电磁波的起伏传播

本文提出了电磁波传播的起伏传播方式概念及其由具有时变初旋转相位角的多频圆极化辐射源形成的起伏传播工作原理，并介绍了起伏传播的一些特点，这种新传播方式导于我们过去熟悉的均匀传播方式的最大特点是沿途电磁波分布的起伏变化，而这一等将使得起伏传播在诸如定点通讯，探测，微波送电等许多传统的电磁波应用方面获得应用。     

电磁波的起伏传播

本文提出了电磁波传播的起伏传播方式概念及其由具有时变初旋转相位角的多频圆极化辐射源形成的起伏传播的工作原理,并介绍了起伏传播的一些特点.这种新传播方式异于我们过去熟悉的均匀传播方式的最大特点是沿途电磁波分布的起伏变化,而这一特点将使得起伏传播在诸如定点通讯、探测、微波送电等许多传统的电磁波应用方面获得应用.     

视距传播中地面起伏对传播中值衰减的影响

电波在以视距传播方式进行传播时,在低空大气层中还要受到地面的影响。文中具体分析了视距传播下几种传播模型中地面起伏对电波传播衰减中值的影响,并进行了相应的计算,最后给出相应地面起伏度与传播衰减中值之间的关系。     

不均匀体漂移速度沿路径不均匀分布对穿越电离层电波的影响

在穿越电离层的电波传播中，在传播路径上，不但电子浓度是变化的，而且遇到的不均匀体的漂移速度也是变化的。在多次散射理论下，利用多重相位屏近似以及电子浓度起伏横向自相关函数的二次函数近似，可以得到同时考虑以上效应时电波的互相关函数的解折表达式。通过与多站接收测量得到的互相关函数的比较，可以得到不均匀体漂移速度沿路径的分布。利用得到的互相关函数，可以得到不均匀体漂移速度的不均匀分布对电波的影响。     

基于FDTD的罗兰-C信号地波传播特性的时域分析

罗兰-C信号为载波调制的高斯脉冲,而传统方法对其传播特性预测均是基于100 kHz单频信号结果。文章采用FDTD方法计算了实际罗兰信号的时域特性,并和100 kHz连续单频信号结果进行了比较。结果显示:对于地形起伏不大的传播路径,两种信号结果吻合得很好,而对于地形起伏较大路径,脉冲信号结果与单频信号结果存在较大差异。该结果对地波传播特性工程测量与分析具有一定指导意义。     

利用射电天文技术监测空间传播环境

在空间时代,沿射线路径的总折射率是研究电波传播的重要参数,常规射电天文技术可监测此参数。基于不同类型射电望远镜和信号起伏,简述了从观测数据中提取传播煤质信息的方法。总结了这种监测方法的特点,与其有关的基础知识,提出取媒质信息的要点和存在的问题。     

甚低频横跨极区传播相位的观测与研究

对甚低频横跨极区传播相位观测数据进行了分析与研究,得出了甚低频跨极区传播相位的日、季变化的主要特征及规律:(1)在夏至前后的一段时间内,传播相位日变化往往呈现"驼峰"形状;(2)昼夜间的传播相位差季节变化明显;(3)传播相位的随机起伏大.     

无线电监测测向常用名词术语

（接上期）44电波的干涉衰落电波在传播过程中．多路径波在参考点引起的合成电场矢量的起伏变化,由此产生的接收信号的起伏变化。电波的干涉衰落又称为选择性衰落。电波的干涉衰落现象存在较为普遍,无论在短波还是在超短波传播中都会经常发生。     

复杂地形下激光大气闪烁与衰减的同步观测研究

在湍流大气中传播的激光强度起伏(闪烁效应)与衰减,作为随机介质中波传播研究的重要问题,一直受到人们的重视.影响该领域进一步发展的主要因素在于:目前仍没有解决随机介质中波传播问题的普适理论;复杂地形下影响大气湍流的因素较多,使得实际大气湍流状态十分复杂.因此,系统地开展复杂地形下光传播的实验研究,不仅能满足工程应用的需求,而且对理论发展具有重要意义. 湍流大气中激光强度起伏的强度、概率分布及频谱特征等特性均已被广泛、深入研究,但这些特性与激光大气衰减特性间的关系,至今未见报导.为此,我们研制了一套激光大气闪烁与大气衰减同步观测测量系统,采用光电倍增管作为探测元件,建立了背景测量装置.在合肥地区进行了传输距离为500 m复杂地形条件下的系统实验研究. 我们对光强起伏的强度、概率分布、频谱特征以及激光大气衰减等特性进行了初步实验研究,发现在不同天气条件下,每天固定的某一时刻的光强起伏特性与大气衰减特性具有不同的变化趋势与规律,并对产生这一不同变化趋势的原因进行了探讨.(OG16)     

湍流大气中准直激光束的光斑特征Ⅰ.特征半径

利用数值方法研究了不同光传播起伏条件下湍流大气中准直激光束的光斑特征 ,结果表明 ,光斑稳定半径(Rrbt)和等效半径 (Reff)具有很好的相关性 ,虽然Rrbt 可应用于更一般的光斑 ,但它们都不适合衡量光斑质量 ;而锐度半径 (Rshp)可以恰当地反映光斑质量。对于准直激光束 ,在极弱起伏条件下 ,约有一半的光斑其质量变好或变差 ,但变化幅度很小 ;随起伏条件的加强 ,越来越多的光斑质量变差 ,但仍有一部分光斑质量变好 ,且程度越来越高     

求解目标地波散射特性的方法研究

本文把求解半空间散射问题的FDTD技术与地波传播理论相结合，研究任意复杂目标的地波散特性。入射地波设置在FDTD计算区域中的总场边界上并在散射场输出边界面上提取散射近场数据，然后利用等效及镜像原理，计算无地波衰减时的远区散射场，通过引入地波衰减因子，把该远区散射场转换为远区地波散射场，文中给出了方法验证例子以及一个较复杂目标的单站RCS计算结果。     

An idea for electromagnetic "feedforward-feedbackward" media

An idea for a new class of complex media that we name feedforward-feedbackward (FFFB) media is presented and some of the results of our theoretical work in analyzing plane wave propagation in the axial direction through these media are described. The concept of FFFB media, as introduced here, was inspired by the theoretical research of Saadoun and Engheta (1992, 1994) on a variation of artificial chiral media. Like chiral media, to our knowledge there are no naturally occurring FFFB media for the microwave frequency band; for this reason we introduce an idea for artificial FFFB media. The focus of this paper is on one conceptualization of such media, namely dipole-dipole FFFB media. First, we present the calculation of the necessary constitutive parameters for studying axial plane wave propagation. Then we solve the macroscopic Maxwell equations in the k domain for axial plane wave propagation in an unbounded source-free crossed-dipole FFFB medium. Finally, we present the dispersion equation for this medium in this case, discuss some of the physical properties of its roots and certain features of the polarization eigenstates, and speculate some of the potential applications of this medium.     

Analysis of Series-Connected Discrete Josephson Transmission Line

Employing a generalized resistive–capacitive shunted junction model for Josephson junctions (JJs), the nonlinear wave propagation in the series-connected discrete Josephson transmission line (DJTL) is investigated. A DJTL consists of a finite number of unit cells, each including a segment of superconducting transmission line with a single array stack, or generally a block including an $N$ identical lumped JJ element. As the governing nonlinear wave propagation is a system of nonlinear partial differential equations with mixed boundary conditions, the method of the finite difference time domain is used to solve the equations. By this numerical technique, the behavior of wave propagation along the DJTL can be monitored in time and space domains. Cutoff propagation, dispersive behavior, and shock-wave formation through the DJTL is addressed in this paper.      

Vessel extraction in medical images by wave-propagation and traceback

This paper presents an approach for the extraction of vasculature from angiography images by using a wave propagation and traceback mechanism. We discuss both the theory and the implementation of the approach. Using a dual-sigmoidal filter, we label each pixel in an angiogram with the likelihood that it is within a vessel. Representing the reciprocal of this likelihood image as an array of refractive indexes, we propagate a digital wave through the image from the base of the vascular tree. This wave "washes" over the vasculature, ignoring local noise perturbations. The extraction of the vasculature becomes that of tracing the wave along the local normals to the waveform. While the approach is inherently single instruction stream multiple data stream (SIMD), we present an efficient sequential algorithm for the wave propagation and discuss the traceback algorithm. We demonstrate the effectiveness of our integer image neighborhood-based algorithm and its robustness to image noise.     

Propagation of Waves in a Plasma in a Magnetic Field

The propagation of electromagnetic waves in a plasma in a magnetic field as given by the Appleton-Hartree theory is discussed in terms of the wave normal surfaces instead of the more conventional propagation vector plots, and the "ordinary" and "extraordinary" waves are defined in terms of their polarizations instead of using a continuity argument. This gives a different picture of "a wave" which has some advantages. In particular, "whistlers" become obvious, as are regions of high reflection and high absorption. The Appleton-Hartree theory is then extended to include the effect of electron temperature, and this results in a third wave whose velocity is of the order of electron thermal motions.     

Theory and application of wave propagation and scattering in random media

This paper presents a review of basic theories and recent advances in the studies of wave propagation and scattering in random media. Examples of the random media include the atmosphere, the ocean, and biological media whose characteristics are randomly varying in time and space. The study of electromagnetic, optical, and acoustic waves in such media has become increasingly important in recent years in the areas of communication, detection, and remote-sensing. Topics covered in this paper are divided into "waves in randomly distributed scatterers," "waves in random continua," and "remote-sensing of random media." Transport theory with various approximate solutions and multiple scattering theories are discussed and their relationships are clarified. Included in the analyses are propagation characteristics of intensities, wave fluctuations, pulse propagation and scattering, coherence bandwidth, and coherence time of communication channels through random media. Remote-sensing techniques include recent advances in the use of inversion techniques to deal with ill-posed problems.     

基于小波变换的酉ESPRIT波束赋形技术研究

为了解决酉旋转不变子空间波束赋形算法在低信噪比环境下性能急剧下降的问题,提出了基于小波变换的酉ESPRIT算法。该算法在分析运算特点后有效调整运算顺序以控制计算量,利用小波变换去噪原理,能在低信噪比及阵元数有限的条件下正确估计来波方向。仿真结果表明,与传统ESPRIT算法及酉ESPRIT算法相比,该算法在低信噪比且不增加阵元数的情况下能更加准确地分辨来波方向。     

基于异常反射型超表面的天线反射面设计

在实际应用中有许多场合需要对电磁波的传播进行人工调控, 传统的光学器件或者人工电磁介质都是采用三维结构, 但传统的三维结构介质很难与其他器件或设备进行集成.利用相位非连续人工电磁表面则可以在传播路径的介质分界面上引入相位突变, 进而实现人工调控电磁波.文章在此基础上在微波波段利用构造分界面相位梯度提出了一种异常反射型超表面的设计, 用作天线反射面可将圆极化波高效地转化为交叉极化波, 为微波段的电磁波人工调控提供了新的手段.根据广义反射定律及斜入射时相位突变修正设计的天线反射面, 在X波段可以实现对入射电磁波的人工调控, 并通过仿真分析验证了该设计方法的准确性, 为人工电磁表面作为天线反射面提供了一种设计思路.     

热带无线电波传播研究进展及核心影响因素分析

热带无线电波传播研究是近年来电波领域重要的研究分支之一。综述了国际电信联盟(ITU)和国际无线电科学联盟(USRI)关于热带无线电波传播的主要研究内容和进展情况,分析了与热带无线电波传播相关的核心影响因素,包括雨衰、对流层闪烁、大气波导、植被衰落、盐度等,最后,指出了关于热带无线电波传播需要进一步研究的方向。相关内容可为热带无线电波传播的深入研究提供参考。