Planar near-field to far-field transformation using an equivalentmagnetic current approach

An alternative method is presented for computing far-field antenna patterns from near-field measurements. The method utilizes the near-field data to determine equivalent magnetic current sources over a fictitious planar surface that encompasses the antenna, and these currents are used to ascertain the far fields. Under certain approximations, the currents should produce the correct far fields in all regions in front of the antenna regardless of the geometry over which the near-field measurements are made. An electric field integral equation (EFIE) is developed to relate the near fields to the equivalent magnetic currents. The method of moments is used to transform the integral equation into a matrix one. The matrix equation is solved with the conjugate gradient method, and in the case of a rectangular matrix, a least-squares solution for the currents is found without explicitly computing the normal form of the equation. Near-field to far-field transformation for planar scanning may be efficiently performed under certain conditions. Numerical results are presented for several antenna configurations     

Planar near-field to far-field transformation using an array ofdipole probes

A method is presented for computing far-field antenna patterns from measured near-field data measured by an array of planar dipole probes. The method utilizes the near-field data to determine some equivalent magnetic current sources over a fictitious planar surface which encompasses the antenna. These currents are then used to find the far fields. The near-field measurement is carried out by terminating each dipole with 50 Ω load impedances and measuring the complex voltages across the loads. An electric field integral equation (EFIE) is developed to relate the measured complex voltages to the equivalent magnetic currents. The mutual coupling between the array of probes and the test antenna modeled by magnetic dipoles is taken into account. The method of moments with Galerkin's type solution procedure is used to transform the integral equation into a matrix one. The matrix equation is solved with the conjugate gradient-fast Fourier transformation (CG-FFT) method exploiting the block Toeplitz structure of the matrix. Numerical results are presented for several antenna configurations to show the validity of the method     

Reconstruction of Equivalent Currents Distribution Over Arbitrary Three-Dimensional Surfaces Based on Integral Equation Algorithms

A technique for the determination of the equivalent currents distribution from a known radiated field is described. This Inverse Radiation Problem is solved through an Integral Equation algorithm that allows the characterization of antennas of complex geometry both for near field to far field (NF-FF) transformation purposes as well as for diagnostic tasks. The algorithm is based on the representation of the radiating structure by means of a set of equivalent currents over a three-dimensional (3D) surface that can be fitted to the arbitrary geometry of the antenna. The innovative formulation uses an integral equation involving the electric field due to the currents tangential components to the represented antenna 3D surface. For that purpose, both the magnetic and electric equivalent currents are considered in the integral equations. Regularization techniques are also introduced to improve the convergence of the proposed iterative solution. The paper concludes with several results related to the practical verification of the Equivalence Principle and the characterization of a horn antenna.     

Far-field predictions from near-field measurements using an exactintegral equation solution

This paper presents a new approach to derive far-field data needed in antenna and EMI/EMC testing from near-field measurements. An exact integral equation solution to the wave propagation problem is used to transform the near-field data to the far field. The method requires near-field measurements on two closed surfaces enclosing all sources and inhomogeneities. The approach is validated with numerical simulation of measurements of fields radiated from a known antenna     

近地垂直对数周期天线的性能分析

采用混合位电场积分方程结合矩量法分析近地垂直对数周期天线,运用微波网络理论处理馈源问题,分别利用二级离散复镜像法和一种快速方法计算近、远场的Sommerfeld积分,较好地解决了分层媒质中电磁辐射与散射问题中的棘手问题,其方法简练,理论完备,结果与有关文献吻合良好,证实了该方法的正确性和通用性。     

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     

Radial field retrieval in spherical scanning for currentreconstruction and NF-FF transformation

A near-field to far-field (NF-FF) transformation is addressed for the case of spherical scanning using equivalent magnetic currents (EMCs) and matrix methods. It is based on the decoupling of the field components and the iterative retrieval of the radial component of the electric field. The technique is applied for far-field calculation as well as for the estimation of the current distribution of the antenna under test (AUT) using spherical near-field facilities. Results from measured near-field data of several antennas are presented and compared to those of the analytical solution via a spherical wave mode expansion method     

Quantitative microwave imaging with a 2.45-GHz planar microwavecamera

This paper presents microwave tomographic reconstructions of the complex permittivity of lossy dielectric objects immersed in water from experimental multiview near-field data obtained with a 2.35-GHz planar active microwave camera. An iterative reconstruction algorithm based on the Levenberg-Marquardt method was used to solve the nonlinear matrix equation which results when applying a moment method to the electric field integral representation. The effects of uncertainties in experimental parameters such as the exterior medium complex permittivity the imaging system geometry and the incident field at the object location are illustrated by means of reconstructions from synthetic data. It appears that the uncertainties in the incident field have the strongest impact on the reconstructions. A receiver calibration procedure has been implemented and some ways: to access to the incident field at the object location have been assessed     

Single integral equation for electromagnetic scattering bythree-dimensional homogeneous dielectric objects

A single integral equation formulation for electromagnetic scattering by three-dimensional (3-D) homogeneous dielectric objects is developed. In this formulation, a single effective electric current on the surface S of a dielectric object is used to generate the scattered fields in the interior region. The equivalent electric and magnetic currents for the exterior region are obtained by enforcing the continuity of the tangential fields across S. A single integral equation for the effective electric current is obtained by enforcing the vanishing of the total field due to the exterior equivalent currents inside S. The single integral equation is solved by the method of moments. Numerical results for a dielectric sphere obtained with this method are in good agreement with the exact results. Furthermore, the convergence speed of the iterative solution of the matrix equation in this formulation is significantly greater than that of the coupled integral equations formulation     

Estimation of Current Distribution on Multilayer Printed Circuit Board by Near-Field Measurement

A method of estimating the current distribution on a microstrip transmission line of a multilayer printed circuit board (PCB) by measuring the near-field distribution is proposed. The microstrip transmission line on the PCB is divided into electrically small segments. An electric field integral equation (EFIE) is established to relate the near field to the current on these segments. The current is estimated by measuring the near field, evaluating the mutual impedance between the current segments, and solving the EFIE. The measurement parameters for near-field measurement are optimized by numerical analysis. Experimental results are presented and compared with the numerical results, confirming the validity of this method.     

A singular integral representation of the electromagnetic field: Physical regularization of the ill-posedness of the near-field zones of antennas

A classification of ill-posedness in the antenna theory is presented in the case when the basic principles of electromagnetics are not fulfilled, which is observed in the near-field radiation zones. The Tikhonov regularization of Fredholm integral equations of the first kind is compared with the physical regularization (a self-consistent method). The ill-posedness arising in the analysis of the electromagnetic field (EMF) of an electric dipole, an equidistant array on its basis, frame antennas, and a cylindrical helical antenna is considered. In particular, for the EMF of an electric dipole, the gap between the field and surface current density is removed and the limits of applicability of the method of induced electromotive forces of coupled dipoles are established. It is shown that unidirectional radiation in the azimuthal plane is typical of a ring frame antenna with a certain dimension of the ring. For a cylindrical helical antenna, the calculation of the surface current density is reduced to a hypersingular integral equation with a quadratic Cauchy singularity. The analysis of the behavior of the current flowing along the helix made it possible to explain the peculiarities of the antenna pattern for various helical turns.     

Electromagnetic Scattering by Arbitrarily Shaped PEC Targets Coated with Anisotropic Media Using Equivalent Dipole-Moment Method

The equivalent dipole-moment method (EDM) is extended and applied in the analysis of electromagnetic (EM) scattering by arbitrarily shaped perfect electric conductor (PEC) targets coated with electric anisotropic media in this paper. The scattering targets are discretized into tetrahedral volume elements in the material region and into triangle patches on the conducting surface, where the volume-surface integral equation (VSIE) is set up. Then the method of moments (MoM) is employed to solve the VSIE. In the impedance matrix, the near field interaction elements are computed by the conventional MoM while the far field interaction elements are modeled by the EDM. The proposed approach is sufficiently versatile in handling arbitrarily shaped objects coated with general electric anisotropic media and is easily constructed through a simple procedure. Numerical results are given to demonstrate the accuracy and efficiency of this method.     

基于未知源域重构的无相位近场测量技术

天线的远场对于研究天线辐射特性具有重大意义,由于远场的直接测量有着诸多限制,近场测量技术计算远场因其简洁准确的特点得到广泛应用. 然而,传统的近场测量技术要求获取近场区的幅度和相位分布才能发挥作用,随着天线频率的升高,人们想要在近场区获取准确的相位信息变得十分困难. 为了解决该技术难题,文中提出一种无相位近场测量技术. 利用一个封闭面上的幅度信息重建或猜测出包围待测天线的球面切向电场分布,并采用遗传算法进行全局优化,其最初为四组随机数据,经过数次优化后将逐渐接近准确结果. 仿真结果表明,本文方法能够在忽略相位信号的前提下,计算出准确的远场辐射特性.     

Efficient analysis of electromagnetic radiation from microstrip antenna using volume-surface current contunuity method

The Volume-Surface Current Continuity Method (VSCCM) is presented to analyze electromagnetic radiation from microstrip antenna. The microstrip antenna is discretized into small triangular patches on conducting surface and tetrahedral volume cells in dielectric region. The Method of Moments (MoM) is applied to solve the integral equation. An equation contains the restriction relation between the volume and surface current coefficient is derived from the current continuity equation at those parts where the conducting surface is in contact with the dielectric material. A simple equivalent strip model is introduced in the treatment of the feeding probe in VSCCM. The VSCCM can reduce the unknowns required to be solved in MoM, as well as the condition number of the matrix equation. Numerical results are given to validate the accuracy and efficiency of this method.     

Model of a dipole antenna in free-space and near a perfectly electrical conducting plate

In this paper, an accurate method is developed to model dipole antennas in free-space or near an infinite perfectly electrical conducting plate. The magnetic field integral equation is implemented and point-matching is used to construct the interaction matrix. The edges of the cylinder, which can cause numerical instabilities in the simulated current distribution, are getting a special treatment by choosing a specific set of basis functions. The excitation is modeled based on the equivalence principle and leads to the application of a magnetic surface current over the gap region. The exact kernel of the integral equation is applied and all possible current components are taken into account. When the antenna is close to a conducting plate, a strong modification of the surface current distribution is observed. It is demonstrated that for these configurations, the incorporation of the circumferential component of the surface current distribution is needed. At larger distances, the circumferential surface current can be neglected in the total distribution. The only approximation (or rather assumption) we made is that we impose a longitudinal directed electrical field over the boundary of the gap region. The results are verified experimentally.     

Scattering from a microstrip patch

A solution to the problem of plane wave scattering by a rectangular microstrip patch on a grounded dielectric substrate is presented. The model does not include the microstrip feed, and thus does not include the so-called "antenna mode" component of the scattering. The solution begins by formulating an electric field integral equation for the surface current density on the microstrip patch. The integral equation is solved using the method of moments. Computed data for the patch radar cross section (RCS) is found to be in close agreement with measurements over a broad frequency range. The microstrip RCS versus frequency consists of a number of large peaks which are identified as impedance or pattern factor resonance peaks.     

Analysis of cavity-backed aperture antennas with a dielectricoverlay

A full-wave analysis of cavity-backed aperture antennas with a dielectric overlay is presented. The theoretical approach uses a closed-form dyadic Green's function in the spectral domain. The aperture equivalent magnetic currents are obtained using the surface equivalence theorem and an integral equation is obtained by matching the fields across the aperture. The moment method applied in spectral domain analysis is employed to solve the integral equation for the equivalent magnetic currents with proper combination of subdomain or entire domain expansion functions. Numerical results include the aperture field distribution and antenna parameters such as input impedance, bandwidth, and efficiency. A set of measurements data is compared with results based on the theoretical work     

Spatial sampling and filtering in near-field measurements

A sample spacing criterion and a data minimization technique for measurements made over the surface of a plane in the near field of an antenna are presented. The sample spacing is shown to depend on the distance from the antenna to the measurement plane, and on the extent to which evanescent waves can be neglected. The near-field data minimization technique utilizes two-dimensional spatial filtering to effect a significant reduction in computational effort required to calculate selected portions of the far-field pattern. Far-field patterns of anXband antenna calculated from near-field measurements are presented and compared with those measured on a standard far-field range. The far-field calculations are repeated for several near-field sample spacings and for various post-filter sample rates.     

Probe uncompensated near-field to far-field transformation for scanning over an arbitrary surface

A new technique of transforming the probe uncompensated near-field data to the near or far field is presented, which is useful in nearfield antenna measurements, when the probe antenna scans over an arbitrary surface enclosing the test antenna. The technique presented is analytically simple and sufficiently accurate.     

多层电厚微带天线的阻抗特性

本文提出了一种多层介质电厚微带天线的一种分析方法。从无界空间中的电并矢格林函数出发,导出了分层介质中任意一点具有任意取向的水平电偶极子场的表达式。以此为基础,利用Richmond反作用积分方程,建立了关于贴片上电流分布的积分方程。通过适当选取电流基函数,使本方法适用于电厚介质微带天线。应用Galerkin方法建立矩阵方程。借助于计算机求解该矩阵方程,可得到天线的各项特性。文中用实例进行了验证。