conductivity profiles, is proposed. This scheme is suitable for both continuous and discontinuous profiles, under both the weak scattering and strong scattering conditions. It has also been shown that when the conductivity of the medium equals zero, the reconstructed result of this scheme will reduce to the one formulated by Ladouceur and Jordan (1985). Numerical and closed-form reconstruction examples show the validity of the scheme     

Weak scattering in random media, with applications to remote probing

A simple phase-screen technique has been used to derive statistical characterizations of the perturbations imposed upon a wave (plane, spherical, or beamed) propagating through a random medium. The method is essentially physical rather than mathematical; its application is limited only by the weak-scattering approximation, a limitation which is important only for long optical or acoustical paths. No other wavelength restriction is imposed. Situations treated include arbitrary path geometries, finite transmitting and receiving apertures, anisotropic or lossy media, and polarization effects. Results include, in addition to the usual statistical quantities, time-lagged functions, mixed functions involving phase and amplitude fluctuations, angle-of-arrival covariances, frequency covariance, and other higher order quantities.     

Discussion of radiative-transfer methods applied to electromagnetic reflection from turbulent plasma

The reflected energy flux of electromagnetic waves incident upon an underdense turbulent plasma recently has been extended beyond the weak-scattering Born approximation by two different methods. One approach is based upon summing cumulative small-angle propagation effects between successive large-angle scatterings. A heuristic application of radiative-transfer methods under the same approximations and conditions yields a backscatter cross section that differs by a factor of up to one-half at given plasma thickness. It is shown that the cross section can be made to agree with that of the selective-summation approach by including a class of contributions that appears to have been omitted in the radiative-transfer approach.     

有耗媒质中目标的无相位衍射层析成像方法

提出一种新的应用于有耗媒质的无相位衍射层析成像方法:只需测量总场的幅度,不需相位信息,实现损耗媒质中二维弱散射目标重建.该方法首先基于测量到的场幅值确定出目标函数在复二维空间中的"伪Fourier谱",将"伪Fourier谱"构造关于目标函数的Fourier级数展开系数的方程组,对此病态方程组作正则化处理求解目标函数的...     

Optimal pulse-width modulation for three-level inverters

The three-level neutral-point-clamped voltage source inverter is widely used in high power, medium voltage applications. This paper studies continuous and discontinuous pulse-width modulation for this inverter. Detailed analysis of discontinuous modulation shows that the average switching frequency is not directly proportional to the carrier or sampling frequency, since additional switching transitions occur between different regions of discontinuity. At typical switching frequencies for high power applications (up to 2 kHz) these additional transitions contribute significantly to the inverter's total losses, so that a proper comparison of the harmonic performance can only be carried out under constant loss conditions with varying carrier frequency. This comparison is performed for a typical industrial medium voltage inverter. The paper then considers the major issues of neutral-point voltage balancing and loss distribution within the inverter, for the identified optimal modulation schemes.     

论飞机尾流散射特性的度量方法

由于航空安全的推动,飞机尾流的雷达探测成为雷达技术领域的新兴前沿课题。作为一类极为特殊的时变、非均匀、分布式、弱散射介质目标,飞机尾流的散射特性极具理论研究价值,同时对尾流的雷达探测技术研究具有重要指导意义。在尾流散射特性研究领域,尾流散射能力的度量存在体散射率、线散射率和雷达散射截面(RCS)直接度量三种方法。然而,目前尚未见这三种度量方法的物理基础、特点、内在关系、相互转换关系的分析。对上述问题进行了讨论,为以后的飞机尾流电磁散射特性研究和工程应用提供参考。     

Reduction of radar image artifacts caused by target inlets

The standard radar weak-scatterer model is inappropriate to targets with inlets and cavities and range-resolved images created using this model often display artifacts associated with these structures. Since inlets and cavities (typically) make a strong contribution to the radar field scattered from aircraft targets, these artifacts can confound the image interpretation process. Many of the more complete and accurate scattering models require extensive knowledge about the cavity/inlet shape and size and, moreover, are numerically intensive-features that make them unsuitable for many imaging applications. We examine an older (first-order) model based on a weak-scattering modal expansion of the structure which appears to be well suited to radar imaging     

"Buried modes" in planar media with nonmonotonically varying index of refraction

When guiding is due to a continuously varying, rather than discontinuous, index of refraction,n(z), andn(z)peaks at some point inside the propagation medium, there may result "buried modes," i.e., modes whose exponential decay takes place inside the guiding medium rather than, as is usual, in an adjacent medium. The conditions for obtaining these modes are discussed in the Wentzel-Kramer-Brillouin (WKB) approximation and the mode equations given for both these modes and the usual ones for an illustrative case.     

Amplitude and phase scintillation measurements on 8.2 km line-of-sight path at 30 GHz

The letter summarises results of amplitude and phase scintillation measurements at 30 GHz on an 8.2 km line-of-sight path in the Netherlands. The measured results compare favourably with those obtained from weak-scattering theory.