Near-to-far-field transformation by a time-domain spherical-multipole analysis

The contribution deals with a new time-domain near-to-far-field (NFF) transformation which is particularly suited for the finite-difference time-domain (FDTD) method. The far field is derived from the tangential field components on a surface enclosing the scatterer by employing a time-domain spherical-multipole representation. The necessary time-domain convolution is performed as "on the fly," in parallel with the FDTD time-stepping algorithm. The efficiency of the method is improved by incorporating a temporal linear interpolation of the near-field data. With the once obtained multipole amplitudes an analytic series representation of the time-domain far field is achieved, which allows a physical interpretability of the result. Moreover, this expansion serves as an ideal basis for a systematic and efficient post processing. The proposed technique may also be useful for other numerical and for asymptotic methods.     

Planar near-field scanning in the time domain .2. Sampling theoremsand computation schemes

For part 1 see ibid. vol.47, no.9, p.1280 (1994). Two computation schemes for calculating the far-field pattern in the time domain from sampled near-field data are developed and applied. The sampled near-field data consists of the values of the field on the scan plane measured at discrete times and at discrete points on the scan plane. The first computation scheme is based on a frequency-domain near-field to far-field formula and applies frequency-domain sampling theorems to the computed frequency-domain near field. The second computation scheme is based on a time-domain near-field to far-field formula and computes the time-domain far field directly from the time-domain near field. A time-domain sampling theorem is derived to determine the spacing between sample points on the scan plane. The computer time for each of the two schemes is determined and numerical examples illustrate the use and the general properties of the schemes. For large antennas the frequency-domain computation scheme takes less time to compute the full far field than the time-domain computation scheme. However, the time-domain computation scheme is simpler, more direct, and easier to program. It is also found that planar time-domain near-field antenna measurements, unlike single-frequency near-field measurements, have the capability of eliminating the error caused by the finite scan plane, and thus can be applied to broadbeam antennas     

A two-dimensional time-domain near-zone to far-zone transformation

A 2D version of a time-domain transformation useful for extrapolating 3D near-zone finite-difference time-domain (FDTD) results to the far zone is outlined. While the 3D transformation produced a physically observable far-zone time-domain field, this is not convenient for the 2D case. However, a representative, 2D far-zone time-domain result can be obtained directly. This result can then be transformed to the frequency domain using a fast Fourier transform, corrected with a simple multiplicative factor, and used, for example, to calculate the complex wideband scattering width of a target. If an actual time-domain far zone result is required, it can be obtained by inverse Fourier transform of the final frequency-domain result     

Formulation of spherical near-field scanning for electromagneticfields in the time domain

Spherical near-field scanning techniques are formulated for electromagnetic fields in the time domain so that a single set of time-domain near-field measurements yields the far field in the time domain or over a wide range of frequencies. Probe-corrected as well as nonprobe-corrected formulations are presented. For bandlimited time-domain fields, sampling theorems and computation schemes are derived that give the field outside the scan sphere in terms of sampled near-field data     

Far-field time-domain calculation from aperture radiators using theFDTD method

It is often desirable to determine the far-field radiation pattern from an aperture in a metal plate. The use of time-domain methods, like the finite-difference time-domain (FDTD) method, is problematic because of the necessity of modeling the entire space of interest. This is often circumvented by using the equivalence principle to determine the far field by equivalent currents generated in the near field. However, these transformations are often mathematically complex. This paper presents an extremely simple formulation for calculating the time-domain fields from some types of aperture radiators. By using wavelet analysis, the pertinent parameters can be compressed and stored to be used later to resynthesize the radiated fields     

Time-domain extrapolation to the far field based on FDTDcalculations

A scheme to extrapolate scattered fields calculated by the finite-difference time-domain (FDTD) method to the far zone is developed. It could replace the usual extrapolation scheme for a single frequency, in that it yields a spectrum with only one incident pulse. The method comprises the following steps: (1) using an incident pulse; (2) performing an FDTD calculation and using the currently calculated data on S to extrapolate to the far field; and (3) Fourier transforming the far-field time response to obtain the frequency-domain response. The approach seems to be more convenient in time-domain calculations and more in agreement with the spirit of FDTD     

Reducing the computational overhead of the near-field transformthrough system identification

The system identification technique is applied to the output of a time-domain near to far-field transform employed with the FDTD algorithm. The technique is used to characterise the far field of a microstrip antenna, the accuracy of the results is evaluated, and the computational savings and overheads involved are discussed     

A parallel FFT accelerated transient field-circuit simulator

A novel fast electromagnetic field-circuit simulator that permits the full-wave modeling of transients in nonlinear microwave circuits is proposed. This time-domain simulator is composed of two components: 1) a full-wave solver that models interactions of electromagnetic fields with conducting surfaces and finite dielectric volumes by solving time-domain surface and volume electric field integral equations, respectively, and 2) a circuit solver that models field interactions with lumped circuits, which are potentially active and nonlinear, by solving Kirchoff's equations through modified nodal analysis. These field and circuit analysis components are consistently interfaced and the resulting coupled set of nonlinear equations is evolved in time by a multidimensional Newton-Raphson scheme. The solution procedure is accelerated by allocating field- and circuit-related computations across the processors of a distributed-memory cluster, which communicate using the message-passing interface standard. Furthermore, the electromagnetic field solver, whose demand for computational resources far outpaces that of the circuit solver, is accelerated by a fast Fourier transform (FFT)-based algorithm, viz. the time-domain adaptive integral method. The resulting parallel FFT accelerated transient field-circuit simulator is applied to the analysis of various active and nonlinear microwave circuits, including power-combining arrays.     

矩形波导斜切口面辐射特性的FDTD分析

应用时域有限差分(FDTD)法,结合时域近场 远场转换公式,分析矩形波导斜切口面的辐射特性.分析的斜切口面是把矩形波导管两个窄边同时渐削,宽边尺寸保持不变形成的辐射口面.应用FDTD法分析,较精确地解决了波导边缘的绕射问题.计算了主模激励时主面辐射方向图,与实验结果基本吻合.本文为这种波导斜切口面天线的分析设计提供了一种较精确、方便有效的方法.     

实际强激光远场靶面上光束质量的评价因素

提出了能量型应用的强激光光束质量的一种评价标准。光束质量的评价标准与实际的应用类型有关。影响实际强激光远场光束质量的因素十分复杂。将其主要因素分为系统内相位像差类、非相位像差类和大气传输影响等三类独立的因素，并探讨了在同一评价标准下，对它们进行解析表达的方法     

Formulation of probe-corrected planar near-field scanning in thetime domain

Probe-corrected planar near-field formulas in the time domain are derived for both acoustic and electromagnetic fields, so that a single set of near-field measurements in the time domain yields the fields of the test antenna directly in the time domain. The time-domain probe-corrected formulas are first derived by taking the inverse Fourier transform-of the corresponding frequency-domain formulas, and then by using a time-domain expansion for the fields of the test antenna and a time-domain receiving characteristic of the probe. Because these general formulas, which involve a double integral over the scan plane and an infinite time-convolution integral, are rather complicated, we consider a special probe whose output due to an incoming time domain plane wave is proportional to the time derivative of the field of that plane wave. For this special “D-dot probe”, the probe-corrected formulas simplify to give the time-domain far-held pattern as a double spatial integral of the time-domain output of the probe over the scan plane multiplied by the angular dependence of the inverse receiving characteristic of the probe. Time-domain reciprocity relations are derived for reciprocal probes, and their time-domain receiving characteristics are related to their far fields. Finally, a time-domain sampling theorem is derived and a numerical example illustrates the use of the time-domain probe-corrected formulas     

飞秒激光触发光电导天线产生太赫兹波的研究

研究了光电导天线产生太赫兹波的辐射特性,利用麦克斯韦方程及其边界条件,计算了近远场的电场强度;采用电磁波时域有限差分方法（FDTD）,在Matlab系统软件中,用C语言编写程序计算光电导偶极天线的辐射太赫兹波的空间电磁场分布,并在计算机上以伪彩色图形显示,这种电磁场的可视化结果为天线的设计和改进提供了直观的物理依据。     

Physical interpretation of Petermann's strange spot size for single-mode fibres

Petermann's mathematical definition of spot size using the modal near field and its derivative is proved to be ?2 times the inverse of the RMS width of the observable modal far field. The theory of fibre optics is given in terms of the transformed field, which determines the far field. Dispersion, splice loss and excitation efficiency are discussed.     

Ultrawide-band modeling of transient radiation from aperture antennas

A new method for the numerical calculation of transient field radiated through aperture-type antennas (slot, open-ended waveguide, and horn) is described. The finite-difference time-domain method is applied for the near-field prediction in the close surrounding of the antenna and a proper data-fitting procedure of the aperture field, involving interpolating functions with separation of space- and time dependence, permits: 1) to calculate "off-line" the radiated field without the need to store a great amount of data; 2) to avoid, in the case of far field, the numerical evaluation of radiation integral; and 3) to obtain approximate far field formulas which are still separable with regard to space and time. The method enables a full data reusability in calculation of field pattern over a wide angular range at a same time, or of the transient response at fixed observation points.     

The far field of a borehole radar and its reflection at a planarinterface

The far held of an electric Hertzian dipole located in a borehole is expressed as the product of a transmission coefficient and the far field of a dipole in a homogeneous region. The derivation uses the method of steepest descent, and gives physical insight into the wave phenomena that are important for borehole radar. The received field inside the borehole due to a planar interface located in the vicinity of the borehole is calculated using geometrical optics and the principle of reciprocity. The resulting expression is validated by numerically comparing it with a finite-difference time-domain solution. In an idealized situation, the reflection from the oil-mater boundary in a horizontal well can be detected with a borehole radar     

矩形微带天线的设计及FDTD分析

根据理论计算和实际修正,设计出工作于1.79GHz的矩形微带天线。利用基于非均匀网格的时域有限差分方法对天线进行建模和仿真,并分析了天线的频率特性、远场辐射方向图以及阻抗特性和驻波比。     

FDTD modeling of transient microwave signals in dispersive and lossy bi-isotropic media

In this paper, we present a novel finite-difference time-domain model of transient wave propagation in general dispersive bi-isotropic media with losses. The special properties of these materials may lead to new applications in microwave and millimeter-wave technology. While their frequency-domain properties have been well described in the literature, their time-domain behavior has only been modeled thus far for special sub-classes and monochromatic time dependence. We have validated our method by first computing time-harmonic wave propagation through a bi-isotropic medium and comparing it with analytical results. Agreement is typically better than 1%. We have then computed transient field propagation in a general lossy dispersive bi-isotropic medium.     

Near-field far-field transformation in time domain from optimalplane-polar samples

This paper presents a new approach to the time-domain near-field far-field transformation technique introduced by Hansen and Yaghijan (1994) and is based either on a time-domain or frequency-domain scheme. The approach presented here attempts to overcome the main drawbacks of this technique related to the computer time and memory requirements, which could make unrealistic the application of the technique to cases of practical interest. To this end, the advanced representation of the (time and frequency domain) near field previously introduced by the authors, which requires a minimum number of nonequispaced field samples, are exploited. This leads to new relationship between the near-field measured samples and the far field, which requires a minimal set of time-space measurements. Various computational schemes are considered and compared showing that the presented algorithm requires a reduced measurement effort, computer time, and memory occupancy, while allowing a lower far-field reconstruction error for a fixed number of measurements     

Transient analysis of EM pulse penetration into a conducting layer using wavelet-based implicit TDIE and iterative IMC technique

The penetration of a wide-band electromagnetic pulse into a conducting layer is formulated in terms of an implicit time-domain integral equation and cast into matrix form via the method of moments. Though such a wide-band problem indeed calls for a time-domain solution, one may argue that the frequency-dependent attenuation of the field penetrating the conductor could suggest a frequency-domain approach that would greatly reduce the number of unknowns. The proposed via media is to use spatio-temporal wavelet functions instead of the standard pulse basis. Owing to their multiresolution property, both in the spatial and temporal domains, these functions can span the field propagating inwardly with substantially fewer terms. The reduction in the number of basis functions used is effected by the impedance matrix compression technique, which automatically omits the basis functions whose coefficients would be insignificant due to the attenuation. Reducing the number of basis functions renders the matrix equation much smaller and the overall solution far more efficient.     

单极天线和偶极子天线辐射特性数值计算*

采用时域有限差分法(FDTD)在柱坐标系下对同轴线馈电单极天线辐射特性进行数值计算,推导了二维扩展柱坐标下修正的Maxwell方程,应用辅助微分方程FDTD法对修正的Maxwell方程进行了差分离散,以TM波在自由空间中的传播为例,仿真了电磁波在自由空间中传播过程并分析了坐标伸缩完全匹配层的性能,建立了单极同轴线馈电天线仿真模型,仿真分析了同轴线馈电单极天线的近场、远场及阻抗特性。在三维直角坐标系下建立了偶极子天线仿真空间模型,数值计算了偶极子天线不同切面的远场方向图,仿真结果与客观实际相同,验证了空间模型及相关理论的正确性。