Calculating currents in a conductor from the measured scattered magnetic field

A theoretical method for calculating currents in a wire antenna when it is illuminated with an electromagnetic wave, is proposed. The current is determined from the scattered field observed near the antenna. The purpose of such calculations is to evaluate currents even when the wire antennas are not directly accessible to current measurement. The tlm method has been used to simulate the scattered field. Theoretical results obtained with our method and tlm simulations are in very good agreement.     

Influence of Inhomogeneities in a Stratified Ground on the Field of a Circular Loop Antenna

This paper extends earlier works on three-dimensional scattering from a finite obstacle in a stratified halfspace, treated by means of the T-matrix method. In this paper we consider a general source configuration above the ground, that excites the subsurface medium and its inhomogeneity. Explicit expressions are given for the magnetic dipole, the circular, and rectangular antenna loops. Numerical examples, which show the effect of an extended source on the ground, are given for the circular antenna loop.     

馈电网络对微带天线阵列时域散射特性的影响

利用时域有限差分(FDTD)方法,研究了入射脉冲分别为水平和垂直极化时,探针馈电和微带馈电两种结构对于微带贴片阵列天线后向散射场的影响.给出了散射场的时域波形以及频谱特征.结果表明:微带天线阵列的每个贴片都通过馈电探针直接端接负载时,阵列的散射场峰值和能量都有所减小;而阵列通过馈电网络进行馈电时,馈电网络末端有无端接负...     

无载波脉冲探地雷达天线及其实验研究

简要地概述了设计无载波脉冲探地雷达天线的理论依据和方法，并讨论了天线的类型和技术现状。对一种比较实用的无载波脉冲探地雷达天线的近地面工作特性进行了实验测试。结果表明，天线的性能在一定程度上受土壤特性的影响，天线向地下传输的能量与天线距离地面的高度ｈ有很大关系。在ｈ≤４０ｃｍ时，随着天线高度ｈ的升高，天线向地下传输的能量急剧地减小。借助于该实验，还对天线辐射出的无载波脉冲在土壤中的传播特性进行了研究     

A New Technique for Measuring an Electromagnetic Field by a Coil Spring

A thin coil spring whose length can be varied periodically by mechanical means has been used as a probe to measure the electric field intensity. The vibrating probe acts as a reradiating antenna with periodically varying length that modulates and scatters the component of the E/sup bar/ field which is parallel to the axis of the spring. The scattered signal is picked up by a receiving antenna (the sending antenna was used also as a receiving antenna) and amplified with an amplifier that is locked-in with the frequency of the mechanical vibration of the coil. Measurements were made with coil springs of three different dimensions, and a gain constant K, which should be independent of the product of the axial length of the coil and the magnitude of the vibration, was experimentally checked. Measurements of the field of a half-wave dipole antenna as measured with the spring probe are in general agreement with theory. An expression for the ratio between the scattered and incident waves is obtained based upon the field-pattern method and the approximations involved in the reciprocity-theorem method are clarified. It is also demonstrated (in the appendix) that the effective length of a thin helical antenna with a triangular current distribution equals one-half of the axial length of the helix.     

Electromagnetic Scattering by an Inhomogeneous Impedance Cylinder

In this paper, the electromagnetic scattering problem related to a circular cylinder with inhomogeneous impedance boundary is solved. This kind of problem may have practical applications such as antenna design since one can obtain a certain radiation pattern by choosing thesurface impedance in an appropriate way. The approach presented in this paper is based on the series representations of the scattering and incident fields. The impedance function is also expanded into a Fourier series. By using orthogonality properties of some special functions the problem is reduced to the solution of a system of linear equations. The problem isalso solved through the extended boundary condition method (T-matrix method). Since an extensive treatment of the T-matrix method is available in the open literature the paper is weighted towards the Fourier series method. The results of both methods are compared. Some illustrative examples showing the effects of different parameters on the scattered field are given.     

Scattered EM Field Responses of Canonical Scatterers Illuminated by an Impulse-Radiating Antenna (IRA)

This paper investigates the possibility of using the impulse-radiating antenna (IRA) as both a transmitter and receiver of electromagnetic energy for the purpose of target identification. Of specific interest is estimating the induced open-circuit voltage at the IRA's source terminals, when it is illuminated by the scattered EM field from simple conducting bodies excited by an incident field from the same IRA acting as a transmitter. In this study, several different canonical scatters are considered, including thin wires, spheres, conducting boxes, and an infinite conducting plate. For a 7 kV pulse excitation of the IRA, received peak transient voltages at the antenna ranged from a few volts to about 400 volts, depending on the scatterer.     

Optimum load for a linear antenna fed by a short duration pulse

An analysis is described to calculate the optimum load which maximizes the radiated electromagnetic field from a linear dipole antenna fed by a very short pulse. For each frequency in the band, the analysis is carried out by solving Pocklington's equation via a Galerkin moment method, together with a variational technique. A constraint on the energy of the radiated signal is used as an isoperimetric condition, so that the solution is physically realizable. The optimum solution is also compared with the response of an unloaded antenna.     

计算轴对称粒子光散射问题的简化T矩阵方法

首先给出了光散射计算中的T矩阵方法的推导过程,然后分析了轴对称粒子T矩阵的特点——方位模m的独立性。并由此特点对原始的T矩阵进行了简化和变形,去除了其中的零元素并将其分成关于方位模m的小矩阵。然后运用基于方位模m的独立方程求得散射场系数。最后以球形粒子为例得到散射场的仿真结果与用微元法有很好的吻合。     

Electromagnetic detection of buried dielectric targets

Scattering from subsurface objects is studied using the T-matrix method. The scatterer is represented by a dielectric ellipsoid, and the source is assumed to be an electric dipole in a borehole. This approach accommodates rotations of the scatterer relative to the source and also permits relaxation of the two-dimensional constraint inherent in some models. In addition, it provides a framework within which a large number of subsurface scattering problems can be studied. To illustrate how this approach can be applied, the fields scattered from highly eccentric ellipsoids are found. In addition, polarization results demonstrate that a method based on the polarization of the scattered field may provide a more robust diagnostic of scatterer location than methods using only a single field component. An experiment based on a scale model of a subsurface tunnel is described. Results from the analytic model compare favorably with those obtained from the experiment     

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     

Free-space power combining and beam steering of ultra-widebandradiation using an array of laser-triggered antennas

A mode-locked laser is used to synchronize jitter-free ultrawideband (UWB) pulse generation at an array of UWB antenna elements. The jitter-free pulses radiated by each element add together in free space to produce a radiated field pattern that is steerable via optical true-time-delay techniques. The results from a three element array experiment are presented and used to develop a model for an N-element phased array. A transmission-line model is presented for a single array element, which includes the functions of energy storage, as well as UWB pulse generation and radiation     

The effects of a nearby biological body on the parameters of asmall hand-held transmitter operating in the HF band (3 MHz-30 MHz)

The effects of a nearby biological body on the efficiency and radiation patterns of a small hand-held radio operating in the HF band are determined by computer modelling. The close proximity of the radio operator's body results in the scattered field from the body reradiating and perturbing the known free-space current distribution on the antenna to some extent. However, due to the extremely small electrical size of the radiating system, this body-to-radio coupling is assumed to be minimal and henceforth neglected. Thus, a direct moment method is used to solve a volumetric electric field integral equation with the near field of the radio used as the excitation field. Once the equivalent volumetric currents are determined, the efficiency and radiation patterns are found using these currents and the unperturbed free space current of the radio antenna     

La synthèse d’antennes à deux réflecteurs conformés à alimentation décalée

A potentially economic method for upgrading the gain of the large earth reflector antenna Cassegrain system to a gain comparable to that obtainable with a dualshaped reflector antenna system is presented herein. It involves a redesign of only the subreflector portion of a Cassegrain antenna or the introduction of a subreflector feed system for a paraboloid. A pair of offset subreflectors are synthesized which will give a controllable high gain amplitude distribution in the aperture of the large paraboloid. The synthesis method that is used is based on an approximate formulation for an offset dual shaped high gain antenna where the geometrical optics energy was scattered from a subreflector and then from a second large reflector which reflected a uniform phase distribution. In the present offset dual shaped subreflector (DSS) antenna, the second reflection is from a smaller subreflector and it scatters a spherical wave that feeds a hyperboloid or feeds a large paraboloid directly. Excellent results are shown for the approximate synthesis of the DSS.     

Near-field to near/far-field transformation for arbitrarynear-field geometry utilizing an equivalent electric current andMoM

Presented here is a method for computing near- and far-field patterns of an antenna from its near-field measurements taken over an arbitrarily shaped geometry. This method utilizes near-field data to determine an equivalent electric current source over a fictitious surface which encompasses the antenna. This electric current, once determined, can be used to ascertain the near and the far field. This method demonstrates the concept of analytic continuity, i.e., once the value of the electric field is known for one region in space, from a theoretical perspective, its value for any other region can be extrapolated. It is shown that the equivalent electric current produces the correct fields in the regions in front of the antenna regardless of the geometry over which the near-field measurements are made. In this approach, the measured data need not satisfy the Nyquist sampling criteria. An electric field integral equation is developed to relate the near field to the equivalent electric current. A moment method procedure is employed to solve the integral equation by transforming it into a matrix equation. A least-squares solution via singular value decomposition is used to solve the matrix equation. Computations with both synthetic and experimental data, where the near field of several antenna configurations are measured over various geometrical surfaces, illustrate the accuracy of this method     

Antenna patterns of nonsinusoidal waves with the time variation ofa Gaussian pulse

The development of antenna theory for nonsinusoidal electromagnetic waves has been based on the idealized rectangular pulse. In practice, an antenna that is designed to operate in the mode of an electric hertzian dipole would radiate a pulse that best approximates a Gaussian one when the driving current consists of a linear transient. The principle of radiation of nonsinusoidal electromagnetic waves with the time variation of Gaussian pulses is discussed. The properties of the Gaussian pulse are presented, i.e., the autocorrelation function, energy spectral density, and spectrum. Antenna patterns, such as peak-amplitude pattern, peak-power pattern, energy pattern, and slope pattern are derived for a Gaussian pulse received (or radiated) by a linear array antenna. Computer plots of the derived antenna patterns are presented that show a considerable improvement in the angular resolution capability over that of the antenna patterns that have been derived for a rectangular pulse     

Spaceborne Mesh Reflector Antennas With Complex Weaves: Extended PO/Periodic-MoM Analysis

Reflector antennas with complex mesh weave patterns are numerically analyzed by using extended physical optics (PO) combined with the periodic method of moments (MoM). The method applies the periodic MoM with Rao-Wilton-Glisson (RWG) basis and testing functions. It calculates the transmission coefficient matrix (T-matrix) of the realistic, complex weave pattern of the planar mesh, in order to calculate the modified PO current on the mesh reflector. When analyzed, the planar grid mesh is found to be compatible with the commonly used wire-grid model. The modified PO current on the locally planar mesh is implemented on the curved reflector surface for the diffraction analysis of the mesh reflector antennas. The far-field pattern of the offset parabolic antenna with grid mesh, modelled using periodic MoM, agreed well with those obtained from the wire-grid model. T-matrix calculated from the tabulated T-matrix data using the interpolation method is sufficiently accurate and compares well with the one calculated using the direct method, allowing users to choose one of the T-matrix calculation methods depending on the problem. The analysis of the mesh reflectors with two different complex weave patterns (single Satin and single Atlas at Ka band) is performed to exemplify the application of the extended PO/periodic MoM algorithm. It is shown that differences were observed in cross-polarization discrimination (XPD) for circular polarization operations depending on the weave patterns. This result demonstrates the usefulness of the proposed technique in accurate performance prediction of mesh reflector antennas with complex weave patterns     

低频/甚低频电磁脉冲测量系统研究

为了准确地测量电磁脉冲（electromagnetic pulse,EMP）的波形,开展低频/甚低频（low-frequency/very low-frequency,LF/VLF）EMP的形成和传播机理研究,识别不同EMP的特征,并以此为依据来识别、确认雷电及核爆炸等现象,提出了一种高分辨率的EMP原始波形测量系统方案,同时针对LF/VLF这一较低的频段提出了一套有效的系统标校方法,实现了对EMP的连续、准确测量和快速有效判别,并能通过网络实时上传数据.系统采用正交环磁场天线和平板电容电场天线实现信号的接收,设计了低噪声高保真的信道调理电路和高速数据采集电路来实现信号的采集,利用高精度的授时模块为EMP信号标记时间戳,最后结合多点监测波形实现EMP定位估计.实测结果表明:该系统能够给出高分辨率、高精度的LF/VLF EMP波形;利用该系统组网可以实现远距离EMP源的定位,定位精度与目前近距离的定位手段相当.通过长期的运行,验证了该系统具有高可靠、低失真、判别准确、实时性强等特点.     

Transient radiation field of a circular loop antenna

For a circular loop antenna excited by a frequency-hand-limited rectangular voltage pulse, the time variation of the electric field at remote points in various directions has been computed. The computations are for a loop of radiusbmade from wire of radiusa, for which the thickness parameterOmega = 2 ln (2pi b/a) = 15. The moment method was used in obtaining the current distribution on the loop due to each harmonic component in the excitation. Then the transient field is obtained by using the harmonic current distribution due to each frequency component of the exciting voltage pulse.