Near-to-far-field Transformation Research Articles

Reconstruction of the Far-Field Pattern of Volumetric AUTs from a Reduced Set of Near-Field Samples Collected along a Planar Spiral with a Uniform Step.

An efficient near-to-far-field transformation (NTFFT) technique, wherein the near-field (NF) measurements are acquired along a planar spiral with a uniform step to make the control of the involved positioners easier, is developed in this article. Such a technique is tailored for quasi-spherical, i.e., volumetric, antennas under test and makes use of a reduced number of NF data. An effective two-dimensional sampling interpolation algorithm, allowing the accurate reconstruction of the input NF data for the standard NTFFT with plane-rectangular scan, is obtained by setting the spiral step equal to the sample spacing required for interpolating along a radial line according to the spatial bandlimitation properties of electromagnetic fields, and by properly developing a non-redundant representation along such a spiral. Tests results are reported to demonstrate that the proposed NTFFT technique retains the same accuracy as the standard plane-rectangular one.

A Spherical Near-to-far-Field Transformation Using a Non-Redundant Voltage Representation Optimized for Non-Centered Mounted Quasi-Planar Antennas

This research falls in the antenna measurements related topic, and deals with the problem occurring in the classical spherical near-to-far-field (NTFF) transformation, when it becomes unpractical to mount the antenna under test (AUT) with its center at the center of the scanning sphere. This issue reflects in a growth of the number of near-field (NF) samples to be acquired, since this number depends on the radius of the minimum sphere, which contains the antenna, and is centered at the scanning sphere center. The non-redundant sampling representations of the electromagnetic field are conveniently exploited, to develop an effective spherical NTFF transformation for non-centered AUTs with quasi-planar geometry, requiring a minimum amount of NF samples, and nearly the same as that for a centered mounting of the AUT. Then, the NF data needed to perform the classical NTFF transformation are determined in efficient way from the acquired non-redundant NF samples by employing an accurate 2-D sampling interpolation scheme. Thus, it is possible to significantly save measurement time. Some simulation and laboratory results are reported to show the effectiveness of the developed technique, which takes into account a non-centered AUT mounting.

Reconstruction of the far field radiated by an offset mounted volumetric AUT from non‐redundant spherical spiral near‐field measurements

This work provides an experimental assessment of the near to far-field transformation (NTFFT) with spherical spiral scan, which properly accounts for the mounting of a volumetric AUT in offset configuration. Such a NTFFT, based on the nonredundant sampling representations of electromagnetic fields using a spherical modelling of the antenna under test (AUT), requires the same number of near-field (NF) spiral data when the AUT is mounted both in onset and offset configuration, because this number depends only on the area of the modelling sphere. The reported experimental results prove the effectiveness of the developed NTFFT and of the related two-dimensional optimal sampling interpolation scheme used to reconstruct, from the spiral NF samples, the NF data at the raster grid points required by the classical spherical NTFFT.

An SVD-Based Approach to Reconstruct the Planar Wide-Mesh Scanning NF Data From Inaccurately Probe-Positioned Samples

A laboratory validation of an efficient singular value decomposition (SVD) approach to correct known errors in the probe positioning, affecting a nonconventional plane-rectangular near-to-far-field transformation (NTFFT) with the planar wide-mesh scanning, is here given. Such an NTFFT uses a reduced number of near-field data since those required by the standard plane-rectangular one are precisely recovered from them through a nonredundant representation of the voltage measured by the probe, achieved by considering the antenna as contained in an oblate spheroid. The values of the voltage at the sample points dictated by the representation are unknown and are efficiently retrieved from the positioning-errors affected ones through the SVD-based procedure. Some results of experimental tests, carried out at the University of Salerno and validating the efficacy of the proposed approach, are reported.

Correction of Known Position Errors in a Spherical Near to Far-Field Transformation for Long Antennas

Background: This paper provides the experimental validation of an efficient iterative procedure to correct known position errors in a spherical near to far-field (NTFF) transformation for elongated antennas which uses a minimum number of NF measurements. Method: This transformation exploits a non-redundant sampling representation of the voltage detected by the probe obtained by shaping a long antenna with a prolate ellipsoid. The uniform samples, those at the points set by the representation, are accurately reconstructed from the acquired not regularly distributed (non-uniform) ones by using an iterative scheme, which requires a one to one relationship between each uniform sampling point and the corresponding non-uniform one. Then a 2-D optimal sampling formula is adopted to evaluate the input data needed to perform the traditional spherical NTFF transformation from the retrieved non-redundant uniform samples. Conclusion: Finally, laboratory proofs have been reported to demonstrate the validity of the presented technique from a practical viewpoint.

A boundary integral method for numerical computation of radar cross section of 3D targets using hybrid BEM/FEM with edge elements

This contribution presents the boundary integral formulation for numerical computation of time-harmonic radar cross section for 3D targets. Method relies on hybrid edge element BEM/FEM to compute near field edge element coefficients that are associated with near electric and magnetic fields at the boundary of the computational domain. Special boundary integral formulation is presented that computes radar cross section directly from these edge element coefficients. Consequently, there is no need for near-to-far field transformation (NTFFT) which is common step in RCS computations. By the end of the paper it is demonstrated that the formulation yields accurate results for canonical models such as spheres, cubes, cones and pyramids. Method has demonstrated accuracy even in the case of dielectrically coated PEC sphere at interior resonance frequency which is common problem for computational electromagnetic codes.

Fast and Accurate Far-Field Prediction by Using a Reduced Number of Bipolar Measurements

This letter gives the experimental assessment of a near-to-far-field transformation (NTFFT) technique allowing the fast and accurate reconstruction of the antenna far field from a nonredundant, i.e., minimum number of near-field (NF) data collected through a bipolar scanning. Such a NTFFT relies on a nonredundant sampling representation of the voltage detected by the scanning probe, attained by shaping the antenna with an oblate ellipsoid, a model suitable to deal with quasi-planar antennas, as typically are the measured ones in a planar NF facility. The NF data necessary to execute the traditional plane-rectangular NTFFT are then effectively recovered from the acquired (nonredundant) bipolar ones by exploiting a 2-D optimal sampling interpolation formula. This makes possible to achieve a remarkable acquisition time saving, which is a very relevant outcome, because this time is nowadays much greater than that needed to carry out the NTFFT.

On the FDTD Near-to-Far-Field Transformations for Weakly Scattering Objects

Recently published papers on the near-to-far-field (NTFF) transformation in the finite-difference time-domain (FDTD) method have demonstrated accuracy improvements for weakly scattering objects. The improvements are achieved by modifications of the “standard” NTFF transformation which is based on interpolation of the spatially shifted fields in the FDTD grid. In this paper, it is shown that accurate results can be obtained for such weakly scattering objects without the suggested improvements, provided that the correct FDTD NTFF transformation is used. “Correct” is referred to a transformation procedure that is consistent with the FDTD version of the equivalence principle. This NTFF transformation procedure was published in the IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION in 1998. Recently published results for weakly scattering objects have been reproduced using this NTFF transformation.

Correction to “An RWG Basis Function Based Near-to Far-Field Transformation for 2-D Triangular-Grid Finite Difference Time Domain Method”

Correction to “An RWG Basis Function Based Near-to Far-Field Transformation for 2-D Triangular-Grid Finite Difference Time Domain Method”

An RWG Basis Function Based Near- to Far-Field Transformation for 2-D Triangular-Grid Finite Difference Time Domain Method

A near- to far-field transformation (NTFT) based on the Rao-Wilton-Glisson (RWG) basis function is proposed for a 2-D triangular-grid FDTD method. The surface equivalence theorem is applied and the equivalent current density, J/sub z/ in the TM/sub z/ case or M/sub z/ in the TE/sub z/ case, on the radiation contour is interpolated by using RWG basis functions. The interpolation is valid for the NTFT in either the time or frequency domain. Formulations for the interpolation scheme and the NTFT are presented for the TM/sub z/ case. The scattering by an infinite dielectric circular cylinder due to an electric line source is simulated by using the triangular-grid FDTD as well as the proposed NTFT. Numerical results in both the time domain and frequency domain are demonstrated and verified by comparison with analytical solutions.