Near-field Far-field Transformation Research Articles

A Multi-Probe Antenna Measurement Technique With Echo Suppression Capability

Multipath propagation limits the obtainable accuracy of antenna measurements in non-anechoic environments. For reliable results of the antenna radiation pattern, appropriate postprocessing of the acquired data should be performed in order to separate the line-of-sight signal from the multipath signals. In this paper, a measurement technique which facilitates the removal of multipath contributions during data postprocessing is described. This is achieved by measuring additional field data with multiple probe antennas such that enough information is available to resolve the direct contributions of the antenna under test and the multipath signals. The data so acquired is then processed by a well adapted plane wave based near-field far-field transformation algorithm, with the echo sources integrated by means of auxiliary sources. The performance of the technique is demonstrated using simulated and true echoic measurements for spherical and planar measurement geometries.

A fully probe corrected near-field far-field transformation technique employing plane-wave synthesis

Abstract. The far-field behavior of an antenna under test (AUT) can be obtained by exciting the AUT with a plane wave. In a measurement, it is sufficient if the plane wave is artificially generated in the vicinity of the AUT. This can be achieved by using a virtual antenna array formed by a probe antenna which is sequentially sampling the radiating near-field of the AUT at different positions. For this purpose, an optimal filter for the virtual antenna array is computed in a preprocessing step. Applying this filter to the near-field measurements, the far-field of the AUT is obtained according to the propagation direction and polarization of the synthesized plane wave. This means that the near-field far-field transformation (NFFFT) is achieved simply by filtering the near-field measurement data. Taking the radiation characteristic of the probe antenna into account during the synthesis process, its influence on the NFFFT is compensated. The principle of the plane-wave synthesis and its application to the NFFFT is presented in detail in this paper. Furthermore, the method is verified by performing transformations of simulated near-field measurement data and of near-field data measured in an anechoic chamber.

Efficient Near-Field Far-Field Transformation for Nonredundant Sampling Representation on Arbitrary Surfaces in Near-Field Antenna Measurements

A nonredundant sampling representation on arbitrary surfaces in near-field antenna measurements together with a direct near-field far-field transformation is described. The utilized sampling criterion depends on the electrical size of the antenna under test (AUT) along with the separation between the AUT and the measurement surface. The minimum number of samples required is directly linked with the number of unknowns in the linear system of equations derived considering spherical expansion of radiated AUT fields. Equivalent plane wave sources are used to represent the AUT fields. Plane wave translations can be directly performed for any arbitrary measurement grid with the same efficiency. Thus, nonredundant sampling representations can be utilized on any scan surface. In contrast, other fast field transformation techniques would lose their efficiency. The proposed sampling is applied to near-field measurements and synthetic data using a variety of antennas and the results are compared to the case of standard sampling. Remarkable decrease in the number of measurement points is observed with a negligible change in the accuracy of the transformed pattern, thereby greatly reducing the measurement time and the computational effort.

Experimental Testing of Nonredundant Near-Field to Far-Field Transformations with Spherical Scanning Using Flexible Modellings for Nonvolumetric Antennas

This paper deals with the experimental testing of effective probe compensated near-field-far-field (NF-FF) transformations with spherical scanning requiring a minimum number of NF data. They rely on nonredundant sampling representations of the voltage measured by the probe, based on very flexible source modellings suitable for nonvolumetric antennas characterized by two dimensions very different from the other one. In particular, a cylinder ended in two half-spheres is adopted for modelling long antennas, whereas the quasi-planar ones are considered as enclosed in a rotational surface formed by two circular “bowls” having the same aperture diameter, but eventually different bending radii. The NF data needed to perform the classical spherical NF-FF transformation are then accurately and efficiently retrieved from the acquired nonredundant ones via optimal sampling interpolation formulas. A remarkable reduction of the number of the required NF data and, as a consequence, a significant measurement time saving can be so obtained. The experimental tests have been carried out at Antenna Characterization Lab of the University of Salerno and both the NF and FF reconstructions are resulted to be very good, thus confirming the accuracy and reliability of these NF-FF transformations from the experimental viewpoint too.

Experimental Assessment of an Effective Near-Field–Far-Field Transformation With Spherical Spiral Scanning for Quasi-Planar Antennas

Experimental tests on a near-field-far-field (NF-FF) transformation technique with spherical spiral scanning suitable for quasi-planar antennas are presented in this letter. Such a technique is based on the nonredundant sampling representations of the electromagnetic fields and employs an oblate ellipsoid to model the antenna under test. A two-dimensional optimal sampling interpolation formula is then employed to recover the NF data at any point on the scanning sphere and, in particular, at those needed by the classical spherical NF-FF transformation. It is so possible to lower the number of needed measurements, as well as the time required to acquire the data. Both the reported NF and FF reconstructions assess the effectiveness of the technique.

Scattering centers modeling of non-anechoic measurement environments

Abstract. Near-field measurements are established techniques to obtain the far-field radiation pattern of an Antenna Under Test via near-field measurements and subsequent near-field far-field transformation. For measurements acquired in echoic environments, additional post-processing is required to eliminate the effects of multipath signals in the resulting far-field pattern. One of such methods models the measurement environment as a multiple source scenario whereby the collected near-field data is attributed to the AUT and some scattering centers in the vicinity of the AUT. In this way, the contributions of the AUT at the probe can be separated from those of the disturbers during the near-field far-field transformation if the disturber locations are known. In this paper, we present ways of modeling the scattering centers on equivalent surfaces such that echo suppression is possible with only partial or no information about the geometry of the scatterers.

Far-Field Pattern Reconstruction from Positioning Errors Affected Near-Field Data Acquired via Helicoidal Scanning

In this paper, an effective technique to compensate the positioning errors in a near-field—far-field (NF-FF) transformation with helicoidal scanning for elongated antennas is presented and validated both numerically and experimentally. It relies on a nonredundant sampling representation of the voltage measured by the probe, obtained by considering the antenna as enclosed in a cylinder ended in two half-spheres. An iterative scheme is used to reconstruct the helicoidal NF data at the points fixed by the representation from the acquired irregularly spaced ones. Once the helicoidal data have been retrieved, those needed by a classical NF-FF transformation with cylindrical scanning are efficiently evaluated by using an optimal sampling interpolation algorithm. Some numerical tests, assessing the accuracy of the approach and its stability with respect to random errors affecting the data, are reported. Experimental tests performed at the Antenna Characterization Lab of the University of Salerno further confirm the validity of the proposed technique.

Two Techniques for Compensating the Probe Positioning Errors in the Spherical NF-FF Transformation for Elongated Antennas

Two effective approaches for compensating the positioning errors in a near-field-far-field (NF-FF) transformation technique with spherical scanning for long antennas using a nonredundant number of data are presented. The transformation technique relies on the nonredundant sampling representations of the electromagnetic fields and on the optimal sampling interpolation (OSI) expansions, and assumes the antenna under test as enclosed in a prolate ellipsoid, a source modelling particularly suitable to deal with elongated antennas. In order to evaluate the NF data at the points fixed by the nonredundant representation from the acquired irregularly spaced ones, the former approach employs the singular value decomposition method, whereas the latter makes use of an iterative technique. The former can be applied when the irregularly samples lie on nonuniform parallels, thus allowing to reduce the starting two-dimensional problem into two independent one-dimensional ones. The latter can be employed also when such a hypothesis does not hold, but requires the existence of a one-to-one correspondence associating at each uniform sampling point the nearest irregular one. In both the cases, the NF data needed by a probe compensated NF-FF transformation with spherical scanning are efficiently evaluated by using an OSI algorithm. Numerical tests assessing the effectiveness of the proposed approaches and their stability with respect to random errors affecting the NF data are shown.

Hybrid Fast Fourier Transform-plane wave based near-field far-field transformation for “body of revolution” antenna measurement grids

[1] Near-field measurement and transformation techniques are widely applied to characterize radiation patterns of antennas. Spherical and cylindrical near-field measurements have been researched extensively and various techniques with different probe compensation capabilities and complexities exist. Among those techniques applicable for (almost) arbitrary probes, the crucial computational efficiency has been achieved through the use of Fast Fourier Transform based preprocessing of the measurement data. It is shown in this paper that the Fast Fourier Transform based preprocessing can also be utilized in conjunction with the plane wave based fully probe-corrected near-field far-field transformation with low numerical complexity. The collection of probe signals is split into smaller subsets for individual orthogonal azimuthal Fourier modes by an Inverse Fast Fourier Transform. These smaller subsets can be transformed to the far field very efficiently with full probe correction. The technique presented in this paper is applicable for arbitrary “body of revolution” antenna measurement grids, including the important cases of cylindrical and spherical measurement grids. The “body of revolution” grids are rotationally symmetric around the z-axis and the probe signals must be available equidistantly in ϕ.

Cylindrical near-field far-field transformation in a half-space with conditions of dielectric and lossy ground

The direct measurement of the radiation pattern of an antenna is often unreachable. A near-field sampling followed by a near-field/far-field (NF/FF) transformation is therefore substituted. This process is, in most cases, applied to antennas placed in free-space or over a perfect electric conductor. On the contrary, few results concern real environment conditions. Nevertheless, it is well known that the nature of the soil can drastically change the radiating characteristic of an antenna. The lack of information about the behavior of the antenna in real conditions can give rise to substantial errors in the evaluation of the performances of electromagnetic devices. The aim of this paper is then to propose a new NF/FF transformation able to reliably supply the field radiated by an antenna placed over a plane, homogeneous, dielectric, and lossy interface. The method is based on the cylindrical-wave expansion of the electric field. The real soil is taken into account by means of its reflection coefficients. First results are given for simple antennas and for both transverse magnetic and transverse electric polarizations.

Assessment of Irregular Sampling Near-Field Far-Field Transformation Employing Plane-Wave Field Representation

Near-field antenna measurements are accurate and common techniques for determining the radiation pattern of an antenna under test. The minimum near-field sampling rate is dictated by the electrical size of the antenna. Equidistant sampling is usually applied for planar, cylindrical, and spherical measurements. Certain applications either rely on or benefit from near-field sampling on irregular grids. To handle irregular measurement grids, near-field transformation algorithms such as equivalent-current methods or the multilevel fast multipole accelerated plane-wave based technique are required, which do not rely on regularly sampled data. In this contribution, the plane-wave-based near-field transformation is applied to spherical, cylindrical, and “combined” near-field measurements employing irregular sampling grids. The performance is assessed by various simulated near-field-measurement scenarios.

Photonic Probes and Advanced (Also Phaseless) Near-Field Far-Field Techniques

We present innovative near-field test ranges, named compact-near-field (CNF) and very-near-field (VNF). These use photonic probes, and advanced near-field far-field (NFFF) transformations from amplitude and phase (complex) or phaseless measurements. The photonic probe allows AUT-probe distances of less than one wavelength. This drastically reduces test-range and scanner dimensions, improves the signal-to-clutter ratio and the signal-to-noise ratio, and reduces the scanning area and time. In both the cases of complex and phaseless measurements, the neat-field-to-far-field transformation problem is properly formulated to further improve the rejection of clutter, noise, and truncation error. The advantages of the compact-near-field and very-near-field test ranges are discussed and numerically analyzed. Experimental results are presented for both planar and cylindrical scanning geometries.

Phaseless spherical near-field antenna measurements for low and medium gain antennas

Abstract. The characterisation of antenna radiation patterns by measurements in the near-field and a following near-field far-field transformation require accurate amplitude and phase data. Especially at higher frequencies phase measurements are demanding in terms of instrumentation and measurement accuracy. Phaseless techniques which require amplitude only data on one or more measurement surfaces are therefore of special interest. In this paper a phaseless spherical algorithm based on spherical modal expansion is presented. The algorithm works with amplitude only near-field data measured on two spheres of different radii and is therefore not restricted to certain types of antennas as for planar measurements for example. Simulated as well as measured results of low and medium gain antennas are shown.

Near-field to far-field transformation utilising multilevel plane wave representation for planar and quasi-planar measurement contours

A fully probe-corrected near-field far-field transformation employing plane wave expansion and diagonal translation operators has recently been presented, which is able to efficiently carry out transformations for arbitrary measurement point locations. Here, the algorithm is discussed in its multilevel version for the application to planar and quasi-planar measurement contours. The measurement points are grouped in a multilevel box structure and translations of the plane waves, representing the antenna under test, are only carried out to the box centres on the highest level. The plane waves are further processed through the different levels towards the measurement points with a decreasing sampling rate using a disaggregation and anterpolation procedure. This reduces the overall complexity to O(N q log N), N being the total number of measurement points, which is comparable to the classical fast Fourier transform-based algorithms for planar measurements. The proposed algorithm includes a full correction of arbitrary near-field probes and is able to cope with irregular measurement grids and is further optimised for highly directive antennas within this contribution.

Hybrid multilevel plane wave based near-field far-field transformation utilising combined near- and far-field translations

Abstract. The radiation of large antennas and those operating at low frequencies can be determined efficiently by near-field measurement techniques and a subsequent near-field far-field transformation. Various approaches and algorithms have been researched but for electrically large antennas and irregular measurement contours advanced algorithms with low computation complexity are required. In this paper an algorithm employing plane waves as equivalent sources and utilising efficient diagonal translation operators is presented. The efficiency is further enhanced using simple far-field translations in combination with the expensive near-field translations. In this way a low complexity near-field transformation is achieved, which works for arbitrary sample point distributions and incorporates a full probe correction without increasing the complexity.

Multilevel Plane Wave Based Near-Field Far-Field Transformation for Electrically Large Antennas in Free-Space or Above Material Halfspace

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> A recently presented fully probe-corrected near-field far-field transformation employing plane wave expansion and diagonal translation operators enables near-field far-field transformation for arbitrary measurement contours and arbitrary antennas. A multilevel extension, inspired by the multilevel fast multipole method, is presented that is suitable for the efficient transformation of electrically large antennas with a size of tens or even hundreds of wavelengths. The measurement points are grouped in a multilevel fashion and translations are carried out to the box centers on the highest level only. The plane waves are processed through the different levels to the measurement points using a disaggregation and anterpolation procedure resulting in a reduced overall complexity. In the second part of this paper, the influence of perfectly conducting ground planes and dielectric halfspaces, as an approximation for ground effects in a real measurement setup, is investigated. As such ground reflected waves are assumed, which propagate from the investigated antenna to the field probe and add to the direct wave contributions. The far-field conditions required for these assumptions are achieved by a source box grouping scheme. By this extension ground effects are directly considered within the near-field far-field transformation. Transformation results using simulated and measured near-field data are shown. </para>

THE UNIFIED THEORY OF NEAR-FIELD-FAR-FIELD TRANSFORMATIONS WITH SPIRAL SCANNINGS FOR NONSPHERICAL ANTENNAS

The unified theory of near-field–far-field transformations with spiral scannings for quasi-spherical antennas is extended in this paper to the case of nonspherical ones, i.e., antennas with two dimensions very different from the third one. To this end, these antennas are no longer considered as enclosed in a sphere, but in a proper convex domain bounded by a rotational surface. The extension, heuristically derived by paralleling the rigorous procedure valid for the spherical source modelling, allows one to overcome its main and serious drawbacks. In fact, the corresponding near-field–farfield transformations use a reduced number of near-field measurements and, above all, allow one to consider measurement surfaces at a distance smaller than one half the antenna maximum size, thus remarkably reducing the error related to the truncation of the scanning zone. These are very important features, which make the spiral scannings more and more appealing from the practical viewpoint. Some examples of the application of this theory to spirals wrapping the conventional scanning surfaces employed in the near-field–far-field transformations are reported, and the accuracy and robustness of the far-field reconstructions are assessed.

Extrapolation of the Outside Near-Field Data in the Cylindrical Scanning

This article deals with an effective technique for reducing, in a significant way, the error related to the truncation of the scanning zone in the cylindrical near-field–far-field transformation. Such a reduction is obtained by extrapolating a proper number of data external to the near-field measurement region via the singular value decomposition method. The approach is based on the nonredundant representations of the electromagnetic field and on the optimal sampling expansions. Numerical examples are reported for assessing the accuracy of the technique and its stability with respect to random errors affecting the data.

Fully Probe-Corrected Near-Field Far-Field Transformation Employing Plane Wave Expansion and Diagonal Translation Operators

Near-field antenna measurements combined with a near-field far-field transformation are an established antenna characterization technique. The approach avoids far-field measurements and offers a wide area of post-processing possibilities including radiation pattern determination and diagnostic methods. In this paper, a near-field far-field transformation algorithm employing plane wave expansion is presented and applied to the case of spherical near-field measurements. Compared to existing algorithms, this approach exploits the benefits of diagonalized translation operators, known from fast multipole methods. Due to the plane wave based field representation, a probe correction, using directly the probe's far-field pattern can easily be integrated into the transformation. Hence, it is possible to perform a full probe correction for arbitrary field probes with almost no additional effort. In contrast to other plane wave techniques, like holographic projections, which are suitable for highly directive antennas, the presented approach is applicable for arbitrary radiating structures. Major advantages are low computational effort with respect to the coupling matrix elements owing to the use of diagonalized translation operators and the efficient correction of arbitrary field probes. Also, irregular measurement grids can be handled with little additional effort.

Directivity Computation by Spherical Spiral Scanning in Near Field Region

A procedure for evaluating the directivity of an antenna under test from near-field measurements on a spherical scanning surface is proposed. It uses a formula in closed form for computing the total radiated power from the knowledge of the radiation intensity samples on the far-field sphere. These last are determined via an efficient probe compensated near-field–far-field transformation technique based on a nonredundant sampling representation of the probe voltage. The samples are collected on a spiral wrapping the near-field spherical surface from pole to pole. With respect to the classical spherical scanning, this innovative way of collecting the data allows the reduction of the time required for the samples acquisition by means of a continuous movement of the positioning systems. The effectiveness of the proposed procedure is assessed by numerical simulations.