未装机状态下单拖曳VLF机载天线电磁安全性仿真分析

针对仿真计算机载VLF拖曳天线的近场辐射特性,分析对单拖曳VLF天线建模仿真的正确性,并进行未装机状态下的单拖曳VLF天线建模仿真。     

干旱区地表L波段微波辐射特性分析穿透深度的影响

利用２０１２年黑河中游HiWATER实验区戈壁和大满超级站的土壤水分和温度廓线数据,基于LＧMEB微波辐射传输模型,分析了L波段微波穿透深度随土壤水分的变化.在此基础上,分析了不同深度土壤水分和温度作为LＧMEB模型输入,对L波段微波发射率和亮度温度的影响.研究结果表明:L波段微波穿透深度随土壤水分、温度的变化,呈现明显的差异,在本研究区L波段微波穿透深度在４~２２cm 之间;在土壤水分较高的超级站,计算的L波段亮度温度与０~４cm土壤层的亮度温度更为接近,而在土壤水分较低的戈壁站,计算的L波段亮度温度与０~４０cm土壤层的亮度温度更为接近.因此,在利用L波段微波数据反演土壤水分时,应考虑穿透深度的影响,以提高反演精度.     

有耗媒质微波传播衰减特性的模拟与实验

以普通砖墙介质为实例,利用电磁波传输理论与计算机模拟相结合,对微泼在有耗媒质中的能量衰减,进行了较为糸统的研究和分析.同时对不同频率的微波穿过相同介质后的能量衰减,以及相同频率的微波穿过不同介质后的能量衰减进行了实测.其结果可应用于提高生命探测雷达穿透能力的研究中.     

海中低频水平磁偶极子在空气中产生的电磁场

采用电磁波进行跨海水-空气界面通讯时,合理的天线形式是影响通信性能的重要因素之一。为了研究环形天线在跨海水-空气界面通讯中的特点,根据磁偶极子性能,考虑海底-海水-空气三层各向同性的分层模型,推导出置于海水中的水平磁偶极子空气中激发的电磁场分量的解析表达式。进一步采用数值方法,计算了顶层空气中电磁场的分布和传播规律。结果表明,海水中的低频水平磁偶极子在空气中激发的电磁场的各个分量在垂直面内有不同的方向性,且在距发射源较近时有快速的衰减,随着距离增大,衰减逐渐减缓,但在一定高度和水平范围内有可测量的量级,说明环形天线在此类型通讯中是可行的。     

表层海水中声纳定位的声速问题

海水温盐跃层和深度方向上的梯度变化,导致海水声速的跃层和梯度变化,这种变化既有系统性,又有随机性,声速的不确定性导致水下定位系统的不稳定性问题。本文针对浅水水下定位系统中声速大小和声线弯曲问题进行研究,提出了用于几何法定位的平均海水声速线估计定位法。另外,基于射线声线理论构成的距离观测方程和时间观测方程都是不可导的非线性方程,该方程组的最小二乘问题是一类特殊的非线性最小二乘问题。本文提出了先分组迭代、后统计分析的"两步法"实现对这类特殊非线性最小二乘问题的求解。     

水下窄脉冲超宽带信号的传播特性仿真

超宽带信号具有分辨率高、截获概率低、穿透能力强等优点,将在水下目标探测、定位及通信等领域具有良好的应用前景;基于水下等离子体声源提出了窄脉冲超宽带信号产生方法,建立了以传播损失和海洋环境噪声为主要影响因素的超宽带脉冲传播信道模型;结合湖试实测数据,利用Matlab仿真软件和Welch谱分析法,仿真分析了不同频率的超宽带脉冲在水下传播不同距离后的功率谱密度以及传播特性;研究成果对水下等离子体超宽带脉冲的工程应用具有重要的参考价值,同时对水下等离子体超宽带脉冲声源的设计和研制也有一定的借鉴意义.     

无线信号在马尾松林中传播特性分析

为解决马尾松林中无线传感器网络的部署、定位和功率控制等问题,研究433 MHz无线信号在马尾松林中的传播特性.实地测量自然生长状态下马尾松林中水平方向距地0.25 m～2.0 m的8个高度40个水平测试点的接收功率.结果表明在不同接收高度下,可以用对数衰减模型来预测433 MHz无线信号在马尾松林中的路径损耗.433 MHz无线信号随着天线高度增加,路径损耗减小,当h=2.0 m时,路径损耗最小.理论值与实测值间的相关系数为0.8599～0.9531.给出适用于马尾松林环境下对数损耗模型的范围及条件,为无线传感器网络在马尾松林环境中的部署、定位和功率控制等提供了依据.     

无线信号在马尾松林中传播特性分析

为解决马尾松林中无线传感器网络的部署、定位和功率控制等问题,研究433 MHz无线信号在马尾松林中的传播特性.实地测量自然生长状态下马尾松林中水平方向距地0.25 m～2.0 m的8个高度40个水平测试点的接收功率.结果表明在不同接收高度下,可以用对数衰减模型来预测433 MHz无线信号在马尾松林中的路径损耗.433 MHz无线信号随着天线高度增加,路径损耗减小,当h=2.0 m时,路径损耗最小.理论值与实测值间的相关系数为0.8599～0.9531.给出适用于马尾松林环境下对数损耗模型的范围及条件,为无线传感器网络在马尾松林环境中的部署、定位和功率控制等提供了依据.     

人体通信不同信道传播特性的研究

植入式通信系统是人体体域网（BAN）的重要组成部分,在医疗监测、疾病诊断治疗和残疾人辅助方面有重要的应用前景。基于时域有限差分的方法（FDTD）原理建立多层非均匀介质人体组织模型,通过在体表、体内植入不同方式信道,研究植入式人体通信信道传播特性。首先,建立多层非均匀介质人体组织模型,考虑到组织介电特性与频率有关,随频率呈α、β、γ色散,需对介质Debye模型进行非线性参数拟合;然后,在人体通信链路体表-体表、体表-体内、体内-体表和体内-体内植入四种信道,分析0.3~100 MHz频段人体通信信道的衰减特性;最后,通过信道植入实验（猪肉）验证了非均匀介质建模的可行性。分析植入式人体信道路径损耗特性：在植入信道传播特性方面,信道路径损耗与信道长度相关且随频率呈高通特性,在信道植入方式方面,体内信道较体表植入更具有优势。     

煤矿巷道壁粗糙度对电磁波传播的影响

根据多波模传输理论推导出多波模综合作用下因巷道壁粗糙度引起的电磁波衰减计算公式,并采用多波模模型仿真分析巷道壁粗糙度对电磁波衰减特性的影响。Matlab仿真结果表明,巷道壁粗糙度对电磁波衰减特性的影响随频率升高而增大,由于波导效应和巷道壁粗糙度的共同作用,煤矿井巷电波传播存在一个0.4～2GHz的低衰减频段。     

Optimal control of electromagnetic wave displacement with an unknown velocity of propagation

The aim of this paper is to control an electromagnetic wave that penetrates in a medium with some missing information about its physical properties. The missing value of wave velocity of propagation leads us to use averaged control notion which is recently introduced by E. Zuazua, also the boundary Dirichlet condition is unknown which requires using the notion of no-regret control introduced by J. L. Lions. In this work, we combine these two techniques where we introduce the notion of averaged no-regret control to solve our optimal control problem with missing data. The averaged no-regret control will be characterised by an optimality system.     

超低频水下遥控系统设计

针对水下设备在布防后难以控制的问题,提出了利用超低频电磁波实现对水下设备的远程遥控。在分析了超低频电磁波传播特性的基础上,提出采用超导量子干涉器(SQUID)作为超低频遥控信号的接收装置,对水下超低频接收系统进行了设计,并给出了接收机的软、硬件实现方案。     

Effects of external magnetic field on propagation of electromagnetic wave in uniform magnetized plasma slabs

A simple method is proposed to describe the propagation of electromagnetic waves in magnetized uniform plasma slabs. Using this method, the reflection, absorption and transmission coefficients of such plasmas for right-hand circularly waves are studied and the effects of the continuously changing external magnetic field on the power of the electromagnetic waves propagated in magnetized plasma slabs with fixed parameters are presented. Our method enables more detailed numerical analyses which are useful in practical applications pertaining to the control of the reflection or absorption coefficients of electromagnetic wave through a uniform magnetized plasma slab by adjusting the external magnetic field.     

Coupled simulation of wave propagation and water pumping phenomenon in driven concrete piles

The purpose of the paper is to simulate the water pumping phenomenon that may cause damage to driven concrete pile below water. The cracked concrete is modeled as water saturated porous media, where the cracked region is given a high permeability. A FE program based on a coupled hydro-mechanical theory has been developed to perform simulations. The simulations clearly demonstrate the water pumping phenomenon, where water is pressed out from the crack by the compressive wave induced by the pile driving hammer, and then sucked back again by the following tensile wave. Further, the principal tensile stress distribution indicates that further cracking is likely to occur at the surface of the pile around the crack, which is in agreement with the damages seen on piles in reality.     

Output feedback control design for station keeping of AUVs under shallow water wave disturbances

In this paper, we consider the problem of autonomous underwater vehicle (AUV) station keeping (SK) in shallow water area. During SK, an AUV is required to maintain position and orientation with respect to a fixed reference point at the sea floor. When AUV operates in shallow water, high‐frequency disturbances due to waves will significantly affect the motion of the AUV. In order to derive wave disturbance information for control purposes, a nonlinear observer is first designed to estimate the shallow water wave velocities and AUV relative velocities by using position and attitude measurement. Using the observer estimates, a nonlinear output feedback controller is subsequently synthesized by applying observer backstepping technique. Global exponential stability (GES) of the proposed nonlinear observer–controller design is proved through Lyapunov stability theory. Simulation studies on a model based on an actual AUV were performed to verify the performance of the proposed nonlinear observer and output feedback controller. Copyright © 2008 John Wiley & Sons, Ltd.     

Finite-difference time-domain simulation of electromagnetic propagation in magnetized plasma

The electromagnetic propagation through a homogeneous magnetized plasma slab is studied using the finite-difference time-domain method based on Z transforms. The reflection and transmission coefficients of the magnetized plasma layer for the right-hand circularly polarized wave are computed. The comparison of the numerical results of the Z transform and recursive convolution algorithms with analytic values indicates that the Z transform algorithm is more accurate than the recursive convolution algorithm. The dependence of the absorption coefficient on frequency is presented.     

基于分步小波变换的对流层电波传播特性和分析软件

为满足对流层无线通信系统设计和优化的需要，基于抛物型波动方程和分步小波变换，研究了对流层电波传播特性，开发了电波传播特性分析软件。首先，通过建立数值求解的计算场景，给出了一种基于分步小波变换的对流层电波传播特性分析方法；其次，基于提出的分析方法和Matlab，开发了对流层电波传播特性分析软件。数值计算表明，提出的分步小波变换方法收敛性比分步傅里叶变换方法好；对流层传播损耗与天线高度和仰角密切相关，天线仰角越小传播损耗也越小，天线高度越大传播损耗越小；蒸发波导环境下的传播损耗比标准大气环境下的传播损耗要小。此外，开发的分析软件图形用户界面友好，操作简单、灵活。     

强电磁干扰下远距离综合无线通信设计

针对大功率电磁干扰环境下以及复杂烟雾条件下的排爆机器人作业时的通信控制问题,提出了一套集无线通信、红外通信、激光通信于一体的多手段综合无线通信系统.并对各通信方式采取了多项改善措施以提升通信效果:一是采用功耗低的2.4 GHz频段无线通信,以提高无线射频的抗干扰能力;二是使用大功率红外发射管设计了大功率红外发射电路,利用反射镜与凸透镜凝聚红外光束,在接收端使用多接收器旋转阵列的方式多方向采集信号并利用遮光罩屏蔽杂散光线,从而提升红外通信距离;三是利用视频标注的方式解决激光通信对准的难题.通过试验验证了所设计电路良好的通信效果,达到了设计目标,并为类似的工程项目提供了参考.     

Efficient numerical algorithms for the solution of “good” Boussinesq equation in water wave propagation

This paper proposes three fast and high accuracy numerical methods for solving a nonlinear partial differential equation (PDE) describing water waves and called the Boussinesq (Bq) equation. We numerically solve the Bq equation with fourth-order time-stepping schemes in combination with discrete Fourier transform. We discretize the original PDE with discrete Fourier transform in space and obtain a system of ordinary differential equations (ODEs) which will be solved with fourth-order time-stepping methods. After transforming the equation to a system of ODEs, the linear operator is not diagonal, but we can implement the methods such as diagonal case which reduces the CPU time. Comparing numerical solutions with analytical solutions demonstrates that those methods are accurate and readily implemented. Also we investigate the conservation of mass for Bq equation.