多分层介质中平面电磁波的传播特性分析

多分层介质中平面电磁波的传播和散射是一个重要研究课题。在电磁波正入射,各区域均由双轴介质组成情况下,用代数方法对多分层介质中平面电磁波的传播特性进行分析,用波阻抗法和等效传输原理计算出了各层介质的反射系数和透射系数的理论公式。结果表明代数方法易于理解但是计算量较大,波阻抗法计算量降低了一半。     

分层介质中平面电磁波的传播与Zenneck波

平面电磁波作为一种理想的电磁波型,其在均匀介质中的传播特性分析是电磁场理论基础教学的核心内容.特别是均匀平面波由理想介质入射到与导电媒质平面分界面时的传播规律,是地球物理勘探、陆地移动通信工程及光纤传输技术的理论基础.本文分别以理想介质与理想介质和导电介质构成的两层平面分层媒质为例,应用场匹配方法,给出反射波和传输波分析求解的统一描述.与传统的处理方法相比,具有数学分析简洁严密与物理概念准确清晰的特点.     

地下目标探测的LFMCW RD成像方法分析

在地下目标探测过程中,由于地下介质与空气的介电特性不一致,发射信号的传播速度和传播路径会发生改变,传播速度不再保持恒定,传播路径也不再是沿直线传播。由于采用连续波体制,此时传统的回波信号模型和成像处理方法都不再适用。根据地下目标成像的特点,针对线性调频连续波体制探地雷达,给出了地下目标成像的接收回波模型,并以此为基础,建立了以分层成像处理为基础的RD成像处理方法,并通过仿真进行了验证。     

基于偏移技术的探地雷达SAR成像方法

BP方法是探地雷达最常用的成像方法,通过调整回波延时完成成像过程。由于地下介质与空气的介电特性不一致,电磁波在传播过程中传播路径和传播速度会发生改变,在调整回波延时过程中,需要实时计算电磁波的传播路径,而且BP方法本身具有计算量大的缺点,造成成像效率低。为了提高计算效率,本文采用地震波处理中的偏移技术完成地下目标的成像。通过实测数据的验证,偏移成像方法不但可以实现地下目标的成像,而且计算量小,成像效率高。      

植入式人体通信中电磁波在不同组织分界面上的传播特性研究

通过计算电磁波在不同人体组织分界面的入射情况,研究人体信道电磁波跨越不同组织分界面时的传播特性.文章首先利用电磁场边界条件推导均匀平面电磁波向单层与多层导电媒质入射时的反射系数、透射系数及电磁波表达式;然后以人体手臂为研究对象,采用4阶Cole-Cole模型获取10 Hz~100 GHz人体组织介电参数,计算向单层、多层组织入射时各个分界面上的最大入射角、反射系数、透射系数与平均能流密度,结果显示,垂直入射是电磁波透射多层人体组织的主要路径,皮肤与空气分界面的反射系数最大,频率小于1 MHz或大于10 GHz的电磁波能量比较难在人体内外传播,电磁波平均能流密度传播随频率呈带通特性,带宽及中心频率与器件植入位置有关等;最后采用COMSOL Multiphysics建立电磁波向3层人体组织垂直入射的有限元仿真模型,仿真结果与理论计算结果相比误差小于0.000 08,验证了理论推导与计算的正确性.     

基于非恒Q衰减模型探地雷达的回波仿真

电磁波在地下介质层中的传播特性及回波仿真技术对于反演地下介质层结构具有十分重要的意义。文中研究了探地雷达信号在有耗介质层中传播的回波仿真问题,基于Debye色散模型定义介质层电磁参数,讨论电磁波在地下介质层中的非恒Q衰减特性和模拟回波信号,得到了探地雷达脉冲在层状介质中传播的回波信号模型和反射回波曲线。仿真结果与当前通用的探地雷达数据仿真软件GprMax进行对比,证明了该方法的有效性。     

电磁波传播特性虚拟仿真实验教学

本文在电磁波传播特性理论分析的基础上,结合EastWave构建了均匀平面波在自由空间、理想导体分界面、理想介质分界面、半波长夹层介质和四分之一波长匹配层中传播的虚拟实验教学模型,对其入射、反射和折射进行了虚拟仿真,直观形象化地呈现电磁波传播过程以及遇到边界时电磁波实时动态变化.通过虚拟仿真实验教学验证了电磁传播特性,改善了教学效果,提升了学生理论应用于实践的能力.     

非平行分层等离子体鞘套电波传播的复射线方法

基于损耗媒质的复Snell定律提出采用复射线方法研究电磁波在非均匀且非平行分层的等离子体媒质中的传播特性.该方法考虑非均匀平面电磁波的复射线(包括等幅度面的传播射线和等相位面的传播射线), 追踪电磁波复射线在每层媒质中的传播路径以及它们透过媒质分界面时的折射射线, 同时根据复射线的传播方向计算电磁波在每层媒质中的传播衰减, 数值累计整个传播过程中的传播衰减即可获得电磁波穿过等离子体鞘套的总衰减.由钝锥体仿真流场数据简化出非平行分层几何模型并采用复射线方法进行计算分析.计算结果表明:飞行器头部至尾部的传播衰减相差巨大且呈现迅速减小趋势, 非平行分层非均匀等离子体媒质存在某特定入射角, 能使传播衰减达到最小值.     

无线电传播简介:专用术语,室内传播和路径损耗计算及实例

通过对传播路径损耗的估算来预测无线通信系统在其工作环境下的性能;解释了自由空间传播损耗的计算;电磁波在介质中的发射和反射系数的理论计算是预测反射和发射系数的工具.下面的一些实例和模型是在2.4GHz工作频率时给出的.     

时域有限差分法在有耗介质电磁能量吸收问题中的应用

本文论述了用时域有限差分法计算平面电磁波照射下有耗介质中电磁能量的吸收问题。讨论了影响计算精度的各种因素。计算了均匀和分层均匀球体中能量的吸收和分布,并把计算结果与精确的解析分析进行了比较。还讨论了理想导电面的模拟问题,并把结果与镜象理论进行了对比。所有的结果都说明,在一定条件下,时域有限差分法能给出满意的结果。     

Transmission des signaux perpendiculairement à un milieu stratifié périodique: étude fréquentielle

The authors consider the propagation of electromagnetic and elastic waves in a medium consisting of alternating layers of two different homogeneous isotropic and nonabsorbing substances. The waves are assumed harmonic and plane and their propagation is perpendicular to the layers. They show the existence of frequency ranges for which the wave propagation is possible and other ones where the waves are vanishing. The dispersion property of this medium is analysed; for low frequencies, the propagation velocity is shown smaller than the expected mean velocity. At last, a detailed work gives the transfer functions for transmission and reflection of the double linear filter constituted of the layered medium set between two homogeneous isotropic media. They pay attention to the isochronous layered medium.     

Cutoff Phenomena for Guided Waves in Moving Media

This paper treats the propagation of electromagnetic waves in the interior of a waveguide that is filled with a moving medium. The medium is assumed to be homogeneous, isotropic, and lossless, and to move with a constant velocity along the axis of the waveguide. The Maxwell-Minkowski equations for the electromagnetic fields are solved by means of a pair of vector potential functions similar to those frequently used for stationary media. The fields inside the waveguide are derived for both rectangular and cylindrical waveguides. The well-known cutoff phenomenon for a waveguide is found to be modified in an interesting way when the medium inside the waveguide is moving The results show that for a slowly moving medium (a medium for which n/sp, Beta/<1, where n is the index of refraction and /spl Beta/ is the velocity of the medium divided by the velocity of light in vacuum/, there are two critical frequencies, separating three frequency ranges in each of which there is a different type of propogation. For a high-speed medium (n/spl Beta/<1), it is found that there is no cutoff phenomenon at all, although there is one critical frequency separating two frequency ranges in which the propagation is different.     

Seismic velocity estimation

The estimate of propagation velocities in the reflection or refraction seismic method is essential to the effective imaging of the subsurface. Wave propagation in a fully elastic medium gives rise to several propagation modes, among them are the longitudinal and transverse compressional waves (P-waves and S-waves), which are commonly used in the reflection and refraction seismic methods. With appropriately sorted data, the arrival time of a wave returning from a particular reflecting interface in the subsurface will vary as a function of source-receiver offset. This variation in arrival time with offset is called "moveout" and is controlled by the propagation velocity. The velocity of propagation enters the processing of reflection seismic data first as an essential parameter of a time coordinate transformation required before data of varying source-receiver offsets can be stacked to enhance signal-to-noise ratio. Velocities estimated in this manner are called "moveout velocities." Velocities also enter the processing sequence as a parameter of an imaging operation called "migration." Early efforts at velocity estimation were only accurate enough to provide parameters to process data, but high-quality data collected using present-day technology allow us to make accurate enough estimates of propagation velocity to infer subsurface geology. Complications arise, however, due to the effects of reflector structure and lateral velocity gradients. Current developments in seismic velocity estimation include measurement of shear wave (S-wave) velocities, use of wide-angle arrivals for more accurate P-wave velocity estimates, and methods requiring areal coverage (three-dimensional seismic).     

基于PVDF压电薄膜的超声波测厚技术探究

开展了基于PVDF压电薄膜的超声波测厚技术研究,通过分析超声波是弹性机械波的一种,在同种均匀介质中,其传播声速一定,当从一种介质进入另一种介质时,在结构表面会发生反射作用。因此,我们可以认为超声波从被测试件的表面发出到接收到另一底面反射波信号的传播间隔时间与被测厚度值成正比关系,从而测量厚度的问题转化成测量超声波在试件中传播时间的问题。基于PVDF压电薄膜的超声波测厚技术将为快速便捷的测量厚度问题提供一种新的手段,并为将来测厚技术系统的开发奠定技术基础,具有工程应用价值。     

Propagation Velocity of Electromagnetic Signals

The propagation velocity of electromagnetic signals has been a vexing problem for about a century. The often-mentioned group velocity fails on two accounts, one being that it is generally larger than the velocity of light for waves in the atmosphere; the other that its derivation implies a transmission rate of information equal to zero. The reason why this problem has resisted a solution for so long is that Maxwell's equations fail for signals propagating in a lossy medium. The propagation velocity is of interest only in a lossy medium, since its value in a loss-free medium has been known since d'Alembert's solution of the wave equation. For infinite signal-to-noise ratios and ideal receivers, the propagation velocity of signals in media with ohmic losses equals the velocity of light; it decreases with decreasing signal-to-noise ratio and eventually approaches zero.     

On Guided Waves in Moving Anisotropic Media

Based upon the Maxwell-Minkowski theory, the equations governing the propagation of electromagnetic waves in a cylindrical waveguide of an arbitrary cross section filled with a moving anisotropic medium are derived. The governing equations are reducible to a pair of coupled wave equations in the axial components of the electric and magnetic fields, which in turn can be solved through the solution of a single second order scalar homogeneous Hehnholtz equation. For a general anisotropic medium no pure TM or TE modes can exist in the waveguide. However, if the moving medium is uniaxially anisotropic, TM and TE modes are possible. It is interesting to note that the cutoff frequencies are always lowered by a factor which depends upon the velocity of the medium and is independent of the guide geometry. The formulas for the characteristic wave impedance and power flow in a waveguide for a moving uniaxial medium, if expressed in terms of the new cutoff frequency, have the same forms as those for a moving isotropic medium. The propagation characteristics of waveguides of rectangular and circular cross sections filled with a moving uniaxial gyroelectric medium are discussed.     

Acoustoelectric domain propagation in two-dimensional photoconducting CdS

It is known that the initial saturation of current and subsequent current oscillations in photoconducting CdS is caused by the presence of high electric field domains traveling at the acoustic shear-wave velocity. The behavior of travel ing domains in nonuniform two-dimensional CdS samples is here studied for the first time. Experimental data showing both the uniform domain propagation in homogeneous region and the nonuniform domain propagation in inhomogeneous region of the two-dimensional samples reveal that domain formation and propagation are consistent with the acoustoelectric model. Local domains originating from the inhomogeneous part of the sample do not propagate in the direction of the applied field and thus may increase the effective traveling path.