Near-field To Far-field Research Articles

Relativistic Bistatic Scattering of a High-Speed Moving Plasma Coated Object

Accurate modeling of relativistic electromagnetic scattering characteristics from high-speed motion of plasma coated objects is crucial for the development of hypersonic aircraft and their applications in the identification and surveillance of moving stealth targets. Nevertheless, a solution for bistatic polarized radar cross sections (RCSs) from a 3-D object with plasma coated layer in motion has yet to be obtained. This manuscript proposes a solution to this problem by employing a combination of the auxiliary differential equation (ADE) method with Lorentz finite-difference time-domain (FDTD) method. Utilizing the Lorentz transformation, this paper presents the transformation of parameters of the incident plane wave and dimensions of the object between the laboratory system that remains static and the rest system that remains stationary relative to the object in high-speed motion. The near-zone electromagnetic fields near the object are computed using the ADE method in the rest system, after which the near-field to far-field (NF-FF) transformation is employed to obtain the far-zone polarized scattered field. By applying Lorentz transformation to the coordinates, this paper presents a solution for the polarized scattering from moving plasma coated objects. Especially, radial components of the polarized scatterings are analyzed. The proposed method is validated through several numerical experiments, demonstrating its efficiency and accuracy.

Prediction of the far field radiated by a flat antenna under test from a reduced set of near‐field bi‐polar measurements

In this article, a bi-polar near-field-to-far-field (NFTFF) transformation with probe compensation, which is very convenient from the data reduction standpoint when characterising a flat antenna under test (AUT), is developed. It takes advantage from the non-redundant sampling representations of the electromagnetic fields to devise a sampling representation on the scanning plane, which requires the knowledge of the bi-polar NF samples at a reduced number of sampling points. To account for the AUT geometry, it properly adopts as modelling surface a disc having diameter equal to the largest size of such an AUT. This surface allows a more effective AUT modelling from the NF data reduction standpoint than the formerly proposed modelling surfaces for quasi-planar antennas (the oblate ellipsoid or the two-bowls), because, unlike these last, it has the capability to best fit the AUT geometry, thus reducing very significantly the involved volumetric redundancy. An ad hoc developed two-dimensional optimal sampling, interpolation formula is then employed to reconstruct the plane-rectangular NF data which would be necessary to execute the classical NFTFF transformation developed by Leach and Paris. Some results of numerical simulations and experimental proofs are reported to demonstrate the efficiency of the so developed technique.

Two approaches to get efficient NF/FF transformations from inaccurately positioned planar wide-mesh scanning samples

This paper aims to give the experimental proof of two effective procedures for the compensation of severe and pessimistic positioning errors, which alter the gathered near-field samples in a near-field to far-field (NF/FF) transformation with planar wide-mesh scanning (PWMS). This scanning is so called since it has a sampling lattice whose meshes are wider and wider as their distance from the measurement plane centre grows, so that it allows a drastic reduction of the acquisition time as compared to the classical plane-rectangular (PR) scan. The here considered NF/FF transformation with PWMS relies upon a nonredundant sampling representation of the voltage detected by the scanning probe, attained by modelling the antenna under test with a double bowl, that is, a surface got by joining together two bowls with the same circular aperture, and exploits an optimal sampling interpolation formula to precisely reconstruct, from the gathered PWMS samples, the huge NF data required by the PR NF/FF transformation. The retrieval of the NF samples at the points prescribed by the representation is got from the collected PWMS samples, corrupted by known positioning errors, by employing either a singular value decomposition based procedure or an iterative algorithm. Some experimental results, which fully assess the effectiveness of both the developed procedures, are shown.

A spherical near field to far field transform for large antenna arrays

ABSTRACTThe simulation or measurement of large antenna arrays requires a spherical near field to far field (NFFF) transform, which suffers from the underflow when calculating the high degree associated Legendre functions by double precision. This paper proposes to normalize the associated Legendre functions by the first non-zero term in the recursion and set the extremely small terms to zero. Numerical experiments show that the proposed method is much more accurate and stable than the commonly used SPHEREPACK library. Besides, the pattern of a linear array of 600 elements is evaluated by the proposed method with the truncation degree up to 985, which is validated against the Method of Moment (MoM).

Transient UWB Antenna Near-Field and Far-Field Assessment From Time Domain Planar Near-Field Characterization: Simulation and Measurement Investigations

In this paper, we present the experimental validation of time domain (TD) near-field to near-field (NFNF) and near-field to far-field (NFFF) transformations for ultrawideband (UWB) antenna transient characterization. The used computation schemes for near-field to near- or far-field transformations are based on the Green’s function representation of the radiated field where NF and FF are directly calculated in the TD. The first step of the validation process comprises the electromagnetic simulation results dedicated to evaluate the accuracy of NFFF and NFNF transformations. The second step uses near-field measurement data of a Vivaldi antenna to validate the developed computation schemes, whereas the measurement and calibration procedures are fully described. The measured NF data using Supelec planar and cylindrical near-field facilities are also compared with the electromagnetic simulation transient results of the Vivaldi antenna.

A SEMI-ANALYTICAL METHOD TO CALCULATE THE ENTRIES OF THE METHOD OF MOMENTS MATRIX FOR THE MIXED POTENTIAL INTEGRAL EQUATION OF A SOURCE RECONSTRUCTION PROBLEM

In this article, the mixed potential integral equation is discretized using the Rao-Wilton- Glisson basis functions in order to obtain a method of moments matrix equation for a source reconstruction problem. The weighting functions used in the setup of the moments equation are Dirac delta functions. The entries of the moments matrix are computed using a semi-analytical method which is applicable to any method of moments problem with point matching. The analytical calculation is made possible by employing a differentiation property of the scalar Green function and the properties of the mesh elements of the source plane. The semi-analytical method makes it easier to increase the accuracy of the moments matrix elements. The accuracy of the method is shown by comparing the results obtained using the semi-analytical method to those obtained by a fully numerical procedure. The source reconstruction for a specific electromagnetic field distribution is the procedure of either setting up equivalent electromagnetic problems in order to determine equivalent sources or determining the actual sources by solving the related integral equations. The source reconstruction method is used in several areas which can be mentioned as near-field to far-field (NF-FF) transformation, microwave imaging, antenna design, antenna characterization, antenna diagnostics, electromagnetic interference estimation in electronic circuits, corrections in antenna measurements, radome defect localization. The most important aspect of the source reconstruction method is that it takes the electromagnetic problem into consideration just as it is, with all of the real world factors included through the electromagnetic field measurement results. The literature on the source reconstruction method can be divided into two parts with respect to the type of the integral equation formulation for the problem. In the first group, a single integral equation formulation is done whereas in the second group, a dual-integral equation formulation is preferred. In the single equation formulation, the equivalent sources obtained using the equivalence principle are related to the field measurement results using the radiation integrals. In the dual equation formulation, an extra equation is set up using the Love's equivalence principle at the measurement surface. Some of the articles in the single equation formulation group are to be cited now. In (7-30), several NF-FF transformation formulations using the source reconstruction method and the related applications are studied. In (7), equivalent magnetic currents are constructed on a plane including the aperture of an antenna using the near electric field data on a plane. These currents are then used to calculate the far-field. In (8), equivalent magnetic currents are built on a plane including the aperture of an antenna again. However, the near electric field data is not necessarily to be obtained on a plane and can be gotten from a surface with an arbitrary geometry. In (9), an equivalent electric current is set up on a

Efficient reconstruction of the pattern radiated by a long antenna from data acquired via a spherical-spiral-scanning near-field facility [measurements corner

This paper provides the experimental assessment of an effective near-field-to-far-field (NF-FF) transformation technique with spherical-spiral scanning, particularly suitable for long antennas. Such a technique allows a remarkable measurement-time saving, due to the use of continuous and synchronized movements of the positioning systems, and due to the reduced number of required near-field measurements. This is made possible by a non-redundant sampling representation of the voltage measured by the probe, obtained by using the unified theory of spiral scans for non-spherical antennas, and adopting a prolate-ellipsoidal source modeling. The near-field data needed by the classical spherical near-field-to-far-field transformation are then efficiently retrieved from those acquired along the spiral by an optimal sampling interpolation formula.

An Experimental Validation of the Near-Field - Far-Field Transformation with Spherical Spiral Scan [AMTA Corner

This paper concerns the experimental validation of a probe-compensated near-field-to-far-field (NF-FF) transformation technique using spherical spiral scanning. This allows one to significantly reduce the measurement time, by means of continuous and synchronized movements of the positioning systems of the probe and antenna under test. Such a technique relies on the non-redundant sampling representations of the electromagnetic fields. It makes use of a two-dimensional optimal sampling interpolation formula to recover the near-field data needed to perform the classical spherical near-field-to-far-field transformation. The very good accuracy obtained both in the near-field and the far-field reconstructions establishes the effectiveness of the approach.

Azimuthal Mode Decomposition Method for Cubical Surface Scanning With the Application to Spherical-Mode Field Synthesis

In this letter, a method of azimuthal mode decomposition is presented for cubical surface scanning. This method can be applied either in near-field-to-far-field (NF-FF) transformation in near-field antenna measurements or in synthesizing the fields using the virtual array principle. The method is presented for spherical-wave-mode field synthesis application in this letter. The method allows a reduction in the computational burden of the matrix inversion for synthesizing the fields. Requirements for the sampling locations on the cubical scanning surface and for the probe orientations and polarizations at each sampling location allowing the azimuthal-mode decomposition are provided.

Finite-difference time-domain and near-field-to-far-field transformation in the spectral domain: application to scattering objects with complex shapes in the vicinity of a semi-infinite dielectric medium

We present the study of a spectral-domain near-field-to-far-field (NFTFF) transformation, taking into account an interface in the vicinity of a particle. This technique is associated with a three-dimensional finite-difference time-domain (FDTD) model, which solves the Maxwell equations in the time domain. Moreover, material properties are considered with the use of dispersion models. First, particular attention is paid to the description of the modeling, especially concerning the NFTFF transformation using the dyadic Green tensors. Second, several simulation cases are considered to evaluate the ability of the developed technique to model the scattering by different kinds of "particles/interface" configurations and for various illuminating waves. Then validation test cases are used in order to assess the model accuracy through comparisons with T-matrix simulations. Finally, perspectives to this work and its application to near-field detection devices are discussed.

Near-field and far-field modeling of scattered surface waves. Application to the apertureless scanning near-field optical microscopy

The detection of surface waves through scanning near-field optical microscopy (SNOM) is a promising technique for thermal measurements at very small scales. Recent studies have shown that electromagnetic waves, in the vicinity of a scattering structure such as an atomic force microscopy (AFM) tip, can be scattered from near to far-field and thus detected. In the present work, a model based on the finite difference time domain (FDTD) method and the near-field to far-field (NFTFF) transformation for electromagnetic waves propagation is presented. This model has been validated by studying the electromagnetic field of a dipole in vacuum and close to a dielectric substrate. Then simulations for a tetrahedral tip close to an interface are presented and discussed.

Probe-distortion correction for the sources reconstruction method

This paper introduces a probe-correction method for near-field antenna measurements. The proposed technique is intended to be used jointly with a sources reconstruction method for antenna diagnosis and near-field-to-far-field (NF-FF) transformation. The method is based on the introduction of the probe antenna's pattern as a weighting function into the field integral equations relating the measured near field and the antenna's equivalent sources. In this way, the equivalent currents are reconstructed without distortion due to the probe's pattern, as well as the fields calculated from those currents. The theoretical formulation and practical examples based on antenna measurements are presented in this paper.

On the Comparison Between the Spherical Wave Expansion and the Sources Reconstruction Method

A comparison between two well-known near field to far field (NF-FF) transformation methods is presented: the spherical wave expansion (SWE) and the sources reconstruction method (SRM). The goal of this paper is to show the methods performance for NF-FF transformation pointing out the advantages and limitations of each one. First, the computational cost is studied, showing its dependence with the antenna under test (AUT) electric size. Second, the NF-FF transformation accuracy of each method is compared for a fixed AUT electric size, analyzing also the influence of the main parameters that affect the SRM and SWE accuracy. Third, the SRM capabilities are presented, with focus on the NF-NF transformation inside the minimum sphere. Practical applications of the SWE and SRM are illustrated by means of an example of NF-FF transformation and antenna diagnostics problem using measured NF data.

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.

Comparison of Different Heuristic Optimization Methods for Near-Field Antenna Measurements

A comparison between different modern heuristic optimization methods applied to antenna far-field radiation pattern reconstruction from planar near-field data is presented in this paper. The antenna under test is represented by means of equivalent magnetic currents (EMC) whose components are optimized using several heuristic algorithms such as simulated annealing (SA), genetic algorithms (GA), and particle swarm optimization (PSO), as well as a traditional local optimization method, the Nelder Mead downhill simplex algorithm. Several schemes for GA (classical real-valued and binary encoding, and their hybrid versions) and PSO (global or local topologies with synchronous or asynchronous updates of the swarm) have been considered in the analysis and the pros and cons of each one are reported and discussed. A study of the performance and limitations of each algorithm using as a canonical problem a small size antenna aperture, along with results of near-field to far-field (NF-FF) transformation are also included

A Near-Field to Far-Field Transformation for Spheroidal Geometry Utilizing an Eigenfunction Expansion

This work presents a near-field to far-field (NF-FF) transformation for antenna and scatterer radiation evaluation. The transformation allows practical computation by making use of a sampling surface in the near-field that is spheroidal in shape: namely a prolate or oblate spheroid. The resulting vector wave equation does not support orthogonal vector solutions in spheroidal coordinates and instead rectangular field components are solved for using the scalar wave equation in spheroidal coordinates. The new transformation only requires knowledge of the completely-specified near-field electric field along the spheroidal transformation surface and does not need any information associated with the corresponding magnetic field. The benefit of using a spheroidal near-field geometry is its ability to closely conform to both linear and planar radiating structures while still permitting evaluation of the full far-field radiation pattern. Our approach makes use of an eigenfunction expansion of spheroidal wave-harmonics to develop two distinct, yet closely related, NF-FF transformation algorithms for each type of spheroidal surface. The spheroidal NF-FF transformation is validated and performance assessed using a well-characterized radiation structure. By applying the prolate and oblate algorithms to a radiating structure with known analytical near- and far-field electric fields, viz., a filament dipole with sinusoidal current distribution, we are able to setup and conduct multiple numerical tests that serve as a proof-of-concept for the spheroidal NF-FF transformation.

Radial field retrieval in spherical scanning for current reconstruction 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.

An efficient technique for spherical near-field to far-field transformation and evaluation of far fields of perfectly conducting scatterers

An efficient technique for probe-directivity-uncompensated, spherical, near-field-to-far-field (NF-FF) transformation is presented. The technique does not make use of the spherical vector waver functions (SVWF) in the conventional manner, but seeks to obtain the NF-FF transformation exactly, viewing it as an electromagnetic scattering problem. The problem is formulated by considering the NF-FF transformation, when the FF observation points are restricted to a single phi -plane at a time. The analytical expressions derived toward this end assume a considerably simplified form offering a high level of computational efficiency. The technique is validated by considering arrays as test antennas with a maximum dimension of 150 lambda . Based on the principles employed for the spherical NF-FF transformation, an efficient technique for computing the FF patterns of a perfectly conducting scatterer illuminated by an arbitrary source using the technique of physical optics (PO) is also developed. In order to validate this, typical numerical computations are performed, and the results are discussed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Evaluation of Fourier transform integrals using FFT with improved accuracy and its applications

A new technique that significantly minimizes the aliasing error encountered in the conventional use of the fast Fourier transform (FFT) algorithms for the efficient evaluation of Fourier transforms of spatially limited functions (such as those that occur in the radiation pattern analysis of reflector antennas and planar near field to far field (NF-FF) transformation) is presented and illustrated through a typical example. Employing this technique and a discrete Fourier series (DFS) expansion for the integrand, a method for computing the radiation integrals of reflector antennas and planar NF-FF transformation integrals at arbitrary observation angles with optimum use of computer memory and time is also described.