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Showing papers in "Electromagnetics in 1987"



Journal ArticleDOI
TL;DR: In this paper, the transmission line model for the leader stroke is applied to the return stroke of a lightning channel, where the charge stored in the corona is converted into current, by a discharge wave.
Abstract: The transmission-line model previously developed for the leader stroke is applied to the return-stroke of a lightning channel. The charge stored in the corona is converted into current, by a discharge wave. The dependence of propagation velocity on coronal charge and radius leads to front steepening and the development of a jump discontinuity or electromagnetic shock in a finite time.

55 citations


Journal ArticleDOI
TL;DR: In this paper, the results from a detailed analysis of electromagnetic and current measurements during 39 lightning strikes to an airborne CV-580 aircraft are presented and discussed, concluding that 35 of the strikes were triggered by the presence of the aircraft in the vicinity of a highly charged region.
Abstract: The results from a detailed analysis of electromagnetic and current measurements during 39 lightning strikes to an airborne CV-580 aircraft are presented and discussed. It is concluded that 35 of the strikes were triggered by the presence of the aircraft in the vicinity of a highly charged region. The triggering mechanism appears to indicate that the negatively charged aircraft approached a positive charge center and encouraged the formation and propagation of leaders from the positive charge center and the aircraft. The remaining four events are shown to be cloud-to-ground strikes in which the aircraft was in the main channel in two cases and in a branch in two others. Representative examples of electromagnetic and current data are presented and discussed in detail to justify these conclusions.

26 citations



Journal ArticleDOI
TL;DR: In this article, a phenomenology analysis of the new results obtained from the study of natural and triggered lightning in New Mexico (TRIP 82), at St. Privat d'Allier in Haute-Loire, France (1983), and in South West France (Front 84) is first described.
Abstract: This article reviews the current knowledge on the very high frequency electromagnetic radiation of lightning. A phenomenology analysis of the new results obtained from the study of natural and triggered lightning in New Mexico (TRIP 82), at St. Privat d'Allier in Haute-Loire, France (1983), and in 1984 in South West France (Front 84) is first described. The authors then present the spectral and temporal characteristics of the VHF-UHF radiation which can be applied to the electromagnetic compatibility of systems. From a synthesis of the experimental and theoretical contributions, they propose a hypothesis on the lightning initiation process and on the mechanisms generating lightning radiation.

19 citations


Journal ArticleDOI
TL;DR: In this article, a transmission line model for the lightning return stroke is presented, and the model reproduces the double exponential shape of the observed field in the radiation zone, without imposing a priori the time behavior of the current pulse.
Abstract: We present a transmission line model for the lightning return stroke. This model incorporates a model for the corona surrounding the charged lightning channel as well as a model for the evolution of the channel incorporating hydrodynamics, energy losses, joule heating. We find that the model reproduces the “double exponential” shape of the observed field in the radiation zone, without imposing a priori the time behavior of the current pulse. The decrease in field at late times precedes any decrease in current, and is due to a lengthening of time scale. The current pulse is substantially altered as it progresses up the lightning channel, and the radiation zone signal is not of the same shape as the current pulse at any position or time.

16 citations


Journal ArticleDOI
TL;DR: The NASA F-106 has acquired considerable data on the rates-of-change of electromagnetic parameters on the aircraft surface during over 700 direct lightning strikes while penetrating thunderstorms at altitudes ranging from 15,000 to 40,000 feet as mentioned in this paper.
Abstract: The NASA F-106 has acquired considerable data on the rates-of-change of electromagnetic parameters on the aircraft surface during over 700 direct lightning strikes while penetrating thunderstorms at altitudes ranging from 15,000 to 40,000 feet. These in-situ measurements have provided the basis for the first statistical quantification of the lightning electromagnetic threat to aircraft appropriate for determining lightning indirect effects on aircraft. The data are presently being used in updating previous lightning criteria and standards developed over the years from ground-based measurements. The new lightning standards will be the first which reflect actual aircraft responses measured at flight altitudes. Nonparametric maximum likelihood estimates of the distribution of the peak electromagnetic rates of change for consideration in the new standards are obtained based on peak recorder data for flights which have multiple strikes. The linear and nonlinear modeling techniques developed provide a ...

15 citations


Journal ArticleDOI
TL;DR: In this paper, a curved structure consisting of an imperfectly conducting circular cylinder coated by any number of coaxial thin anisotropic layers separated by isotroplc regions of different materials is considered.
Abstract: A curved structure consisting of an imperfectly conducting circular cylinder coated by any number of coaxial thin anisotropic layers separated by isotroplc regions of different materials is considered. Each thin anisotroplc layer is represented by a sheet with an anisotropic Jump Impedance. Thus, the structure is a generalization to the anisotropic case of Jaumann absorbers on curved surfaces. For a normally incident and obliquely polarized plane wave, the scattered field is determined via a chain-matrix algorithm. An optimization procedure is developed to minimize the radar cross section over an assigned frequency band by varying the geometric and electromagnetic parameters of the structure. Numerical results based on this design procedure are presented and discussed.

14 citations



Journal ArticleDOI
TL;DR: In this paper, the theory and measurement of the electromagnetic correlation between orthogonal linearly polarized returns in two-channel precipitation radars is summarized and the relationship of the properties of precipitation particles to the cross-correlation as a function of polarization direction is discussed.
Abstract: The theory and measurement of the electromagnetic correlation between orthogonal linearly polarized returns In two-channel precipitation radars is summarized. The relationship of the properties of precipitation particles to the cross-correlation as a function of polarization direction is discussed. Also considered is the relationship of correlation in a linearly polarized radar system to the corresponding circular polarisation characteristics. The formulation for the cross-correlation at linear polarization is extended to Include coverage of more general models of the distribution function for precipitation particle canting angle. Typical experimental results showing measured values of cross-correlation with rotating linear polarization are presented.

9 citations


Journal ArticleDOI
TL;DR: In this paper, it was shown that the Wiener-Hopf analysis may involve quantities whose edge singularities are non-integratable, and whose Fourier transforms do not exist in the classical sense.
Abstract: When solving the diffraction problem of an electromagnetic wave incident on a half plane subject to a mixed boundary condition, the analysis can be simplified by using the field components normal to the surface. Although this is the procedure generally employed, it is shown that the Wiener-Hopf analysis may involve quantities whose edge singularities are non-integratable, and whose Fourier transforms do not exist in the classical sense. A method for overcoming the difficulty is presented.

Journal ArticleDOI
TL;DR: In this article, the simultaneous optical and electromagnetic emission as a function of known current have been determined both experimentally and analytically, and the most important result was the determination, through fitting the electromagnetic fields to a particular velocity, that the effective velocity of propagation of the current pulse near the ground was near.9c.
Abstract: In this paper, the simultaneous optical and electromagnetic emission as a function of known current have been determined both experimentally and analytically. The experimental measurements were accomplished at the Kiva 1 and 2 facilities atop S. Baldy Peak located in the Magdelena Mountains in central New Mexico. The facility is capable of recording lightning current, electromagnetic fields, and optical emission to at least a 10 ns resolution. The measurements of all variables are time tied to assure comparison of like quantities. Numerical predictions of the optical output for the measured current were made with the magnetohydrodynamic code, CHARTB. The predictions for the electromagnetic fields were made with a simple analytical model. The most important result of this exercise was the determination, through fitting the electromagnetic fields to a particular velocity, that the effective velocity of propagation of the current pulse near the ground was near .9c.

Journal ArticleDOI
TL;DR: In this paper, the effect of radiation losses in the responses of parallel-line mm-wave open microstrip filters is discussed, based on a method that accounts for the presence of the substrate and the associated surface wave propagation.
Abstract: This paper discusses the effect of radiation losses in the responses of parallel-line mm-wave open microstrip filters. The analysis presented is based on a method that accounts for the presence of the substrate and the associated surface wave propagation. This analysis is very accurate for a wide frequency range, starting from a few GHz up to millimeter and submillimeter waves, where existing techniques fail to apply. The computer codes generated by this work can be effectively used in conjunction with available design packages to evaluate unwanted radiation losses and possibly eliminate them by slightly modifying the geometry of the filters. Comparison of theoretical results from the presented analytical method with experimental data shows excellent agreement.

Journal ArticleDOI
TL;DR: In this article, the effect of thin conducting sheet on the fields of a subsurface vertical magnetic dipole has been analyzed numerically and numerical results for the vertical magnetic field above the source at the surface are presented in parametric form.
Abstract: The effect of a thin conducting sheet on the fields of a subsurface vertical magnetic dipole has been analyzed. The integral representation of the fields has been evaluated numerically, and numerical results for the vertical magnetic field above the source at the surface are presented in parametric form. It is found that the predicted fields give better agreement with previous transmission measurements than do the fields of a homogeneous half-space model.

Journal ArticleDOI
TL;DR: The interaction of aircraft with lightning is discussed with the aid of the test data, and a mathematical model is proposed for calculating the aircraft external response to direct-strike lightning.
Abstract: This paper discusses the lightning data acquired by the F-106B aircraft during 1984 and 1985. Representative data recorded inside and outside the aircraft are given in both the time and frequency domains. Peak values and spectral envelopes are tabulated and graphed. The interaction of aircraft with lightning is discussed with the aid of the test data, and a mathematical model is proposed for calculating the aircraft external response to direct-strike lightning.

Journal ArticleDOI
TL;DR: A nonlinear digital filter technique that uses a circular serial correlation test is shown to provide a preprocessing technique which allows the classic Prony method to be applied to the data.
Abstract: Generally, the response of a second-order system to an impulse excitation can be expressed in terms of a series of exponentially damped sinusoids. This technique is called the singularity expansion method (SEM). Knowing the SEM parameters, it is possible to characterize the system response and to search for conditions required to maximize a particular attribute of the response signal. Thus the goal of this study is to present techniques whereby the SEM parameters can be extracted from measured data, and in particular, data with a low s1gnal-to-noise ratio. A nonlinear digital filter technique that uses a circular serial correlation test is shown to provide a preprocessing technique which allows the classic Prony method to be applied to the data. The technique also estimates the noise level and accomodates the presence of non-stationary noise Results are presented for typical noisy data, as well as for analytical data with added white-Gaussian noise.

Journal ArticleDOI
TL;DR: In this paper, the authors analyzed the characteristics of the VHF (63MHz) radiation and correlated wideband electric field records for 113 cloud-to-ground (CG) flashes and 33 intracloud (IC) flashes in five different storms in South Florida during the summer of 1981.
Abstract: We analyzed the characteristics of the VHF (63 MHz) radiation and correlated wideband electric field records for 113 cloud-to-ground (CG) flashes and 33 intracloud (IC) flashes in five different storms in South Florida during the summer of 1981. Each of the stepped leaders in the CG flashes was preceded by active VHF radiation with an average duration of 34 msec. In 28% of the CG flashes, preliminary variations in the electric field records were observed which are similar to the B,I,L pattern reported by Clarence and Malan (1957). In these cases, the VHF radiation started only a few milliseconds prior to the preliminary electric field change. The VHF pulse repetition rate increased from the beginning of the preliminary breakdown to a few milliseconds prior to the return stroke at which time it decreased. There was a variation in the VHF pulse rate at about the time of stepped leader initiation determined by the first steady electric field change. The VHF radiation started at the same time as the ...

Journal ArticleDOI
TL;DR: In this paper, a spectral formulation for the analysis of arrays of thin conductors arranged in a skew three-dimensional lattice and lying in one of the lattice planes is presented.
Abstract: A spectral formulation for the analysis of arrays of thin conductors arranged in a skew three-dimensional lattice and lying in one of the lattice planes is presented. Two cases are considered, i.e. homogeneous and plane stratified dielectric medium. In the first case, the Fourier transformation is three-dimensional; in the second one, the spectral approach is Limited to the plane of stratification and the transmission line technique is used for the analysis in the longitudinal direction. The eigenmodes of the structure (Bloch waves) are studied in detail and some significant examples of dispersion curves are presented. The concepts of passband and stopband of Bloch waves arc exploited to define a synthesis procedure for high performance multiple grid frequency selective surfaces, which are considered as three-dimensional periodic structures truncated in the direction of a lattice basis vector.

Journal ArticleDOI
TL;DR: In this paper, the authors presented the electromagnetic data obtained from a broadband analysis in frequency range of 200 Hz to 20 MHz, which help to better characterize the signature of electromagnetic pulses emitted by the discharges, and their chronological development.
Abstract: The aim of this paper is to present the electromagnetic data obtained from a broadband analysis in frequency range of 200 Hz to 20 MHz. These results help to better characterize the signature of electromagnetic pulses emitted by the discharges, and their chronological development. They also allow for a better understanding of the inherent characteristics of cloud-to-ground and intracloud discharges.


Journal ArticleDOI
TL;DR: In this paper, the authors summarized some techniques developed for electromagnetic measurements of lightning electromagnetic fields at Langmuir Laboratory, including measurement of transient electric and magnetic fields at the ground surface, use of these fields to estimate location of the lightning and properties of the source and use of time of arrival at different sensors to locate the source.
Abstract: This paper summarizes some techniques developed for electromagnetic measurements of lightning electromagnetic fields at Langmuir Laboratory. These included measurement of transient electric and magnetic fields at the ground surface, use of these fields to estimate location of the lightning and properties of the source, and use of time of arrival at different sensors to locate the source. These measurements are compared to acoustic and photographic measurements of the same events.

Journal ArticleDOI
TL;DR: In this paper, the adjoint problem of the Uyadic Green's Functions (DGFs) is investigated and a more rigorous foundation of their theory and a clarification of the origin and meaninq of their symmetrical properties is obtained.
Abstract: In this work the problem of the Uyadic Green's Functions ( DGF ) is investigated so, that a more rigorous foundation of their theory and a clarification of the origin and meaninq of their symmetrical properties is obtained. The adjoint problem is formulated, the UGFs for all possible cases of boundary conditions for the Vector Helmholtz equation are defined and their symmetrical properties, as well as the assumptions under which these ones hold, proved. The analysis begins with the examination of the basic problem of the DGF for a finite homoqeneous space, which results to a more general statement of the Love equivalence theorem and the investigation of the possibilities on the choice among more DGFs for the solution of a problems: it passes to the problem of two spaces in contact (two layered medium), where the coupling relation between the two DGFs is determined and oroved to be arbitrary in the general case; and, finally, it is generalised for the case of a medium with arbitrary number of layers.

Journal ArticleDOI
TL;DR: In this article, the scattering of electromagnetic waves off a dielectric wedge is approached using an integral equation formulation, which seeks to express the interfacial field as a combination of the geometric optics field and a diffraction field made up of a number of Bessel functions.
Abstract: The scattering of electromagnetic waves off a dielectric wedge is approached using an integral equation formulation. This formulation seeks to express the interfacial field as a combination of the geometric optics field and a diffraction field made up of a number of Bessel functions. Both null field and interfacial integral equations are used to overspecify the problem and thus, seek a least squares approximation to the interfacial field on the wedge interface. The optimum number of Bessel functions used to specify the diffraction field is sought through a minimization of additional interfacial fields of equations. The unboundedness of the wedge interface problem makes it particularly difficult; the use of global basis functions embodying limiting forms of quasi-analytic formulations appears to be an efficacious approach to computing the scattered field. The methodology is quite general, lending itself to the analysis of a wedge with any dielectric constant, having any arbitrary angle.

Journal ArticleDOI
TL;DR: In this article, the scattering of electromagnetic waves from axisymmetric bodies of lossy dielectric in the resonance region is considered, and a theoretical formulation is used in terms of equivalent currents on the surface of the scatterer, leading to a set of surface integral equations which are solved numerically using the method of moments.
Abstract: The scattering of electromagnetic waves from axisymmetric bodies of lossy dielectric in the resonance region is considered. A theoretical formulation to the problem is used in terms of equivalent currents on the surface of the scatterer, leading to a set of surface integral equations which are solved numerically using the method of moments. The excitation is considered to be a circularly polarized plane wave at oblique incidence. Various sizes and shapes are considered which simulate ice, and for which experimental results of the backscattering matrix elements are available. The measurements and the computations are carried out at two different frequencies, 2.86 GHz and 9.6 GHz. Very good agreement between the numerical results and the experimental data is obtained for the backscattering direction. The numerical results of the forward scattering are also calculated and presented.

Journal ArticleDOI
TL;DR: In this article, a method of improving the accuracy of traditional methods (Guru and Chen, Hagmann and Gandhi, and Livesay and Chen) of solving integral equations has been obtained.
Abstract: A method of improving the accuracy of traditional methods (Guru and Chen [6], Hagmann and Gandhi [7]—[8], Hagmann and Levin [9], and Livesay and Chen [13] of solving integral equations has been obtained. We now have exact formulas for the accuracy of protective approximation methods (Gohberg and Feldman [5], Neittaanmaki and Saranen [20]) for solving integral equations. For a class of approximation schemes for estimating the solution of integral equations of electromagnetic scattering, we can develop a systematic procedure for reducing the original infinite rank integral equation to an exact finite rank integral equation capable of being completely analyzed by digital computer. In this paper the method is illustrated for the popular pulse basis function method(Guru and Chen [6], Hagmann and Gandhi [7]—[8], Hagmann and Levin [9], and Livesay and Chen [13]).

Journal ArticleDOI
TL;DR: In this paper, a modified Hamid-Ragheb approximation to the Planck's radiation law is presented, and the absolute percentage errors in constructing the area-coldness distribution due to the proposed and previous approximations are presented.
Abstract: A survey of the inverse black body radiation problem is presented including a modification to the Hamid-Ragheb approximation which leads to a new approximate formula for the Planck's radiation law valid at all frequencies. The modified regime is useful in solving the inverse black body radiation problem in closed form. The absolute percentage errors in constructing the area-coldness distribution due to the proposed and previous approximations are presented. The results suggest that our modified approximation to Planck's law is more suitable than all other approximations at all frequencies.

Journal ArticleDOI
TL;DR: In this article, the natural frequencies of a metallic post with a lightning channel attached were calculated by using an integro-differential equation and the asymptotic antenna technique.
Abstract: The natural frequencies are calculated for a metallic post with a lightning channel attached. Two different lightning channel models are discussed and the asymptotic antenna technique is employed to solve the problem formulated in terms of an integro-differential equation. The results are presented by way of figures and approximate analytic formulas.

Journal ArticleDOI
TL;DR: In this paper, a hybrid high-frequency formulation for determining the scattering characteristics of dielectric bodies has been developed using the principles of geometrical optics and Huygens equivalent sources.
Abstract: A hybrid high-frequency formulation for determining the scattering characteristics of dielectric bodies has been developed using the principles of geometrical optics and Huygens equivalent sources. An application of the method to the problem of determining the near backscattered field due to once internally-reflected rays from dielectric spheroids is presented here, with particular consideration given to the dominating rear-axial and glory rays. The field distribution as predicted by geometrical optics is found to occupy a finite circular aperture and exhibit a “caustic ring” along the outer perimeter of the aperture where the field becomes singular. In order to specify a finite value for the field at the caustic and provide a continuous transition from lit to shadow regions, the Huygens-Fresnel diffraction principle is utilized.

Journal ArticleDOI
TL;DR: In this paper, the propagation path loss between a radio transmitter site and receiver sites located on flat land, in the shadow of mountainous and hilly terrain, near wooded areas and within several types of building structures is predicted.
Abstract: Diffraction techniques are used to predict the propagation path loss between a radio transmitter site and receiver sites located on flat land, in the shadow of mountainous and hilly terrain, near wooded areas and within several types of building structures. These predictions are used to determine the effective coverage range for a radio paging service and, in the reverse problem, are used to determine the required transmitter power and siting for a desired geographical coverage. Extensive field tests at over 50 sites and ranges to 50 km are summarized for comparison with predicted values.

Journal ArticleDOI
TL;DR: The arm-cap Junction of a biconical antenna supports a ring charge, the value of which is calculable in an electromagnetically exact way as mentioned in this paper, using conservation of charge and Causa' law.
Abstract: The arm-cap Junction of a biconical antenna supports a ring charge, the value of which is calculable in an electromagnetically exact way. The field intensity Just off its surface Is singular but integrable. Charge values are calculated Using conservation of charge and Causa' law. Observations Include that, in differential dipole radiators, most of the dipolar current oscillates between the driving source and the ring charge. Radiated power is roportional to the square of the small cap current.