Methods for calculating the electromagnetic fields from a knownsource distribution: application to lightning

Two different techniques (monopole and dipole) for calculating the electric and magnetic fields from a distribution of currents and charges are discussed. Both techniques have been used for calculating the fields from lightning. A simple lightning return stroke current model, consisting of a square current pulse traveling up a vertical antenna above a ground plane, is used to compare the two techniques. Analytical expressions are obtained for the fields using each technique. These expressions are shown to be numerically equivalent, but the authors are unable to prove their equivalence analytically. It is concluded that the monopole and the dipole techniques can both be derived from Maxwell's equations and hence that both are correct. In attempting to dispel the apparent confusion that has existed regarding the validity of the monopole technique, the authors show that the monopole approach, as discussed in the literature, is applicable only to upward-traveling current waves and hence is not particularly useful in the realistic modeling of lightning return strokes     

交变磁偶极子辐射磁矩测量方法研究

为了提高谐变磁偶极子辐射磁矩的测试精度,避免测量角度对磁矩测量结果的影响,提出利用三分量磁通门传感器通过在空间多点测量磁场强度来获得磁偶极子辐射磁矩的新方法,与传统的两点法相比,该方法可以在任意角度下实现对辐射源辐射磁矩的测量,而不需要事先确定角度信息,且提高了磁矩的测量精度.     

The influence of the measurement probe on the evaluation of electromagnetic fields

In this paper, the influence of the measurement probe on the evaluation of the far and near field of an electromagnetic source is characterized. While measuring, the electromagnetic field will be disturbed by the measurement probes themselves. Therefore, not the true, free-space field but the disturbed field will be measured. The disturbance can not be fully taken into account by the calibration, especially in the near field. It is found that in the near field, these disturbances are much higher than in the far field for a large sensitive measurement probe. This paper will show that when the disturbance must be limited to a predefined value (e.g., 5%), a suitable measurement probe with maximum sensitivity can be selected. The characterization was performed using a numerical electromagnetic computational program.     

Semiconductor lasers: An overview part II

The semiconductor laser continues to be one of the biggest success stories for III-V devices. While media attention constantly refers to the contest between silicon and GaAs, in the field of optoelectronics, and laser diodes in particular, III-Vs reign supreme. The semiconductor laser industry is a dynamic one with new materials opening up new wavelength regions and new process technologies promising monolithic integration of laser diodes and electronics.     

Numerical analysis of electromagnetic fields in biological issues

The feasibility of using integral equation techniques to predict EM fields in biological tissues is demonstrated by analyzing the scattering from arbitrary cylinders composed of lossy dielectric materials. For the circular cylinder case, good agreement is obtained between moment method solutions and exact solutions. To show the applicability to arbitrary geometries, an elliptical cylinder is also considered.     

The traveling-wave phototube part II: Experimental analysis

This paper reports a detailed experimental investigation of the traveling-wave phototube (TWP) as a broad-band demodulator of microwave-modulated light. As Part II of a theoretical and experimental analysis of the interaction between a density-modulated photoelectron beam and a slow-wave circuit, it examines the output power, bandwidth and voltage modewidth of the traveling-wave phototube as a function of photocurrent, beam voltage and interaction length. Three types of experiments are described: 1) direct demodulation of light, amplitude-modulated at 3 Gc by a cavity-type modulator, 2) measurement of the microwave shot noise from photoelectron beams initiated by light from both coherent and incoherent sources and 3) comparison of the previous two measurements with the microwave shot noise of a thermionic electron beam. Comparisons of the results of these experiments with the predictions of the theory of Part I are drawn, showing good agreement. In particular, a number of new effects which are predicted by the detailed analysis of Part I, and which had not been previously observed or predicted are reported.     

大型短波广播发射台电磁辐射环境非等距网格测量法

大型短波广播台源项众多,影响区域内的电磁辐射水平分布情况复杂,目前常用的电磁辐射源环境检测方法均无法全面准确地测量出其对周边电磁辐射环境的影响情况,为此提出一种基于最大相对误差控制算法的非等距网格测量法.该方法根据大型短波广播发射台周边电磁场变化规律理论分析结果,控制网格拟合值与网格内理论计算准确值的最大相对误差,考虑现场环境对电磁场变化影响和检测点位布设可行性确定检测网格尺寸,在电磁场强度变化很大的区域采取小网格提高检测取样的抽样率,在电磁场强度变化不大的区域采取大网格降低检测取样的抽样率,不仅保证了测量结果的精准度,同时也合理地降低了实际工作量.     

Computation of electromagnetic fields in large biological bodies byan iterative moment method with a restart technique

The computation of electromagnetic (EM) fields in a large, three-dimensional arbitrarily shaped dielectric or biological body is made practical by a conjugate gradient algorithm with a restart technique. This algorithm allows the operator to run the program for large bodies in a measured and controlled manner, minimizing the computing cost and avoiding the crashes inherent in an extended computer run. It is shown that in achieving convergence, a good initial guess plays only a very minor role while the G⁽ⁿ⁾ and A⁽ⁿ⁾ functions are crucial to the convergence in the conjugate gradient algorithm and must be included in the restart     

An asymptotic representation of electromagnetic fields radiated in magnetoactive plasma

The singularities of an asymptotic representation of electromagnetic fields radiated in magnetoactive plasma with the parameters of the near-Earth ionosphere in the range of helical waves are investigated. The singular points of the dispersion surface of the refractive index of electromagnetic waves in the aforementioned range are determined. At these points, asymptotic representations are obtained for the intensity and power of the radiated electric field. The dependences of the directions of the vectors of the electromagnetic-wave group velocities on the directions of the wave phase velocities are found in the entire range of helical waves.     

An implanted spherical head model exposed to electromagnetic fields at a mobile communication frequency

Can cellular phones and personal communication systems base station antennas affect the active or passive implantable medical devices adversely? Concerns over the possible harmful effects of nonionizing irradiaton upon implanted medical devices have been present for many years. Key issues to address are the questions of whether mobile phones have a detrimental effect on implants, and how the interaction of the handset with the body can be minimized in order to both alleviate public fears and improve handset antenna performance and new implant designs. This paper presents a thorough investigation of the scattering of an electromagnetic (EM) wave from a perfectly conducting implant (a cylindrical wire and a very thin cylindrical disk) of electrically small radius (of resonant length), embedded eccentrically into a dielectric spherical head model by a dipole antenna (0.4 wavelength) at 900 MHz. The dyadic Green's function (DGF) for spherical vector wave functions is employed. Analytical expressions for the scattered fields of an implant embedded head model is obtained. Numerical results from analytical expressions are computed for this problem and then compared with the results from the same model using the finite-difference time-domain, EMU-FDTD electromagnetic simulator. Good agreement is observed between the analytical results on the proposed method in comparison with the FDTD method.     

Generalized moment matching for the linear combination of lognormal RVs: application to outage analysis in wireless systems

Moving from the need for a simple and versatile method for outage computation in various contexts of interest in wireless communications, in this paper we propose a lognormal approximation for the linear combination of a set of lognormal random variables (RV) with one-sided random weights. The approximation is based on a generalization of the well known moment matching approximation (MMA) for the sum of lognormal RVs, and it allows quite simple handling of the power sum of interfering signals even in rather complicated scenarios. Specifically, composite multiplicative channel models with unequal parameters can be handled, and generic (unequal) correlation patterns for some channel components can be handled with reference to any pair of signals. At this stage of the computation, only moments of the random weights are required. The probability density function of the random weight for the useful signal component may be required in computing outage probability, and numerical methods may be only required to solve a single integral at this second stage. The suitability of the approximation is examined by evaluating outage performance for various values of system parameters in some contexts of interest, namely spread spectrum systems and typical reuse-based systems with composite Rayleigh-lognormal and Nakagami-lognormal channels.     

Temporally constrained ICA: an application to artifact rejection in electromagnetic brain signal analysis

Independent component analysis (ICA) is a technique which extracts statistically independent components from a set of measured signals. The technique enjoys numerous applications in biomedical signal analysis in the literature, especially in the analysis of electromagnetic (EM) brain signals. Standard implementations of ICA are restrictive mainly due to the square mixing assumption-for signal recordings which have large numbers of channels, the large number of resulting extracted sources makes the subsequent analysis laborious and highly subjective. There are many instances in neurophysiological analysis where there is strong a priori information about the signals being sought; temporally constrained ICA (cICA) can extract signals that are statistically independent, yet which are constrained to be similar to some reference signal which can incorporate such a priori information. We demonstrate this method on a synthetic dataset and on a number of artifactual waveforms identified in multichannel recordings of EEG and MEG. cICA repeatedly converges to the desired component within a few iterations and subjective analysis shows the waveforms to be of the expected morphologies and with realistic spatial distributions. This paper shows that cICA can be applied with great success to EM brain signal analysis, with an initial application in automating artifact extraction in EEG and MEG.     

Model selection in electromagnetic source analysis with an application to VEFs

In electromagnetic source analysis, it is necessary to determine how many sources are required to describe the electroencephalogram or magnetoencephalogram adequately. Model selection procedures (MSPs) or goodness of fit procedures give an estimate of the required number of sources. Existing and new MSPs are evaluated in different source and noise settings: two sources which are close or distant and noise which is uncorrelated or correlated. The commonly used MSP residual variance is seen to be ineffective, that is it often selects too many sources. Alternatives like the adjusted Hotelling's test, Bayes information criterion and the Wald test on source amplitudes are seen to be effective. The adjusted Hotelling's test is recommended if a conservative approach is taken and MSPs such as Bayes information criterion or the Wald test on source amplitudes are recommended if a more liberal approach is desirable. The MSPs are applied to empirical data (visual evoked fields).     

Three-dimensional analysis of electromagnetic fields in rectangularwaveguides by the boundary integral equation method

A rapidly convergent expression of electromagnetic fields in rectangular waveguides is proposed for three-dimensional electromagnetic field analysis by using the integral equation method. The new method is an improved image expansion method utilizing the rapid convergence of the orthogonal expansion method. By this new method, the slow convergence of the orthogonal method with currents near an observation point can be removed completely. In order to investigate the adequacy of the new expression, the fields produced by the line electric and magnetic current segments are calculated and compared with the values obtained by the orthogonal expansion method. This confirms that the new expression gives accurate numerical values with a short computing time. Electromagnetic fields in a rectangular waveguide with circular metallic and dielectric posts are analyzed by using the new expression. From computed values, equivalent circuits of the metallic and dielectric post are obtained and compared with values obtained by N. Marcuvitz (1951). Reasonably good agreement is obtained     

Calculation methods of electromagnetic fields very close to lightning

In this short paper, we present two methods: Quasi-images formula and finite-difference time-domain (FDTD) method to evaluate the electromagnetic fields very close to lightning channel, which are applicable for poorly conducting ground case, and the numerical results are consistent with each other. Moreover, the vertical electric fields at 15 m obtained by the two proposed methods is in good agreement with the measured result, and the horizontal electric field at a distance of 100 m above finely conducting ground obtained by the FDTD method is identical to the field obtained by accurate Cooray-Rubinstein approximation. With the proposed quasi-images formula, the effects of electrical dispersion of the ground on the lightning generated electromagnetic fields are analyzed and some significant results are obtained.     

The bi-polar planar near-field measurement technique, part II:near-field to far-field transformation and holographic imaging methods

A novel customized bi-polar planar near-field measurement technique is presented in a two-part paper. This bipolar technique offers a large scan plane size with minimal “real-estate” requirements and a simple mechanical implementation, requiring only rotational motions, resulting in a highly accurate and cost-effective antenna measurement and diagnostic system. Part I of this two-part paper introduced the bi-polar planar near-field measurement concept, discussed the implementation of this technique at the University of California, Los Angeles (UCLA), and provided a comparative survey of measured results. This paper examines the data processing algorithms that have been developed and customized to exploit the unique features of the bi-polar planar near-field measurement technique. Near-field to far-field transformation algorithms investigated include both interpolatory and non-interpolatory algorithms due to the a typical arrangement of the bi-polar near-field samples. The algorithms which have been tailored for the bi-polar configuration include the optimal sampling interpolation (OSI)/fast Fourier transform (FFT), Jacobi-Bessel transform, and Fourier-Bessel transform. Additionally, holographic imaging for determination of antenna aperture fields has been incorporated to facilitate antenna diagnostics. Results for a simulated measurement of an array of infinitesimal dipoles and a measured waveguide-fed slot array antenna are included. Appropriate guidelines with respect to the advantages and disadvantages of the various processing algorithms are provided     

外场作用双导线的电磁耦合时域分析方法

基于时域有限差分（finite-difference time-domain，FDTD）法和传输线方程，并结合插值技术，研究了一种高效的时域混合算法，能够快速模拟电磁波照射自由空间和屏蔽腔内双导体传输线的电磁耦合，并实现空间电磁场与双导线瞬态响应的同步计算.该算法先采用FDTD方法模拟双导线周围空间的电磁场分布，结合插值技术构建适用于双导线电磁耦合的传输线方程，再采用FDTD的中心差分格式进行离散，从而求解得到传输线和端接负载上的瞬态响应.同时，分析双导线间距对其电磁耦合的影响，掌握其耦合规律.通过相应数值算例的模拟，并与FDTD方法进行对比，验证了该时域混合算法的正确性和高效性.     

Weapons systems analysis,part II: Simulation techniques and models

The simulation techniques, languages or models best suited for a particular simulation study depend upon the characteristics of the system and the programming skill of the individual conducting the study. This paper identifies, defines, evaluates and classifies a spectrum of simulation techniques that are applicable to weapons systems analysis and air defense weaponry.