The time-domain characteristics of a traveling-wave linear antenna with linear and nonlinear parallel loads

The time-domain characteristics of a traveling-wave linear antenna with linear and nonlinear parallel loads are discussed. The fast Fourier transform (FFT) is used to analyze the antenna with a linear parallel load. A numerical time-stepping finite-difference equation method is used to analyze the antenna with a nonlinear parallel load. The nonlinear effect is treated by the Newton-Raphson iteration technique. The effects of various linear and nonlinear parallel loads are examined. Physical insight into the nonlinear parallel loading of the antenna is also given in terms of detected time-domain sinusoidal electromagnetic (EM) waves.     

A numerical method for calculation of transients in electric circuits

 The present paper describes a numerical method for calculation of transients in electric circuits,using a step by step approach.For a small interval of time △t=const,for each branch j of circuit one calculates α_j=R_j+L_j/△t+△t/C_j,β_j=R_j+△t/C_j and θ=△t/C_j.After that,for each interval △t the current increments △i_(jk),the currents i_(jk)=i_(jk-1)+△i_(jk)and the capacitor\'s voltage u_(cjk)=u_(cjk-1)+Au_(cjk)are calculated. The values of △i_(jk)are the roots of a system of simultaneous linear equations.The results of numerical calculation of transients by the proposed method do not differ from those obtained by analytical ones.The proposed method can be applied to a large number of problems in electrical engineering.A similar approach can be used for the calculation of transients in non-linear electric circuits and in other engineering disciplines(mechanics,thermodynamics,hydraulics etc.).Several numerical examples show how the pro- posed method can be applied.     

Approximate calculation of time-domain effective height for aperture antennas

The time-domain (TD) effective height is a space-time vector operator recently introduced to describe the performance of ultrawide-band antennas. Since generally obtained by measurements or intensive numerical modeling, it is represented through a large set of data not very suitable to the evaluation of the whole TD link. For the particular class of aperture-radiating antennas of separable shapes, such as slot, open-ended waveguides and horns, this paper proposes a systematic TD processing of the aperture field, involving model-based deconvolution and parameter estimation, at the purpose of an efficient calculation and a more manageable representation of the effective height. Following the idea at the base of modal near to far field transformation, as previously presented by the authors, the processing of the aperture impulse response is here addressed with a particular care to the fast varying early transient. The corresponding time-dependent Radon transforms are then approximated by semi-analytical formulas whose accuracy is discussed as for the aperture size and frequency limits. The proposed methods, which are much faster than the conventional approach, are demonstrated by some examples.     

Time domain modeling of nonlinear loads

The behavior of an antenna or scatterer when loaded with a nonlinear element can be changed greatly from that observed under linear conditions. In some cases, the nonlinearity causes effects such as the intermodulation products arising from the nonlinear mixing of two frequencies. In other cases, the nonlinearity may be exploited, for example, to reduce late-time ringing on a pulse-excited antenna. A procedure for treating general nonlinear loads is described and illustrated. This procedure is applied using a computer to several specific types of nonlinearities. The treatment within the framework of the thin-wire approximation to the electric field integral equation is developed. As such, the treatment can be applied to the large class of objects modeled by wires. The nonlinear load types that are considered include those with piecewise-linear voltage-current curves (e.g., one or more diodes), a load with a time varying resistance (which permits modulating the scattered fields), and a general nonlinear load represented by specified voltage-current functions.     

Power-mains transients from connection of resistive loads and apossible capacitive mitigation

The voltage transient disturbances produced in power mains by the connection of resistive loads are analyzed theoretically and experimentally. The effect of a series inductance is included in the circuit model. A fast unidirectional (exponential) waveshape that reduces the instantaneous phase-to-neutral voltage is demonstrated. Differential- and common-mode components are separated, and the basic quantitative formulas are given. The case of the switching of resistive loads when a capacitor is installed across the line on the mains side of the switch is analyzed. The presence of such a capacitor results in the appearance of double-exponential transients that are considerably smoothed with a maximum rate of change without the capacitor. Low capacitance values are shown to be adequate for giving substantial smoothing, and this technique appears to be suitable for mitigating the rate-of-change related effects of the resistive-load switching. The analytical derivations are based on a three-wire circuit representation of the power line described previously by the authors (1990). The reliability of that lumped-constant model for the analysis of the low-frequency disturbances is confirmed     

RCS数值计算的时域有限体积方法

以空间无限长导体圆柱雷达散射截面(RCS)的数值计算为例,分析、研究了时域有限体积(FVTD)方法在电磁场数值计算中的应用方法,通过对不同散射条件下RCS的计算,并对数值计算结果进行了较为深入的分析。结果表明,计算结果与解析结果吻合较好。     

Finite-difference time-domain calculation of the spontaneousemission coupling factor in optical microcavities

We present a general method for the β factor calculation in optical microcavities. The analysis is based on the classical model for atomic transitions in a semiconductor active medium. The finite-difference time-domain method is used to evolve the electromagnetic fields of the system and calculate the total radiated energy, as well as the energy radiated into the mode of interest. We analyze the microdisk laser and compare our result with the previous theoretical and experimental analyses. We also calculate the β factor of the microcavity based on a two-dimensional (2-D) photonic crystal in an optically thin dielectric slab. From the β calculations, we are able to estimate the coupling to radiation modes in both the microdisk and the 2-D photonic crystal cavity, thereby showing the effectiveness of the photonic crystal in suppressing in-plane radiation modes     

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     

Absorbing surface impedances of lossy layers in the finite difference time-domain method

In this paper the time-domain surface impedances of an homogeneous absorber layer, are given for the vertical and horizontal polarizations, or respectively for the electric field perpendicular or parallel to the incidence plane. It turns out that the application of the concept in finite difference time-domain (FDTD) in absorbing surface impedances boundary conditions, gives results in good agreement with analytical Fresnel reflection coefficients.     

基于时域多分辨分析方法的地闪雷电电磁脉冲计算

为提高地闪回击电流辐射场的计算效率,提出基于时域多分辨分析的雷电电磁脉冲计算方法.在二维柱坐标系下,以具有紧支撑的Daubechies小波尺度函数作为时域多分辨算法的展开基函数,推导出该算法在二维柱坐标系下的迭代计算公式;采用完全匹配层吸收边界条件截断计算域.利用该算法计算了与放电通道不同距离处雷电电磁脉冲的各个场分量.将所得结果与时域有限差分法的计算结果进行比较,结果表明: 二者具有较好的一致性;在相同精度要求下,时域多分辨算法不仅具有更好的数值色散特性,还提高了计算效率、降低了计算内存开销.     

Transient response of antennas with nonlinear loads

A method for determining the current flowing through a nonlinear load on an antenna is discussed. This method uses the results of a frequency-domain analysis for the linear portion of the problem. A time-marching procedure then is devised to take account of the effects of the nonlinear element. Sample calculations are illustrated for a step-excited antenna having a nonlinear resistive load.     

Table-based nonlinear HEMT model extracted from time-domain large-signal measurements

This paper presents an empirical table-based nonlinear HEMT model fully extracted from time-domain large-signal measurements. A simple and direct extraction procedure, based on a vector nonlinear network analyzer measurement system with load-pull facilities, demonstrates by experimental results on microwave transistors how a very reduced number of measurements is enough to obtain the current and charge generators to fill a lookup model. Table-based model extraction, implementation, and validation are described in this paper.     

The finite-difference time-domain solution of lossy MTL networkswith nonlinear junctions

We describe a numerical technique to solve lossy multiconductor transmission line (MTL) networks, also known as tube/junction networks, which contain nonlinear lumped circuits in the junctions. The method is based on using a finite-difference technique to solve the time-domain MTL equations on the tubes, as well as the modified nodal analysis (MNA) formulation of the nonlinear lumped circuits in the junctions. The important consideration is the interface between the MTL and MNA regimes. This interface is accomplished via the first and last finite-difference current/voltage pair on each MTL of the network and, except for this, the two regimes are solved independently of each other. The advantage of the FDTD method is that the MTL equations may contain distributed source terms representing the coupling with an external field. We apply the method to previously published examples of multiconductor networks solved by other numerical methods, and the results agree exceptionally well. The case of an externally coupled field is also considered     

A time-domain algorithm for the analysis of second-harmonicgeneration in nonlinear optical structures

A time-domain simulator of integrated optical structures containing second-order nonlinearities is presented. The simulation algorithm is based on nonlinear wave equations representing the propagating fields and is solved using the finite-difference time-domain method. The simulation results for a continuous-wave operation are compared with beam propagation method simulations showing excellent agreement for the particular examples considered. Because the proposed algorithm does not suffer from the inaccuracies associated with the paraxial approximation, it should find application in a wide range of device structures and in the analysis of short-pulse propagation in second-order nonlinear devices     

非线性负载简化模型及其应用

为实现复杂网络电磁脉冲（electromagnetic pulse，EMP）耦合分析，针对非线性负载引起的分析效率和收敛性问题，提出了非线性负载精简建模方法，即将多项式表示的非线性关系用压控元件代替，指定器件动作时间表示开关响应时间，忽略温度等不必要参数，减少模型元器件个数等.采用该方法建立了气体放电管（gas discharge tube，GDT）和金属氧化物变阻器（metal oxide varistors，MOV）的精简SPICE（simulation program with integrated circuit emphasis）模型，并且用3种不同类型的脉冲激励端接非线性负载的传输线，进行了收敛性和仿真效率分析.结果表明，GDT和MOV的简化模型能够很好收敛，MOV的模型分析效率提高约30%，GDT的模型分析效率稍有劣化，但完全避免了理想开关元器件的使用，与实际器件的工作原理一致.这些建模方法具有较强的普适性，可以移植到其他非线性负载的建模上，例如与线缆耦合的精简计算模型相结合，从而提高超大系统的电磁兼容（electromagnetic compatibility，EMC）设计与评估效率.     

Calculations of the dispersive characteristics of microstrips bythe time-domain finite difference method

A direct time-domain finite-difference method is used to recharacterize the microstrip. Maxwell's equations are discretized both in time and space and a Gaussian pulse is used to excite the microstrip. The frequency-domain data are obtained from the Fourier transform of the calculated time-domain field values. Since this method is completely independent of all the above-mentioned investigations, the results can be considered as an impartial verification of the published results. The comparison of the time-domain results and those from the frequency-domain methods has shown the integrity of the time-domain computations. This method is very general and can be applied to model many other microwave components     

TIME-a nonlinear d.c. and time-domain circuit-simulation program

TIME is a Fortran computer program that can perform low-cost nonlinear d.c. and time-response simulation of bipolar transistor circuits. This paper describes TIME's straightforward free- format input, its built-in transistor model, the general analysis method, and its very efficient sparse matrix algorithm. Example analyses illustrate the computational efficiency of the program.     

Finite difference equations at magnetic boundaries

Ampere's Law is used to derive finite difference equations for the complex magnetic vector potential at boundaries between magnetic materials with different permeabilities and conductivities.     

A modified time-domain model for nonlinear analysis of an operational amplifier

A generalized two-pole time-domain model useful for op amp analysis in both linear and nonlinear regions is presented. It models slew enhancement and degradation effects for different types of input devices and voltage steps. This has been accomplished by taking a more limited two-pole model and modifying it to take into account the initial conditions in both the nonlinear and linear regions and the slope at the beginning of either the slewing or linearly settling time periods. Both the slewing period and settling period are accurately predicted by the model. A comparison of this model with SPICE analyses on a typical CMOS op amp circuit gives excellent agreement.