Antenna Under Test Research Articles

Far-Field Estimation Method of Antennas Above Conductive Plane Using a Partially Spherical Near-Field

For evaluating the practical performance of land mobile communication antennas via near-field measurement, it is essential to measure the electromagnetic field, including the housing in which the antennas are mounted. When measuring a heavy housing, scattered waves from the supporting turntable for the antenna under test (AUT) disturb the electromagnetic field. A technique for eliminating the influence of scattered waves is required to evaluate the abnormal AUT radiation performance. This paper proposes a method for estimating the far-field when the antenna exists above an infinite conductive plane, using partially spherical near-field measurements that include the turntable. The proposed method estimates the far-field by combining the equivalent source reconstruction and the far-field estimation with the reflection coefficient method. In the proposed method, the equivalent source distribution surrounding an AUT is reconstructed from a partially spherical near-field to remove the effects of the turntable. Through numerical calculations and experiments, this paper shows that the proposed method can estimate the far-field pattern above an infinite conductive plane, with arbitrary electric constants from a partially spherical near-field. The far-field estimation error is less than -20 dB when the upper hemispherical near-field can be obtained.

Phase‐reconstruction from magnitude‐only data of electrically small antennas for body‐centric wireless communication

AbstractA method for phase retrieval from magnitude‐only data for electrically small antennas intended for use in relation to body‐centric wireless communication, is presented. The method utilizes the spherical wave expansion (SWE) description of the electromagnetic field radiated by the antenna under test (AUT). The expansion coefficients of the SWE are optimized such that errors between the reconstructed and provided data magnitudes are as small as possible. The required additional phase information is acquired from additional magnitude‐only data sets of the AUT in different orientations. These orientations are defined by rotations of the AUT along the Eulerian angles. The performance of the method is evaluated for a realistic configuration, which consists of an inverted‐A antenna in the vicinity of an ear. The sensitivity of the method to the angular resolution of the field data, as well as to the Eulerian rotation accuracy, is investigated. Finally, the reconstruction method is tested on a set of practical antenna measurements conducted in an RF‐anechoic chamber.

GNSS Receiver Antenna Absolute Field Calibration System Development: Testing and Preliminary Results

For high-precision Global Navigation Satellite Systems (GNSS) positioning based on carrier-phase measurements, knowledge of the GNSS receiver antenna electrical signal reception characteristics, i.e., phase center, is crucial. Numerous studies have led to the understanding of the influence of GNSS receiver antenna phase center corrections (PCCs) on GNSS positioning accuracy and other estimated parameters (e.g., receiver clock estimates, ambiguities, etc.). With the goal of determining the PCC model of GNSS receiver antennas, only a few antenna calibration systems/facilities are in operation or under development worldwide. The International GNSS Service (IGS) publishes type-mean PCC models for almost all geodetic-grade GNSS antennas. However, the type-mean models are not perfect and do not fully reflect the signal reception properties of individual GNSS receiver antennas. Relevant published scientific research has shown that the application of individual PCC models significantly improves the accuracy of GNSS positioning and other estimated parameters. In this article, the new automated GNSS antenna calibration system, recently developed at the Laboratory for Measurements and Measuring Technique (LMMT) of the Faculty of Geodesy of the University of Zagreb in Croatia, is presented. The developed system is an absolute field calibration system based on the utilization of a Mitsubishi MELFA 6-axis industrial robot. During calibration, the robot tilts and rotates the GNSS antenna under test (AUT) around a fixed point within the antenna. The antenna PCC modelling is based on time-differenced double-difference carrier-phase observations. Our preliminary results for the Global Positioning System (GPS) L1 (G01) frequency show a submillimeter repeatability of the estimated PCC model and a submillimeter agreement with the Geo++ GmbH calibration results.

An Effective Near-Field to Far-Field Transformation with Planar Spiral Scanning for Flat Antennas under Test.

The goal of this article is to provide numerical and experimental assessments of an effective near-field to far-field transformation (NF-FF T) technique with planar spiral scanning for flat antennas under test (AUTs), which requires a non-redundant, i.e., minimum, number of NF measurements. This technique has its roots in the theory of non-redundant sampling representations of electromagnetic fields and was devised by suitably applying the unified theory of spiral scans for non-volumetric antennas to the case in which the considered AUT is modeled by a circular disk having its radius equal to half of the AUT's maximum dimension. It makes use of a 2D optimal sampling interpolation (OSI) formula to accurately determine the massive amount of NF data required by the classical plane-rectangular NF-FF T technique from the non-redundant data gathered along the spiral. It must be emphasized that, when considering flat AUTs, the developed transformation allows one to further and significantly save measurement time as compared to that required by the previously developed NF-FF T techniques with planar spiral scans based on a quasi-planar antenna modeling, because the number of turns of the spiral and that of NF data to be acquired depend somewhat on the area of the modeling surface. The reported numerical simulations assess the accuracy of the proposed NF-FF T technique, whereas the experimental tests prove its practical feasibility.

An Effective Method for Antenna Radiation Pattern Reconstruction Based on Phaseless Measurement in a Reverberation Chamber

It has been shown that the angular correlation of the antenna can be obtained by rotating it in a reverberation chamber (RC) and the radiation pattern of the antenna under test (AUT) can be reconstructed from the measured correlation coefficients using spherical wave decomposition. However, the correlation coefficient needs to be obtained by measuring the complex-valued S-parameters in an RC, which is no longer applicable to active antenna tests where the phase information is unavailable. To solve this problem, we propose an antenna radiation pattern reconstruction method based on phaseless measurement in an RC. In this method, the power correlation coefficient of AUT is calculated using the signal amplitude received by the antenna at different angles. According to the relationship between complex correlation coefficient and power correlation coefficient of the AUT, the nonlinear least squares method is used to reverse the spherical wave coefficients and reconstruct the antenna radiation pattern. Simulations and measurements are carried out in RC and anechoic chamber, which demonstrated the feasibility and applicability of the proposed method.

Measurement and transformation of continuously modulated fields using a short-time measurement approach

Abstract. Near-field measurements, which are performed in-situ, may suffer from the fact that the antenna under test (AUT) cannot be accessed to transmit or receive a specifically tailored test signal. In some scenarios, it might also be desired to test the AUT during its real operation state, especially when it comes to the verification of antenna systems. Therefore, the need to handle time- and space-modulated fields in combination with a time-harmonic near-field to far-field transformation (NFFFT) arises. For the case where unmanned aerial vehicles (UAVs) carry the field probe in in-situ measurement scenarios, long observation times, required for the resolution of the frequency spectra of modulated fields, are detrimental due to the UAV movement resulting in blurred measurement positions. The short-time measurement (STM) approach, presented in this article, offers a way to transform the measured field data using a time-harmonic NFFFT with short observation times for the collection of the individual field samples. Measurements are shown which demonstrate the applicability of the STM approach for the measurement and transformation of continuously time-modulated fields in different measurement scenarios.

Single-Cut Far-Field Pattern Determination of Polarized Linear Antenna Array via Quasi-Far-Field Mathematical Absorber Reflection Suppression

In this work, a method for rapidly and accurately determining the single-cut far-field (FF) patterns of any polarized linear antenna array, termed as quasi-far-field mathematical absorber reflection suppression (QFF-MARS), is introduced. The proposed QFF-MARS method takes advantage of the cylindrical wave expansion (CWE) theory and is able to exclude interference from scatters at the QFF range. In this method, probe compensation is not needed, owing to the fact that the measurement procedure is the same as that for FF. It is only required that the measurement distance for an antenna under test (AUT) satisfies the FF and QFF conditions for its smaller and larger dimensions, respectively. To validate the proposed method, a 45° polarized antenna array and a phased array have been simulated. The results obtained by the proposed approach are consistent with both theoretical and measured results. This indicates that the proposed approach can directly obtain the single-cut FF patterns of polarized linear antenna arrays at a QFF range, while identifying and extracting range-related multipath effects.

A Fast and Efficient Calibration Method for Phased Array Antennas Using Fourier-Structured Excitation Matrix

This article presents a novel phased array antenna calibration method. Unlike conventional phased array calibration methods equipped with only one probe antenna in the far-field setup, multiple measurement probes are introduced in the proposed algorithm to significantly improve the measurement efficiency. Specifically, a novel construction method for an excitation matrix (which contains excitation coefficients for phased array elements) is proposed, where phase differences introduced by the spatial locations of the multiple probes are perfectly incorporated into the Fourier-structured excitation matrix, thereby presenting the best measurement efficiency without sacrificing the array calibration accuracy. The proposed method can offer robust array calibration results (guaranteed by the low condition number of the constructed excitation matrix with Fourier structure) and high measurement efficiency (offered by multiprobe setups). To experimentally validate the proposed algorithm, a multichannel (i.e., multiprobe) far-field measurement setup was utilized. A four-element open-ended waveguide array operating at 2.6 GHz and two-probe far-field systems were employed as the antenna under test (AUT) and multiple probes, respectively, to validate the effectiveness and robustness of the proposed phased array calibration algorithm. The proposed algorithm will be highly valuable for efficient phased array calibration, especially for future antenna systems equipped with large-scale antenna configurations.

Fast Antenna Characterization With Numerical Expansion Functions Built With Partial Knowledge of the Antenna and Accelerated Asymptotic Methods

We describe a method to use numerical simulations concurrently with measurements (near-field data) to characterize the radiation properties of an antenna (far-field) with a strongly reduced number of samples. Both the simulations and experimental data are dealt with as sources of information toward obtaining the desired characterization. Simulations compensate for data scarceness, allowing to obtain an accurate characterization with fewer data than with measurements only, and the few measured data allow the comprehensive numerical proxy to get tuned to the true antenna under test (AUT). The method uses partial knowledge of the antenna and leverages accelerated asymptotic numerical methods to perform the numerical simulations needed by the algorithm. We report experimental tests of performance, both in terms of accuracy and speed-up with respect to the classical methods, with reflector antennas up to 27 wavelength. It is important to note that good results are obtained both when samples are taken on irregular optimized grids and on regular grids usually resulting from the conventional measurement rigs.

A Novel Calibration-Free Remote Antenna Impedance Measurement

Connecting the antenna under test (AUT) to a network analyzer through a feeding cable is a typical method for determining the input impedance. However, the induced current on the feeding cable obstructs precise measurement in the case of electrically small antennas (ESAs). A remote impedance measurement technique is proposed to address the drawbacks of network analyzer-based measurements. First, the AUT is connected to three standard loads, a short, an open, and a matched load consecutively. Then an electrostatic discharge (ESD) is used as an impulsive source to excite the AUT. Concurrently, a probe connected to an oscilloscope captures the impulse responses. Finally, the input impedance of the AUT is determined by extracting complex natural resonances (CNRs) from three impulse responses. Four different antennas are designed and prototyped. Then, using a full-wave simulator, their input impedances are computed and compared to those estimated using the proposed technique. The input impedances obtained by the proposed technique are very consistent with those from full-wave simulations.

Probe-AUT Multiple Reflection Suppression in the Over the Air Measurement of 5G Antennas Using Matrix Pencil Method

The effect of probe proximity to the antenna under test (AUT) in the fixed-probe spherical near field (SNF) measurement system is investigated. The matrix pencil method (MPM) is utilized to suppress the undesired multiple reflections between AUT and probe, and the optimal angular region for correction is determined in this scenario. The simulation results are compared with a reference set of data in terms of transmission coefficients and radiation patterns to evaluate the performance of the MPM in the probe-generated echo filtering procedure. To verify the functionality of the MPM in practice, two antennas are exploited as different probes to implement a spherical measurement system in a mini-anechoic chamber, which is developed for conducting the over the air (OTA) measurements of the fifth-generation (5G) antennas. By comparing the radiation pattern results, it is observed that applying the MPM to the sampled data leads to more accurate results in a compact near field (NF) measurement setup.

A Near-Field-Based Gain and Pattern Measurement Technique for Probe-Fed Millimeter-Wave Antennas [Measurements Corner

Driven by the perpetual demand for higher data rates, extensive research efforts have been devoted to millimeter-wave (mm-wave) 5G technologies in recent years. Highly integrated mm-wave antennas for 5G user equipment are considered one of the key components in the realization of mm-wave-based mobile networks. An accurate characterization of antennas is as important as antenna design. Despite great advances in measurement capabilities over the years, we are still facing many difficulties in probe-based mm-wave antenna measurements. This article presents a fast, simple, and accurate near-field (NF)-to-far-field (FF) technique for gain and radiation pattern measurements of probe-fed mm-wave antennas. A setup based on an electro-optical (EO) NF measurement system (NeoScan) is utilized to measure the NF of the antenna under test (AUT). The system uses a high-spatial-resolution nonintrusive probe to scan the NF in very close proximity to the antenna’s surface, which significantly alleviates multipath effects and reduces the required scan area. The radiation pattern of the AUT is computed using the NF-to-FF transformation. To obtain the gain of the AUT, a ground-backed dipole antenna is created and used as a gain standard. The absolute gain of the ground-backed dipole is characterized using the image method. Measurement accuracy is checked against the simulation. With the help of the standard gain antenna, the NF-to-FF method can be used to measure both the gain and radiation pattern of any type of probe-fed mm-wave antenna. For demonstration purposes, two test antennas, a horizontally polarized tapered slot antenna, and a dual-band vertically polarized hexagonal bridge antenna operating at 28 GHz are fabricated and characterized. Excellent agreement against full-wave simulation is achieved.

Spherical near-field measurement of the offset vehicle antenna mounted on a vehicle

For the vehicle antenna measurement mounting on a vehicle, the offset mounting of the antenna will increase the maximum spherical wave expansion modes, with the increasing calculation time and the increasing risk of introducing environmental reflection noise. Therefore, basing on the matrix form of the traditional spherical wave expansion (TSWE) algorithm, this paper introduces the offset spherical wave expansion (OSWE) algorithm of the offset mounting antenna under test (AUT) aiming to decrease the maximum modes, shorten the calculation time and reduce the environmental noise with the method of spherical wave expansion. Taking LTE antenna and GPS antenna as examples, this paper analyzes the improved performance of OSWE algorithm comparing with TSWE algorithm on the vehicle antenna mounting on a vehicle, as well as the parameters like equivalent noise level (ENL), omnidirectional standard deviation (σ).

Investigation of active phased arrays performance deficiency due to array planarity and misalignment

ABSTRACT This paper investigates the degradation in the electromagnetic performance of an active phased array antenna and establishes the coupling relationship between the antenna array alignment/planarity to the near-field scanner probe. An analytical framework is presented to evaluate the sensitivity of radiation parameters of active phased arrays antenna due to the deviation of complex excitation vector. A laser-based positioning metrology equipment measures the near-field scanner plane to the antenna under test (AUT) plane in a global coordinate system. The antenna array surface is adjusted and aligned to the near-field scanner probe measurement plane within 1 mm (in S-band) by employing the mechanical mechanism within the positioning accuracies of 50–100 µ. The collimation of a perfectly aligned antenna array results in the alignment of electrical and mechanical boresight of the active phased array radar. The theoretical evidence and experimental results presented powerful tools and guidelines to set a reasonable tolerance for array alignment in the near-field test range (NFTR) to achieve high performances of the active phased array antenna.

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.

Rectilinear Planar Near-Field Measurements From the Perspective of the Synthetic Phased Array

A new far-field valid angle equation for rectilinear planar near-field measurements is derived. The new valid angle equation is derived by viewing the near-field to far-field transformation process as the generation of a pseudo plane wave by a synthetic phased array and subjecting the antenna-under-test (AUT) to the radiation from this synthetic array. The synthetic phased array does not physically exist; rather, the array is formed during the postprocessing of the planar near-field measurements. The resultant valid far-field angle computed with the new equation is compared against previously established and popularly accepted valid angle equations. A brief discussion is offered on the measurement of low-directivity antennas with a planar near-field measurement system and on amplitude tapering of the near-field measurements to improve the quality of the pseudo plane wave inside the test zone.

A Bayesian Compressive Sensing Approach to Robust Near-Field Antenna Characterization

A novel probabilistic sparsity-promoting method for robust near-field (NF) antenna characterization is proposed. It leverages on the measurements-by-design (MebD) paradigm and it exploits some a-priori information on the antenna under test (AUT) to generate an over-complete representation basis. Accordingly, the problem at hand is reformulated in a compressive sensing (CS) framework as the retrieval of a maximally-sparse distribution (with respect to the overcomplete basis) from a reduced set of measured data and then it is solved by means of a Bayesian strategy. Representative numerical results are presented to, also comparatively, assess the effectiveness of the proposed approach in reducing the "burden/cost" of the acquisition process as well as to mitigate (possible) truncation errors when dealing with space-constrained probing systems.

One-Port Planar Near-Field Antenna Measurement and Corresponding Scatterer Compensation Method

A one-port near-field antenna measurement technique with a small wire scatterer is proposed. In the proposed technique, a wire scatterer is scanned with an interval of less than a quarter wavelength, and the measurement is performed at the port of the antenna under test (AUT). A two-dimensional phase unwrapping method is used to remove 180° phase uncertainty. The data obtained in the near-field are transformed to the far-field pattern, and the scattering pattern is compensated for better measurements. The proposed measurement technique was validated by measuring the near-field of an AUT over a planar scanning surface. The far-field radiation pattern obtained using the proposed technique was compared with that obtained from a conventional commercial anechoic chamber. It was observed that they agreed well, especially in the region near the center.

Antenna Radiation Efficiency Estimation From Backscattering Measurement Performed Within Reverberation Chambers

This paper presents a contactless measurement method for antenna radiation efficiency estimation within reverberation chambers (RCs). This method does not require to connect any analyzer to the antenna under test (AUT). The AUT radiation efficiency is obtained from the differential measurement of the RC composite $Q$-factor estimated for two AUT load conditions, namely an open circuit and a \SI[detect-weight]{50}{\ohm} load. Remotely-controlled RF switches are used in order to keep the setup identical for both measurements. The method is experimentally validated in two RCs of different volumes (1~m$^3$ and 19~m$^3$) with a wideband patch antenna in the 1.8~GHz to 2.8~GHz frequency range. The estimated efficiency is found to be in very good agreement with the one obtained through a conventional invasive RC measurement technique. This new method is suitable for miniature and buried antenna characterization without access to its port.

Antenna Characterization From a Small Number of Far-Field Measurements via Reduced-Order Models

The determination of a small number of samples to characterize the antenna far-field with a controlled accuracy is addressed. The radiation operator that maps the equivalent currents to the far-field is built from the outer dimensions of the antenna under test (AUT) and is discretized into the so-called radiation matrix using the methods of moments. This matrix is approximated up to the measurement accuracy by truncating its singular value decomposition (SVD) accordingly. This operation enables to construct numerically a reduced basis of the radiated fields. The dimension of this basis and consequently the number of field samples is shown to be determined by the area of the equivalent current convex surface, in agreement with the fundamental works on the minimum non-redundant samplings. The influence of the field sampling strategy and noise on the far-field pattern reconstruction is investigated to determine a realistic small number of field samples. Finally, the characterization of two radiating structures from the so-derived reduced number of samples validates experimentally the proposed approach and demonstrates its practical relevance.