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。     

不规则地形地波传播衰减因子的改进算法及结果一致性研究

针对低频地波衰减因子预测算法中积分方程算法与其它经典算法在均匀/分段均匀光滑路径模型下求解结果不一致的问题进行研究分析。引入球面校正因子和高程转换技术,对积分方程方法进行改进,提高其与其它经典算法在光滑路径模型下预测结果的一致性。在此基础上,将改进算法用于实际不规则地面模型地波衰减因子预测,并与实测结果进行比较。与原算法相比,改进后的算法更适合考虑地球曲率影响下复杂长距离地面及空中地波衰减因子的求解。     

基于遗传算法的大地电导率反演

大地电导率是目前影响地波传播时延修正量计算精度的关键因素之一。将地波传播时延计算方法中的Millington经验公式作为前向算法,以传播时延修正量的理论预测结果与实测结果误差最小为优化目标,将大地电导率反演问题转化为优化问题,并采用遗传算法对传播路径上的大地电导率进行反演。通过对部分试验区域的仿真实验,证实了这种反演方法的可行性以及所反演电导率的可靠性。     

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。     

低频多跳天波时延估计算法比较

低频天波传播时延的准确预测对其在远程导航授时中的应用潜力挖掘具有重要意义. 为了获得地-电离层波导中低频多跳天波模式的传播时延特性,同时验证典型多径时延估计算法在不同信道环境下的作用性能,文中首先采用时域有限差分(finite-difference time-domain, FDTD)电磁计算方法对不同电离层反射情况下的距发射台400 km处地面接收的低频天地波耦合总场进行正演预测,然后分别基于快速傅里叶变换(fast Fourier transform, FFT)/快速傅里叶逆变换(inverse fast Fourier transform, IFFT)频谱相除、多重信号分类(multiple signal classification, MUSIC)和旋转不变技术信号参数估计(estimating signal parameters via rotational invariance techniques, ESPRIT)三种算法对电磁场数值预测结果进行后处理,解耦得到不同模式（地波、一跳天波、二跳天波、三跳天波及四跳天波）的时延,并在此基础上分析比较了无噪声和信噪比(signal-noise ratio, SNR)为0 dB、?5 dB以及?10 dB情况下三种算法对多跳天波的时延估计结果. 结果表明,波跳次数越高,算法的检测能力越差. 对于文中所模拟的信道条件,在弱噪声（SNR=0 dB）、电离层强反射时,FFT/IFFT算法结果精度最高,时延误差不超过400 ns;而在强噪声（SNR= ?10 dB）、电离层弱反射时,ESPRIT算法稳定性最好,误差范围在5 μs以内.     

ITU-RP.1546-3建议书的相关修正方法

国际电联ITU-R P.1546建议书为我们提供了大量建立在实验数据统计分析基础之上的场强预测数据和推测方法,旨在为VHF和UHF波段地面无线广播电视规划工作及其它无线电规划提供指导;同时为提高场强预测的精度提供了地形起伏、接收环境、接收地点概率变化等因素对场强预测影响的修正方法,在2007年修正的1546-3版本中还加入了有关大气散射对电波传播影响的场强修正方法.本文汇总了该建议书2001～2007年4个版本中提供的几种场强修正方法,并对其进行详细ITU-R P.1546介绍和分析.     

基于分形的粗糙海面三维抛物方程模型及其应用

风驱海浪随机起伏变化是海面环境的典型特征之一, 而较大的风浪通常会给海面无线通信带来重要的影响.传统的抛物方程(Parabolic Equationmethod, PE)模型在预测粗糙海面的电波传播时, 未能充分考虑海浪的电磁散射以及阴影效应等.针对以上不足, 文中基于三维抛物方程, 引入动力学分形方法, 对传统的抛物方程模型进行了改进研究.相比传统的Miller-Brown近似方法, 改进后的预测模型能更好地反映出海浪几何特征对电磁波传播的影响.最后以舰载雷达的有效探测范围为计算背景, 对粗糙海面的电波传播特性进行了仿真分析, 结果表明了该模型在区域级海面环境电波预测的可行性.     

ITU-R P.1546-3建议书的相关修正方法

国际电联ITU-R P.1546建议书为我们提供了大量建立在实验数据统计分析基础之上的场强预测数据和推测方法,旨在为VHF和UHF波段地面无线广播电视规划工作及其它无线电规划提供指导;同时为提高场强预测的精度提供了地形起伏、接收环境、接收地点概率变化等因素对场强预测影响的修正方法,在2007年修正的1546-3版本中还加入了有关大气散射对电波传播影响的场强修正方法。本文汇总了该建议书2001～2007年4个版本中提供的几种场强修正方法,并对其进行详细ITU-R P.1546介绍和分析。     

抛物线方程中不规则地形的分段线性模型的研究与验证

主要研究了对流层电波传播过程中起伏地形的处理方法。阐述了抛物线方程中阶梯地形方法和分段线性方法,提出了新的初始场设置方法,并给出了算法。利用该方法,仿真计算了尖劈地形下电磁波传播问题,并与射线法和UTD方法的计算结果进行了比较。计算结果显示了分段线性方法的优越性。     

基于改进地形重构的SSPE传播预测方法

针对复杂环境下电波传播预测分析难的问题,提出了一种基于地形重构的分步抛物方程(split-step parabolic equation, SSPE)传播预测方法. 该方法采用具有广角传播因子且步长可变的双向SSPE解法,并利用基于改进地形重构的SSPE传播预测方法对真实地形的电波传播特性进行仿真分析和试验验证. 与现有的电波传播预测方法相比,该方法结合了真实地形,通过改进重构算法对地形建模效果进行完善,并利用变步长双向SSPE解法进行传播损耗的预测. 仿真试验结果表明,此预测方法6 dB以内的精度满足了高精度电波传播模型的要求,表明该方法具有实际意义与推广价值.     

Finite-difference approach to the solution of time-domain integralequations for layered structures

A numerical algorithm for the analysis of transient electromagnetic fields in planar structures is proposed based on the time-domain magnetic-field integral equation (MFIE), electric-field integral equation (EFIE), and the marching-on-in-time approach. The field vectors are represented in terms of vector potential functions which are calculated either by integration or by the three-dimensional (3-D) wave equation according to the geometry of the structure. Thus, the algorithm combines the advantages of integral equation techniques and finite-difference schemes. While this approach is applicable to any geometries, it is especially suitable for multilayered planar structures and is competitive to the finite-difference time-domain (FDTD) method in the case of open and radiating problems. Theoretical results are verified by the analysis of a pulse propagation in a homogeneous open-end microstrip line     

The inclusion of wall loss in finite-difference time-domainthin-slot algorithms

The authors present two methods for incorporating slot wall loss into the finite-difference time-domain (FDTD) calculations. The walls are assumed to be good conductors. Loss is only applied to the current component that flows axially along the slot walls, which is generally the dominant component for slots that are long and narrow. The first method modifies an FDTD equation internal to the slot to include a surface-impedance contribution. This method is appropriate for the usual FDTD thin-slot formalisms. The second method includes the losses into a half-space integral equation that can be used by the recently introduced hybrid thin-slot algorithm. Results based on the two methods are compared for a variety of slot parameters and wall conductivities     

TIME-DOMAIN VOLUME INTEGRAL EQUATION FOR TRANSIENT SCATTERING FROM INHOMOGENEOUS OBJECTS-2D TM CASE

This letter proposes a time-domain volume integral equation based method for analyzing the transient scattering from a 2D inhomogeneous cylinder by involking the volume equivalence principle for the transverse magnetic case. This integral equation is solved by using an MOT scheme. Numerical results obtained using this method agree very well with those obtained using the FDTD method.     

TIME-DOMAIN VOLUME INTEGRAL EQUATION FOR TRANSIENT SCATTERING FROM INHOMOGENEOUS OBJECTS-2D TE CASE

This letter proposes a time-domain volume integral equation based method for analyzing the transient scattering from a 2D inhomogeneous cylinder by involking the volume equivalence principle for the transverse electric case. This integral equation is solved by using an MOT scheme. Numerical results obtained using this method agree very well with those obtained using the FDTD method.     

Time-domain volume integral equation for transient scattering from inhomogeneous objects—2D TM case

This letter proposes a time-domain volume integral equation based method for analyzing the transient scattering from a 2D inhomogeneous cylinder by involking the volume equivalence principle for the transverse magnetic case. This integral equation is solved by using an MOT scheme. Numerical results obtained using this method agree very well with those obtained using the FDTD method.     

Time-domain volume integral equation for transient scattering from inhomogeneous objects—2D TE case

This letter proposes a time-domain volume integral equation based method for analyzing the transient scattering from a 2D inhomogeneous cylinder by involking the volume equivalence principle for the transverse electric case. This integral equation is solved by using an MOT scheme. Numerical results obtained using this method agree very well with those obtained using the FDTD method.     

基于FDTD的微带缝隙漏波天线分析

应用时域有限差分法(FDTD)对微带缝隙漏波天线进行分析,直观地得到了微带缝隙漏波天线内部的电场分布。提出了一条有效的经验公式,成功地计算了微带缝隙漏波天线的漏波传播系数,与实验数据吻合较好。比较了不同缝隙宽度和微带宽度对微带缝隙漏波天线性能的影响。证明了在微带漏波天线上开缝可以有效地减少天线尺寸及降低天线的工作频率。     

Finite-difference time-domain calculation with all parameters ofSellmeier's fitting equation for 12-fs laser pulse propagation in asilica fiber

In order to both experimentally and numerically investigate nonlinear femtosecond ultrabroadband-pulse propagation in a silica fiber, we have extended the finite-difference time-domain (FDTD) calculation of Maxwell's equations with nonlinear terms to that including all exact Sellmeier-fitting values. We have compared results of this extended FDTD method with experimental results, as well as with the solution of the generalized nonlinear Schrodinger equation by the split-step Fourier method with a slowly varying-envelope approximation. To the best of our knowledge, this is the first comparison between FDTD calculation and experimental results for nonlinear propagation of a very short (12 fs) laser pulse in a silica fiber     

Hybrid FDTD-MPIE method for the simulation of locally inhomogeneous multilayer LTCC structure

A new hybrid finite-difference time-domain (FDTD) and mixed potential integral equation (MPIE) method is proposed for the modeling of multilayer planar circuits with locally inhomogeneous objects. By using equivalence principle, the original problem can be decomposed into two kinds of regions. The FDTD method is employed to model the locally inhomogeneous objects and construct an interaction matrix to be used in the subsequent model coupling procedure. The MPIE method with less singular kernels is applied to model the layered structure with possible perfect electric conductors. The FDTD model and the MPIE model are coupled together by enforcing the continuity of the tangential electric and magnetic fields on the equivalent surface using a Galerkin testing procedure. Numerical results are presented to validate the proposed hybrid FDTD-MPIE method.     

A Monte-Carlo FDTD Technique for Rough Surface Scattering

A Monte-Carlo finite-difference time-domain (FDTD) technique is developed for wave scattering from randomly rough, one-dimensional surfaces satisfying the Dirichlet boundary condition. Both single-scale Gaussian and multiscale Pierson-Moskowitz surface roughness spectra are considered. Bistatic radar cross sections are calculated as a function of scattering angle for incident angles of 0, 45, 70, and 80 degrees measured from the vertical. The contour path FDTD method is shown to improve accuracy for incident angles greater than 45 degrees. Results compare well with those obtained using a Monte-Carlo integral equation technique