On the transient radiation of energy from simple currentdistributions and linear antennas

Smith (1998) examined the radiation from two simple filamentary current distributions: traveling-wave and uniform. The radiated or far-zone electric field was computed for an excitation that was a Gaussian pulse in time. Two interpretations for the origin of the radiation were presented, based on the far-field results. The present article continues this investigation; however, the emphasis is on an examination of the near field and the related transport of energy away from the current filament. We examine traveling-wave and standing-wave current distributions, because these distributions are frequently used to model practical antennas. Exact analytical expressions are presented for the electric and magnetic fields of the assumed, filamentary current distributions when the excitation is a general function of time. For the filamentary distributions, the current and charge are confined to a line (a line source). There is no radius associated with the filament. The expressions for the fields apply in both the near and far zones, and are used to determine the Poynting vector. For an excitation that is a Gaussian pulse in time, exact analytical expressions are obtained for the energy leaving the filament per unit time per unit length, the total energy leaving the filament per unit length, and the total energy radiated. Graphical results based on these expressions are used to study the energy transport from the filamentary current distributions. The results for the standing-wave current distribution are compared with those from an accurate analysis of a pulse-excited, cylindrical monopole antenna, performed using the FDTD method     

Method of moments analysis of EM fields in a multilayered spheroid radiated by a thin circular loop antenna

This paper presents derivation and computation of electromagnetic (EM) fields inside a dielectric prolate spheroid radiated by a loop antenna. The dielectric spheroid is considered to be multilayered, and a thin circular loop antenna that is loaded by a voltage source radiates on the top of the prolate spheroid. The multiple interaction of transmitted and reflected waves with the spheroid is characterized by applying the method of moments (MoM) to both the circular loop antenna wire and the stratified spheroidal interfaces. The dyadic Green's function in the expansion form of eigenvector wave functions is used to derive the EM fields, so the formulation is quite compact. Different basis and weighting functions are used inside the method of moments procedure for obtaining in an efficient way the unknown current distributions along the antenna wire and the unknown expansion coefficients of their resulted EM fields. Current distributions and the transmitted fields inside the spheroidal model are computed numerically and the convergence issues are discussed.     

Exact solutions of electromagnetic fields in both near and farzones radiated by thin circular-loop antennas: a general representation

This paper presents an alternative vector analysis of the electromagnetic (EM) fields radiated from thin circular-loop antennas of arbitrary radius a. This method, which employs the dyadic Green's function in the derivation of the EM radiated fields, makes the analysis more general, compact, and straightforward than those two methods published recently by Werner (1996) and Overfelt (1996). Both near and far zones are considered so that the EM radiated fields are expressed in terms of the vector-wave eigenfunctions. Not only the exact solution of the EM fields in the near and far zones outside the region (where r>a) is derived by the use of the spherical Hankel function of the first kind, but also the closed-series form of the EM fields radiated in the near zone inside the region 0⩽r   

An exact integration procedure for vector potentials of thincircular loop antennas

A direct integration procedure for far-zone vector potentials of thin circular loop antennas has been known for many years. This method is general in the sense that it leads to simple integrals which have closed form solutions for most commonly assumed loop current distributions. However, a comparable integration technique has not been available for evaluating the more complicated near-zone vector potentials. This paper introduces a systematic approach for the exact integration of general near-zone vector potentials associated with current-carrying circular loop antennas. A particular example is considered where this new integration technique is used to find exact solutions to the vector potential and electromagnetic field integrals for loops with a Fourier cosine-series expansion of the current. The observation is made that degenerate forms of these exact representations lead to simplified expressions for the important special cases of a uniform and cosinusoidal current loop. Two equivalent forms of exact series expansions are derived for the uniform current vector potential and field integrals. It is shown that the familiar small-loop approximations, as well as the classical far-field expressions, may be obtained as limiting cases of the more general exact series representations for the uniform current loop obtained in this paper. Convenient asymptotic far-field expansions are derived for the loop with a cosinusoidal current distribution. Finally, the far-field analysis for the cosinusoidal loop is generalized to loops having an arbitrary current represented by a Fourier cosine series     

Radiation and scattering from thin toroidally knotted wires

The electromagnetic radiation and scattering properties of thin knotted wires are considered in this paper. A special class of knots, called torus knots, are introduced for the purpose of this investigation. The parameterizations available for torus knots are used in conjunction with Maxwell's equations to formulate useful mathematical representations for the fields radiated by these knots. These representations are then used to derive simple closed form far-field expressions for electrically small torus knots. The derivation of a new electric field integral equation (EFIE) suitable for analysis of toroidally knotted wires is also outlined. Finally, it is demonstrated that the well-known expressions for the electromagnetic fields radiated by a circular loop antenna (canonical unknot) may be obtained as degenerate forms of the more general torus knot field representations     

Radiation of an aperture antenna covered by a spherical-shellchiral radome and fed by a circular waveguide

This paper presents a full-wave analysis of the radiation characteristics of an aperture antenna that is flush-mounted on a ground plane and fed by a circular waveguide supporting the dominant TE₁₁ mode. The antenna is covered by a dielectric hemispherical chiral radome. Huygen's equivalence principle and the image theory are utilized to simplify the problem. The magnetic dyadic Green's function for the three-layered geometry is formulated and applied to analyze the radiated electromagnetic fields outside the chiral radome. Both the exact and approximate expressions of electric fields valid for the Fresnel and Fraunhofer zones are obtained using the spherical vector wave functions and their approximations in the far zone. Various chiral materials are assumed and computations of antenna parameters are carried out. The effects of the dielectric chiral radome on the radiation power patterns, sidelobe levels, and 3-dB beamwidths are also discussed numerically     

Near fields of the constant current thin circular loop antenna ofarbitrary radius

Assuming a known (constant) current distribution on the thin circular loop antenna of arbitrary radius in free space, an exact integration of the vector potential is performed without recourse to approximations. The only restrictions on the solution variables are that the observation point distance must be greater than the loop radius and that the polar angle must run between 0 and π. The resulting vector potential infinite series solution possesses a real part composed of linear combinations of complete elliptic integrals of the first and second kind and an imaginary part composed of elementary functions. Thus, it is possible to obtain an exact solution which is valid everywhere that r>a and 0⩽&thetas;⩽π. The electromagnetic field components of the constant current circular loop antenna are then determined by direct series differentiation. These solutions are valid in the near and induction fields, converging rapidly there, and are also valid in the far field, although many terms of the series are needed for convergence     

Time-domain vector representation of monopole transient electromagnetic radiation using Maple software

The analytical time-domain vector representation of the transient electromagnetic radiation of a monopole antenna is computed using the Maple software package. This simulation originates directly from the rigorous formulation of the problem via Maxwell's equations. A ramp-function approximation of the antenna excitation current (of arbitrary waveform) is employed. Using the capabilities of Maple, the problem is easily transformed into a compact Maple code. Maple computing enables analytical time-domain three-dimensional vector expressions for the transient radiation field. The fundamental properties of transient radiation are thus characterized qualitatively and quantitatively, assisting with physical interpretation of the results.     

EM fields generated by lightning channels with arbitrary locationand slope

It is well known that lightning discharges follow a tortuous path; therefore, a general technique able to evaluate the electromagnetic (EM) fields associated with discharge currents flowing into tortuous channels seems to be worthy of consideration. Two techniques have been adopted to find the EM field generated by a current pulse traveling along a single line radiator with arbitrary slope and location above the ground. The first one employs the Fraunhofer approximation, which can provide useful information only on distant radiated fields. The second technique is exact, but applies only to the case of a velocity of propagation v of the current pulse equal to c (velocity of light). Even this solution is indeed inadequate for our purposes since v相似文献   

Comments on “Exact solutions of electromagnetic fields inboth near and far zones radiated by thin circular loop antennas: ageneral representation” [and reply]

For original paper see Li et al. (IEEE Trans. Antennas Propagat., vol. 45, p.1741-8, 1997 Dec.). The paper by Li et al. presents closed form expressions in series form for the electromagnetic (EM) fields for both near and far zones due to thin circular loop antennas. Special cases such as fields due to sinusoidal and uniform current distribution in the circular loop antenna are given in the equations (14)-(18) of Li et al. However, the assumption of taking just the first term in the summation in (18) for electrically small loops compromises the accuracy of the field calculations for the near zone where the observation point is on or around the sphere r=a. Since the current in the loop is filamentary, the magnetic field will be singular in nature at r=a when &thetas;=π/2. However, this behavior is not displayed by the magnetic field, which is computed by taking just the n=1 term of (18b), as shown in Fig. 2(b) and Fig. 3 of Li et al.. The present comment has recast (18b) and (18c) of Li et al. to illustrate the need for more terms for accurate representation of fields in the near field. A reply is given by Li et al. in which they explain the simplification of equation (18) into (20 and then (23)     

Closed-Form Expressions of the Axial Step and Impulse Responses of a Parabolic Reflector Antenna

Electromagnetic pulses radiated by parabolic antennas and similar structures are needed in many applications like air and ground penetrating radar or high power microwaves (HPM) weapons. In this paper, an approach based on Skulkin and Turchin's work and on physical optics approximation in the time domain is developed to determine the radiated fields. Closed-form time-domain expressions of the electromagnetic step and impulse responses, along the axis of a classical parabolic reflector antenna, are derived. A closed-form expression of the impulse response duration is also given. The obtained E-field and H-field formula are valid along the axis, both near and far from the reflector. Using these closed-form expressions, the radiated fields are computed by means of a convolution product of the primary source excitation and the impulse response. Numerical results have been obtained in the case of a causal sine and a generalized Gaussian impulse excitations to illustrate some specific transients effects which occur with such an antenna     

Near-to-Near Transformation in Axisymmetrical Antenna Problems

Expressions for effective near-to-near (NTN) transformation of fields in structures of axial symmetry are presented. The results are applicable in the problems of axisymmetrical antenna feeds working in non-axisymmetrical reflector environment. The solution of the problem is based on an extension of the exact integration procedure for vector potentials of thin circular loop antennas, published previously by Werner. The Werner's procedure is used directly to calculate contribution of components of the equivalent currents at the Huygens surface surrounding the antenna. New formulas are provided for the contribution of and current components. The accuracy of the present approach is verified against the results obtained by direct numerical integration in the 3-D space. Asymptotic behavior of the NTN transformation for large distances is also compared to the known results of the near-to-far transformation.     

Analysis of Radiation from a Traveling-Wave Current and Comparison with the Standing-Wave Sinusoidal Current Filament

The traveling-wave current (TWC) has served as one kind of "canonical" current distribution for simulating the behavior of a wire antenna, as has the standing-wave, sinusoidal current filament (SCF). Besides yielding closed-form solutions for their far-field patterns and radiated powers, the sinusoidal current filament and traveling-wave current can approximate the current on a wire antenna that is appropriately excited. The traveling-wave current's power distributions, as obtained using the induced EMF method and the FARS (far-field analysis of radiation sources) method, together with similar results for the sinusoidal current filament, are discussed in this brief note     

Electromagnetic characteristics of superquadric wire loop antennas

The analysis of antenna configurations in the form of a generalized superquadric loop, which includes circular, elliptical and rectangular loop geometries, is presented in this paper. Use of a Galerkin form of the moment method with piecewise sinusoidal subsectional basis and testing functions provides rapid numerical convergence and accurate representation of the antenna current. A convenient parametric representation for the superquadric curve is developed to allow a subsectional formulation using curved wire segments, rather than the commonly employed piecewise linear segments, to construct the geometry. Both magnetic frill and delta gap source models are implemented to allow a detailed study of input impedance, directivity, radiation pattern and current distribution as a function of various geometrical parameters. The results are shown to compare well with previous results for the special case of a circular loop antenna. Some useful curves are presented to aid in the design of practical superquadric loop antennas     

The Exact Analytical Solution for Large Circular Loops Radiating Around a Dielectric Coated Conducting Sphere

In this paper, the electromagnetic field of electrically large circular loop antennas, with different radii and different nonuniform current values, around a dielectric coated conducting sphere is considered. One or more loop antennas are located on the outer surface of a spherical dielectric shell covering a conducting sphere. Eigenfunction series solutions for the field are assumed in two regions. The current distribution on the wire loop, driven by a voltage source, is determined by Fourier series expansion and all necessary harmonics are taken into account. Exact analytical field expressions in closed forms are derived and field patterns are plotted. The antenna model and formulation presented in this paper offer exact analytical solutions to several loop antenna problems.      

On a new cylindrical harmonic representation for spherical waves

An exact series representation is presented for integrals whose integrands are products of cosine and spherical wave functions, where the argument of the cosine term can be any integral multiple n of the azimuth angle φ. This series expansion is shown to have the following form: I(n)=e^-jkR0/R₀ δ_no-jk Σ_m=1^∞ C(m,n)(k ²ρρ0)/m! h_m⁽²⁾(kR₀)/(kR₀)^m . It is demonstrated that in the special cases n=0 and n=1, this series representation corresponds to existing expressions for the cylindrical wire kernel and the uniform current circular loop vector potential, respectively. A new series representation for spherical waves in terms of cylindrical harmonics is then derived using this general series representation. Finally, a closed-form far-field approximation is developed and is shown to reduce to existing expressions for the cylindrical wire kernel and the uniform current loop vector potential as special cases     

Fast, simple and accurate computation of the currents on an arbitrarily large circular loop antenna

This paper is presenting a very efficient method for computing the currents on a circular loop antenna with arbitrarily large size. Pocklington's integral equation is formulated for the circular loop geometry, and the method of moments with point matching is applied to cast the equation's discrete counterpart. The basis functions are chosen in such a way that the relevant square matrix is circulant, and therefore amenable to exact eigenvalue analysis. Subsequently, the matrix is diagonalized and inverted analytically, yielding simple, analytical expressions for the weights of the basis functions, and hence the current and the input admittance of the antenna. The algorithm utilizes mainly elementary mathematical functions, as opposed to already known expressions in the literature, and yields results in the form of a rapidly convergent, single series, applicable to extremely large loops.     

Analytical form of EM fields radiated by circular aperture antennas of various current distributions

~~Analytical form of EM fields radiated by circular aperture antennas of various current distributions[1] Schelkunoff S. A., Advanced Antenna Theory, John Wiley &Sons, 1952. [2] E|liott R.S., Antenna Theory and Design, Prentice-Hall, Engie-wood Cliffs, N J, 1981. [3] R.W.P. King and G.S. Smith, Antennas in Matter: fundamen-tals, Theory and Applications, MIT Press, Cambirdge, MA,1981. [4] W.L. Stutzman and G.A. Thiele, Antenna Theory and Desigrn,John Wiley & S…     

Transient fields of small loop antennas

A rather simple and general analysis allows us to calculate directly the transient response of many different radiators of practical interest in the time domain without the knowledge of the corresponding time harmonic solution. For example, we present exact expressions for the transient near- and far-field around small circular current loops involving arbitrarily time-varying excitation. The resulting convolution integrals have been numerically evaluated for step function currents with finite rise time, thus showing the influence of finite loop radius on the radiation field in comparison with the ideal magnetic dipole.     

圆柱螺旋线的时域电磁场

给出一个单连通的空间电流线圈在远区辐射的的时域电磁场，应用等效性原理和场的迭加性，求出圆柱螺旋天线的辐射场分析。