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The sampling of the field first-order spatial derivative, in addition to the field itself, enables an increase of the sampling step to twice that of the standard sampling criterion – and thus facilitates a reduction of the measurement time. Here, we investigate so-called derivative probes and their usage for spherical near-field antenna measurements.
The experimental characterization of vehicular wireless power transfer (WPT) systems, requires the measurement of all three components of the near magnetic field. The in-situ characterization of dynamic WPT systems requires the near-simultaneous measurement of these quantities. A WPT system can produce an intense near electric field and hence substantial electric field rejection is required of the magnetic field probes. A conventional shielded loop can provide this rejection, but co-locating three such loops with orthogonal axes presents several challenges. Here such a three-axis shielded loop is presented along with a detailed analysis of the isolation between the component loops and the electric field rejection. Discussion of the use of the probe in dynamic WPT characterization is provided.
In this paper we introduce a novel technique for the efficient production test and measurement of 5G, Massive MIMO array antennas for the purpose of verification, diagnostics, and fault detection that drastically reduces the number of measurements required and the associated acquisition time needed. The technique utilises compressive sensing and sparse sampling combined with a total variation measurement approach that enforces the requisite sparsity on the problem. In this paper, we compare this new spherical near-field total-variation based acquisition approach with the authors existing, analogous, planar technique. Extensive performance comparisons are presented which aggregate results across many test cases which is a necessity, and a consequence of the statistical nature of the compressive sensing technique that is imposed by virtue of the requirements of the Restricted Isometry Property (RIP). Crucially, this paper identifies and addresses a fundamental flaw within the application of many total-variation based methods and especially when used with the difference field between a reference antenna and a production test antenna. This extends the use of a novel analysis process that incorporates an l0 based minimisation strategy to overcome this problem thereby restoring the CS process to very nearly the levels of performance attained in our prior work.
An adaptable patch-like antenna element has been designed, tested, and implemented across several airborne flight applications. The element design improves upon the limited bandwidth of a typical patch antenna on a simple ground plane or within a cavity. The element position within the cavity, in conjunction with the distance from the opposing wall, can be used as degrees of freedom in steering gain along the direction of primary polarization. In addition, the Mono-Patch has been used in RF antenna test couplers for validation of section level and full vehicle RF system testing. When enclosed in shielded enclosures used for near-field coupling, the Mono-Patch antenna element has proven less sensitive to electrical loading than other candidate antennas. Antenna elements of this type have been designed to support both linear and circular polarization. These elements can be used in both single and multi-element array configurations for applications in L, S, and C frequency bands.
Planar near-field (PNF) acquisition always samples a limited or truncated subset of the infinite plane in front of an antenna under test (AUT). That truncation of the sampled field has two primary impacts: Power radiating to or from angles beyond the probing boundary is not fully captured and thus not included and erroneous ripple is injected throughout the pattern when the transformation algorithm sees by default a sudden drop to zero power beyond that boundary. The topic of truncation mitigation in the PNF geometry has been addressed with a variety of algorithms. This paper introduces a new algorithm that is similar to, yet distinct from, some that have come before. The new algorithm makes use of the recently introduced holographic PNF filter [1][2][3], treating the truncation effects like stray signals. Where the most common technique [2][4] uses the known planar-AUT bounds (2D) and a computed “valid region” of the plane-wave spectrum as truth through iterative transformations, this algorithm treats as truth the known AUT volume (3D) and the measured PNF data. The new algorithm is evaluated herein by retruncating a large set of measured PNF data.
The impact of receiver internal leakage on planar near-field measurement uncertainty is significantly influenced by the selection of near-field parameters. Understanding how specific scan parameters affect the far-field leakage level is essential for effective mitigation. This paper establishes quantitative relationships between near-field parameters and the far-field peak amplitudes of both leakage and the antenna under test (AUT), as well as the mean noise level in the far-field pattern, based on empirical results. Systematic scans were performed by altering only one or two specified near-field setup parameters per measurement, and graphical comparisons are provided. Practical approaches for mitigating receiver leakage are demonstrated through a case study involving receiver leakage on a planar scanner with a maximum scan area of 3.6 m x 3.6 m (12 ft x 12 ft). Additionally, a method for estimating the far-field receiver leakage level relative to the beam peak is discussed.
This paper presents an overview of the induced ripples observed in the far-field antenna patterns of the Antenna Under Test (AUT) when measured with an open-ended waveguide antenna probe in a near-field planar system. The author hypothesized that induced ripples in far-field patterns are primarily originated by diffracted fields on the ground plane that supports the collar absorber. This study systematically evaluates the effects of absorber size and quality. Numerical simulations and experimental measurements are employed to validate the author’s hypothesis, providing insights into the relationships between these parameters and their influence on the induced ripples in far-field patterns. Results indicate that collar absorbers with reflectivity better than -30 dB are optimal for achieving accurate element characterization of phased array antennas.
The standard Near-Field antenna characterization allows to reconstruct the Far-Field pattern over the whole visible domain, even if, in many cases, the partial characterization of the Far-Field pattern just along some cuts can be sufficient, and becomes preferred if realized in shorter measurement time with respect to the standard case. A method for Partial Characterization has been proposed. The approach provides a general framework and defines the optimal distribution of the near-field samples required to reconstruct the Far-Field pattern along the cut of interest. The main features of the method are presented, and the performance is verified, experimentally, for two test cases.
In this work, an effective procedure to compensate for 3-D mispositioning errors of the probe, occurring when characterizing a long antenna through a non-redundant (NR) near to far-field (NTFF) transformation with helicoidal scan, is developed. The pro- posed technique involves two steps. The former allows the correction of the mispositioning errors, caused by the deviation of each sampling point from the nominal measurement cylindrical surface, using a phase correction technique called Cylindrical Wave correction. The latter restores the samples at the sampling points required by the NR representation along the scan helix from the previous ones affected by 2-D mispositioning errors, via an iterative scheme. Finally, the compensated near-field samples are effectively interpolated via an optimal sampling interpolation (OSI) formula to accurately recover the input data required to perform the traditional cylindrical NTFF transformation. The OSI representation is here developed by assuming a long antenna under test as enclosed in a cylinder terminated by two half spheres (rounded cylinder), in order to make the representation effectively non-redundant. Numerical results, assessing the effectiveness of the proposed technique, are reported.
It is well-known that any structure in the proximity of a radiating antenna will affect its radiation pattern. This is one of the reasons that a vehicle-mounted antenna tends to be tested while mounted onto the actual vehicle. There is a current discussion regarding how the vehicle should be tested. Traditionally, metallic turntables are used, with the tested vehicle resting on this conductive half-space. Several new spherical near field (SNF) ranges elevate the vehicle over a floor treated with RF absorber to obtain a quasi-free-space pattern. Discussions regarding which method is better are on-going. One of the arguments in favor of the free-space SNF range approach is that, using computational methods, the equivalent radiating currents that radiate the measured near fields, be it over a spherical surface or a non-canonical surface, can be computed. The equivalent radiating currents computed on a triangular element mesh are then imported onto a quadrilateral element mesh on a higher order basis function method of moments (HOBF-MoM) package. Once imported into the HOBF-MoM these currents can be used as excitations to obtain the far field. Within the HOBF-MoMit it is possible to place these equivalent currents in the presence of a metallic (PEC) using symmetry. A new development that allows for the use of an arbitrary Green’s function hence it is possible to get the far field from the computed equivalent currents in the presence of a dielectric half-space. Thus, the theoretical radiation pattern of the vehicle mounted antenna can be computed when the vehicle is on concrete, dirt, or even salt water. In this paper the authors present the latest work performed using this approach to place free space measured antennas over a PEC or dielectric half-space. Results show the potential of this approach. The higher order basis functions allow for the modeling of large structures with reduced number of unknowns. Thus, the antenna under test can be then placed in proximity to not only various half-space materials, but also to towers, buildings, or spacecraft.
In this work, an efficient two-step algorithm to compensate for 3-D probe positioning errors, which occur in a near-field–far-field transformation (NF–FFT) using a minimum number of spherical NF measurements, is developed and numerically assessed. Firstly, a so called spherical wave correction is exploited to correct the phase shifts caused by the deviations from the nominal spherical surface. Then, an iterative technique is employed to recover the NF samples at the exact sampling points from those, altered by 2-D mispositioning errors, attained at the previous step. Once the correctly positioned samples have been retrieved in such a way, an optimal sampling interpolation formula is used to accurately determine the massive input NF data for the classical spherical NF–FFT. Numerical tests will be shown to prove the capacity of the devised method to correct even severe 3-D positioning errors.
Phaseless spherical near-field antenna measurements generally address the challenge of computing complex coefficients describing the antenna under test’s (AUT) radiation behavior from amplitude near-field measurements. The AUT’s far-field (FF) can then be obtained from those complex coefficients. Most of the techniques used in the literature result in a modified sampling method (e.g., two-spheres or sphere with two probes) and a phase retrieval algorithm (e.g., WirtingerFlow or PhaseLift). Sampling methods are chosen to increase the number of independent measurements to aid phase recovery. In our contribution, we introduce the approach of random masks, leaning on the concept of diffraction patterns, which is well-known and used in the phase retrieval theory. Random masks can be seen as intentional random perturbations occurring in the measurements either at the AUT, probe or in between; or as an extension to conventional measurements to increase the number of independent measurements. State-of-the-art phaseless sampling methods can also be interpreted as masks with limited randomness. A general mathematical model is presented, and different types of masks based on random distributions are investigated through simulations on a transformation with spherical wave expansion. Firstly, generic masks are considered to benchmark the achievable reconstruction error, and secondly, masks based on probes are examined.
This work presents a fast Planar Near-Field (PNF) technique for characterizing electrically large antennas from sparse acquisitions over localized angular domains employing a robotic arm in non-anechoic environments. The proposed technique has been designed for the effective characterization of complex antenna systems with multiple beams, such as Reflective Intelligent Surfaces (RIS) or Massive MIMO panels for millimeter wave (mmW) 5G communications although it is particularly suitable for any electrically large antenna with narrow and tilted beams. The near-field to far-field transformation is restricted to a reduced angular domain, and the non-redundant acquisition grid is computed by means of Singular Value Decomposition (SVD) techniques so that the number of unknowns of the inverse problem is highly reduced, significantly decreasing acquisition time. The proposed technique has been validated by means of a numerical example in the X-band and a measurement example in the Ka-bands yielding excellent results.
In this paper, a new approach for planar near-field (NF) measurements for lower band 5G applications is presented employing a customised Vivaldi antenna as a near-field probe. The paper includes a careful analysis of the impact that the absorber collar has on the overall measurement performance of the probe. A 5G, 24-elements, C-band, planar array antenna has been used as an antenna under test (AUT). Full-wave three-dimensional computational electromagnetic simulations (CEM) of the production test, measurement, and calibration of a given planar-near-field measurement setup with, and without, absorber collar, have been undertaken. Here, special attention has been paid to a thorough examination of the presence of scattering, and the standing waves in the simulated near-field measurement. The presence and impact of this phenomenon has been carefully inspected by intensive simulations and compared with results obtained for a standard open-ended waveguide probe (OEWG) probe, as well as with an alternative dielectric probe. The obtained results have demonstrated clear advantages when compared to the alternative solutions with superior results being obtained in terms of the scattering performance. Standing waves and ripple are found to be far less visible with the overall results after probe compensation being noticeably improved when compared with the more commonly used alternatives. We complete this study by verifying the suitability of the proposed Vivaldi probe for Spherical NF measurements by comparing its spherical mode coefficients with that of the ubiquitous OEWG. In conclusion, the Vivaldi probe spans several waveguide bands, and is suitable for planar, cylindrical, and spherical near-field testing applications.
The measurement of active devices or Active Antenna Systems (AAS) necessitates the assessment of transmitter and receiver characteristics, such as radiated power, sensitivity, and occasionally data throughput. The AAS transmit and receive properties can be fully characterized by evaluating two spatial power quantities. Equivalent Isotropic Radiated Power (EIRP) when the AAS operates as a transmitter, and Equivalent Isotropic Sensitivity (EIS) when it functions as a receiver. This testing often requires an Over-the-Air (OTA) measurement setup and is relatively simple to perform when the measurement distance is sufficient for both the probe and AAS to be in the far field. For physically and electrically large AAS’s, this assumption can be hard to satisfy. This paper explores techniques for assessing spatial-directional transmitted and received power-related performance metrics of active devices using spherical near field measurement techniques. This approach can be complemented by phase recovery techniques to enable accurate NFFF transformation. The presented method is validated by experiments on a suitable validation mockup.
The size and optics of a Compact Antenna Test Range (CATR) determine its quiet zone and the lowest frequency at which it will meet nominal quiet zone specifications. If a reflector is not large enough to present a sufficient multi-wavelength surface at a given frequency, a plane wave is not generated. Operating compact ranges at lower and lower frequencies is a continuing desire in the measurement community. The normal solution for both instances is to increase the reflector size. This leads to larger test chambers, hence increasing cost. Collecting spherical near- field (SNF) data in a CATR within its normal operating frequency band is well known. However, this leads to collecting more data than required to obtain principal plane cuts in the CATR. This paper presents a study and empirical data on using low-frequency range antennas that operate down to one half the nominal CATR low frequency using SNF techniques to measure test articles at these frequencies with relative accuracy. The paper includes simulations of the quiet zone performance at low frequencies.
The objective of this paper is to provide some guidelines about the measurement uncertainty of Spherical Near Field (SNF) ranges when they are used to derive near field figure of merits instead of more conventional far field-based metrics. One of the main advantages of the SNF ranges is their flexibility. Indeed, from the NF scanning, the spherical wave expansion is applied, and it can be used as a powerful, accurate and efficient propagation tool, able to evaluate figures of merits at (almost) any distance from the device under test. This feature is particularly useful in the testing of modern antenna systems intended to operate in specific regions of space instead of conventional far field scenarios. Examples are Plane Wave Generators (PWG) which create a uniform field distribution in the proximity of the device, or more generic field synthesizer devices. Despite the flexibility of SNF systems, the evaluation of their uncertainty budgets is normally limited to far field-based metrics. Understanding under which conditions and in which measurement scenarios such uncertainty budgets are applicable to more generic near field metrics is the main topic addressed in this paper.
One of the major contributions to the measurement uncertainty of antenna measurements are multiple reflections between antenna under test (AUT) and probe antenna. In the case of spherical near-field (SNF) measurements, multiple reflections are typically estimated and compensated for by conducting full SNF measurements at different radii and averaging the transformed far-field results. However, the need for several measurements leads to a multiplication of the measurement duration, and subsequently to an increase in costs. Another option is to increase the measurement radius, which might not be possible depending on the positioning equipment. Therefore, a technique to reduce multiple reflections between AUT and probe antenna by intentionally tilting the latter is presented. The technique is evaluated with a robotic antenna measurement system, the flexibility of which allows to almost arbitrarily tilt the probe antenna and perform a spherical measurement in this tilted configuration. It is shown that the magnitude of the reflections can be reduced significantly with this approach, even for small tilt angles. A comparison with the conventional averaging technique indicates that the presented approach reduces the error to a similar level, but at a fraction of the measurement time.
Spherical Near-Field antenna measurements are broadly used for vehicular measurements, which almost always include several antennas. Due to the large size of vehicles and the reduced size of near-field ranges, it is often impossible to displace the vehicle so that the desired Antenna Under Test (AUT) be in the center of the measurement sphere - and when it is possible, it is highly impractical to repeatably displace the vehicle for each of the antennas. Nevertheless, it is often required to retrieve the radiation characteristics of the AUT as if it were centered. In this work, Parallax-based methods for the correction of near-field acquired data are discussed, and a novel method based on the correction of the probe’s relative view angle and distance to the offset AUT is introduced. This method, additionally, does not require any matrix (pseudo)inversion for the calculation of the Spherical Wave Coefficients (SWCs) and can be solved with classical FFT-based Near-Field-to-Far-Field Transformations (NFFFT) based on the Wacker transmission formula.
Transceiver satellites with a ”bent-pipe” payload are commonly used in communication systems. Accuracy of measurement of their main End-to-End (E2E) parameters, such as Saturating Flux Density (SFD), Gain flatness (G/F), Equivalent Isotropic Radiated Power (EIRP) and Gain over Temperature (G/T) depends not only on the test setup, but also on the accessibility of different test points in the payload. In this work, we focus on the error budget for different accessibility levels when the payload is tested in Planar Near-Field (PNF).