Near-field Antenna Measurements Research Articles

Phaseless Probe-Corrected Spherical Near-Field Antenna Measurements Using Two Scan Spheres

Phaseless Probe-Corrected Spherical Near-Field Antenna Measurements Using Two Scan Spheres

Monostatic Free-Space Method for Relative Permittivity Determination Using Horn Antenna Near-Field Measurements

Monostatic Free-Space Method for Relative Permittivity Determination Using Horn Antenna Near-Field Measurements

Multi-frequency Spherical Near-field Antenna Measurements Using Compressive Sensing

Multi-frequency Spherical Near-field Antenna Measurements Using Compressive Sensing

Planar Near-Field Antenna Measurements with a Uniform Step Larger than Half-Wavelength

Planar Near-Field Antenna Measurements with a Uniform Step Larger than Half-Wavelength

Phaseless Spherical Near-Field Antenna Measurements With Reduced Samplings

Phaseless Spherical Near-Field Antenna Measurements With Reduced Samplings

Near-field antenna measurement based on Rydberg-atom probe.

Current near-field antenna measurement methods are commonly based on metal probes, with the accuracy limited and hard to be optimized due to the drawbacks they suffered, such as large volume, severe metal reflection/interference and complex circuit signal processing in parameter extracting. In this work, a novel method is proposed based on Rydberg atom in the near-field antenna measurement, which can offer a higher accuracy due to its intrinsic character of traceability to electric field. Replacing the metal probe in near-field measurement system by Rydberg atoms contained in a vapor cell (probe), amplitude- and phase- measurements on a 2.389 GHz signal launched out from a standard gain horn antenna are conducted on a near-field plane. They are transformed to far-field pattern and agree well with simulated results and measured results by using a traditional metal probe method. A high precision in longitudinal phase testing with an error below 1.7% can be achieved.

Holographic phase-retrieval method of near-field antenna pattern measurement for bolometer-array-equipped millimeter-wave telescopes

Wide field-of-view millimeter-wave telescopes with a bolometric detector array have been developed for cosmic microwave background radiation observations. For the purpose of laboratory verification of these telescopes, several studies have demonstrated near-field antenna measurements using a phase-sensitive detector that replaces a few representative pixels of the focal-plane detector array. We present a holographic phase-retrieval method that enables near-field measurements with the bolometric detector array as it is. We place a reference emitter at a fixed position and scan a signal emitter at the telescope aperture. These two emitters are phase-locked and generate interference patterns (holograms) on the focal plane, from which the amplitude and phase of the aperture field can be retrieved. We experimentally demonstrated this method with a crossed-Dragone telescope with a field-of-view that is 18 deg × 9 deg. In the demonstration, we placed a phase-sensitive detector at three detector positions on the focal plane. The antenna patterns calculated from the hologram, neglecting the directly measured phase information, were consistent with those calculated from both intensity and phase measurements at the −60-dB level at 180 GHz. Applying this method, the antenna patterns for all of the bolometric detectors on the focal plane can theoretically be measured simultaneously.

Optimizing Sensing Matrices for Spherical Near-Field Antenna Measurements

In this article, we address the problem of reducing the number of required samples for spherical near-field (SNF) antenna measurements by using compressed sensing (CS). A condition to ensure the numerical performance of sparse recovery algorithms is the design of a sensing matrix with low mutual coherence. Without fixing any part of the sampling pattern, we directly find sampling points that minimize the mutual coherence of the respective sensing matrix. Numerical experiments show that the proposed sampling scheme yields a higher recovery success in terms of phase transition diagram when compared to other known sampling patterns, such as the spiral and Hammersley sampling schemes. Furthermore, we also demonstrate that the application of CS with an optimized sensing matrix requires fewer samples than classical approaches to reconstruct the spherical mode coefficients (SMCs) and far-field pattern.

High-Speed Antenna Measurement System Using Multi-Probe Array Technique for 5G Applications

In this article, a high-speed antenna measurement system using multi-probe array technique for 5G application is suggested. The spherical near-field scan measurement system for millimeter wave frequency band (3 GHz–40 GHz) and the multi-probe array rotation method is adopted to improve the antenna measurement speed. The proposed measurement RF system consists of the 14 multi-probe array (28 channel with dual-polarization), switch matrix, downconverters, amplifiers/attenuators, digitizers, and signal generators, and is compared to the conventional planar near-field scan antenna measurement system. The measurement speed performance can be improved significantly and the 3D beam pattern of the AUT can be achieved. The designed system was verified through comparison with a verified far-field measurement system. In addition, to increase the measurement efficiency, an effective sampling technique was proposed and verified.

Reliable Linearized Phase Retrieval for Near-Field Antenna Measurements With Truncated Measurement Surfaces

Most methods tackling the phase retrieval problem of magnitude-only antenna measurements suffer from unrealistic sampling requirements, from unfeasible computational complexities, and, most severely, from the lacking reliability of nonlinear and nonconvex formulations. As an alternative, we propose a partially coherent (PC) multi-probe measurement technique and an associated linear reconstruction method which mitigate all these issues. Hence, reliable and accurate phase retrieval can be achieved in near-field far-field transformations (NFFFTs). In particular, we resolve the issues related to open measurement surfaces (as they may emerge in drone-based measurement setups) and we highlight the importance of considering the measurement setup and the phaseless NFFFT simultaneously. Specifically, the influence of special multi-probe arrangements on the reconstruction quality of PC solvers is shown.

An Adaptive Data Acquisition Technique to Enhance the Speed of Near-Field Antenna Measurement

In this article, a new approach is proposed to improve the speed of near-field measurement of antennas. The near-field measurements are mainly performed using a planar near-field scanner, named RFX2. In RFX2, which is an electronically switched probe array, the probes in the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$x$ </tex-math></inline-formula> and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$y$ </tex-math></inline-formula> directions cannot be fabricated at the same place. At each point, only one tangential component of the magnetic field is measured, and the other component is estimated numerically using an interpolation technique. Here, an adaptive data acquisition technique is proposed that sequentially collects samples from the field in the areas with highly dynamic behavior and skips the regions that have smooth near-field variations. Since the newly introduced points at each iteration of the algorithm are not necessarily laid on the probe locations of the RFX2, the values of the data at these points are calculated using an interpolation method. The proposed adaptive algorithm in this work requires remarkably fewer samples to reach the same accuracy as uniform sampling. This method is also utilized in spherical NF measurement over <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\theta $ </tex-math></inline-formula> and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\phi $ </tex-math></inline-formula> plane. The validity of the approach is verified using various numerical and measurement results.

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.

Optimal Sampling in Spherical Near-Field Antenna Measurements by Utilizing the Information Content of Spherical Wave Harmonics

In this article, we propose an efficient method that specifies the minimum number of samples and their locations to measure the spherical near-field (SNF) of antennas utilizing information-theoretic tools. To this end, spherical harmonics are investigated from the information-theoretic point of view. Our study determines the minimum required samples and provides a validation tool to assess the ability of a sampling scheme to recover the antenna characteristics by the measured data. Furthermore, an optimum sampling scheme is proposed by clustering the energy of vector spherical harmonics using the frame theory without any time-consuming iterative, which leads to a highly efficient SNF measurement procedure. The number of samples determined by the proposed method is only 50% of the Nyquist method, which provides significant advantages for storage and processing time. Our method additionally shows a great promise for the recovery of the far-field radiation pattern from near-field samples with the same performance as the Nyquist sampling on a sphere. To derive the proposed sampling scheme with the minimum redundancy, no prior information on the antenna is assumed. Finally, the proposed sampling scheme is validated on measurement data.

Design of Multi-Functional Transmitarray with Active Linear Polarization Conversion and Beam Steering Capabilities

This study designed a multi-functional reconfigurable transmitarray with switchable polarization and beam steering capabilities using only a PIN diode having ON/OFF switching ability. The simulation confirmed that the proposed architecture is capable of 2D beam scanning of up to ±45° for elevation and azimuth while maintaining or converting linear polarization when operating within the X-band. The performance was verified through a far-field beam pattern measurement using the near-field antenna measurement system by producing a transmitarray with an 8 × 8 unit cell arrangement. The maximum level of gain of the proposed architecture was confirmed to be 14.5 dBi, the sidelobe level was below 14.5 dBc, the polarization converting loss was below 1.28 dB, and the polarization isolation level was above 14.7 dB. It can be utilized for the design technique of a radar system demanding small size, weight, power, and cost (SWaP-C) characteristics based on a simplified composition.

Phaseless Spherical Near-Field to Far-Field Transformation Algorithm via Sparsity of Spherical Mode Coefficients

In order to overcome the difficulty in obtaining accurate phase information in antenna near-field measurement, this study proposes a phase recovery algorithm that can be applied to phaseless spherical near-field sampling data. Based on the sparsity of spherical mode coefficients (SMCs), under the condition of unknown sparsity level and low oversampling rate, the proposed algorithm can accurately recover SMC to obtain high-precision antenna radiation characteristics. To improve the recovery performance of the algorithm, a spherical sampling strategy matching with the algorithm is also presented. By calculating the correlation of the measurement matrix and comparing the accuracy of the reconstructed far field, a set of optimal parameter settings including the number of spheres, the distribution of the measurement points, radius difference, and polarization mode is determined. On the basis of the obtained measurement data, the performance advantage of the presented algorithm in recovering SMC is demonstrated.

Numerical and Experimental Investigation of Phaseless Spherical Near-Field Antenna Measurements

This article presents a thorough study of spherical probe-corrected phaseless near-field measurements with the two-scans technique. Such technique is based on retrieving the antenna under test radiation pattern from the measurement of the near-field amplitude signals on two spheres of different radii. The postprocessing of this type of measurements results in a highly nonlinear algorithm, prone to get trapped in local minima and provide incorrect solutions when the measurement conditions are not properly selected. Through a series of numerical simulations, the influence of different measurement parameters on the phaseless technique is analyzed. It will be shown how both the relative and absolute values of the measurement spheres highly affect the convergence of the phase retrieval algorithm. The type of AUT and its radiation pattern characteristic also play a fundamental role in the feasibility of phaseless measurements. Other parameters such as sampling rate, noise, probe correction, polar truncation, and measurement offsets are also investigated. The conducted study allows to extract a set of guidelines to improve the accuracy of phaseless spherical near-field algorithms. In addition, purely phaseless antenna measurement examples are given to demonstrate the algorithm capabilities and limitations, and to validate the developed numerical investigations.

High-order probe compensation algorithm for ultra-wide-band probe in spherical near-field measurement

In order to increase the precision of spherical near-field antenna measurement systems based on wide-band probes, probe compensation algorithms supporting high-order azimuth mode (HOPC) are needed to process the near-field measurement data. In this study, an HOPC algorithm is realized based on the iterative strategy in the reference [ 3 ] for solving the spherical-mode transmission equation with high-order probes. This algorithm is applied to a dual-ridged wide-band horn probe that works in the frequency range of 2–18 GHz. Using dipoles array antennas as examples, the higher precision of the high-order algorithm is illustrated compared with that of the low-order algorithm. The convergence of the algorithm under different conditions is also systematically tested. These results indicate, firstly, the higher-order effects are more remarkable for higher frequency and secondly, when the antenna under test can totally be covered by the 6 dB domain of the main lobe, the iterative process can converge to satisfactory precision within several iterations. Therefore, the algorithm in this paper for HOPC can increase the precision of the spherical near-field system while still preserves high computational efficiency.

Near-Field Antenna Measurements with Manual Collection of the Measurement Samples

Abstract. Near-field measurements are commonly performed in anechoic chambers which limits the flexibility of the measurements and requires high precision equipment to achieve exact results. In this contribution, we investigate a simple near-field measurement setup which does not use any sophisticated positioning system nor operates in a controlled environment. Instead, the probe antenna is moved by an operator person while the probe position is measured by a laser tracker. This implies that the measurement results will have a higher error level in comparison with antenna chamber measurements. However, excellent error levels are not always necessary, especially when it comes to on-site testing of the principle functionality of antennas. Measurement results are shown to illustrate the performance of the system.

Application of Nonuniform FFT to Spherical Near-Field Antenna Measurements

This article investigates on the use of nonuniform fast Fourier transform (NUFFT) to solve spherical near-field transformation problems. Traditional postprocessing algorithms make use of conventional FFT for the transformation of measured grids with uniform sampling. More recently, the processing of irregular sampling grids has been introduced by means of matrix-based algorithms. The benefits of both type of algorithms are combined in this article with the implementation of the spherical wave expansion (SWE) summation and its adjoint with NUFFT. This allows the processing of fields with the efficiency of the FFT approaches and the versatility of the matrix operators. To this end, the NUFFT will be introduced and it will be shown how it can be implemented to accelerate the multidimensional summations required in the SWE calculations. Numerical investigations will be introduced to demonstrate the computational gains achieved with negligible approximation errors. Finally, the NUFFT will be tested on three different spherical near-field postprocessing problems: far-field transformation of near fields measured with spiral scanning, phaseless measurements on spherical surfaces with different sampling grids, and antenna diagnostics.

Under-Sampled Spherical Near-Field Antenna Measurements With Error Estimation

This article proposes an under-sampling scheme in φ for spherical near-field antenna measurements. Reducing the number of samples along one of the spherical coordinates allows to decrease the number of positioner steps. Therefore, the savings in the number of samples are almost directly translated into measurement time savings. Under-sampling usually leads to aliasing; however, it can be controlled by exploiting the properties of the spherical wave expansion. The axial symmetry of conventional antennas leads to narrow azimuthal spectra, where the influence of this aliasing is small. By designing an estimator of the error introduced by the aliasing, the required number of φ samples for a given accuracy can be determined, without any <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">a priori</i> knowledge about the antenna under test. This allows the application of this technique for any kind of antenna regardless of its level of axial symmetry, obtaining different values of measurement time savings depending on the level of symmetry. Several examples using the numerical and measured data are presented, showing significant reductions in the measured samples for different antennas. In addition, the algorithm is successfully tested against antennas with poor axial symmetry to demonstrate its robustness.