Spherical 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

Phaseless Spherical Near-Field Antenna Measurements With Reduced Samplings

Phaseless Spherical Near-Field Antenna Measurements With Reduced Samplings

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.

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.

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.

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.

A 60 GHz First-Order Dual-Port Probe for Spherical Near-Field Antenna Measurements

We present the design, manufacturing, calibration, and testing of a first-order dual-port conical-horn probe for spherical near-field antenna measurements at 60 GHz. The measurement results demonstrate that the probe performance is acceptable, but they also illustrate the challenges for high-accuracy antenna measurements at millimeter-wave frequencies.

An improved method of reducing SWE order for phaseless spherical near-field antenna measurement

This paper presents a method of reconstructing far-field characteristics by using phaseless spherical near-field measurement, which is used to solve insufficient phase information problem in the near-field measurement of the antenna under test. This method firstly samples the amplitude of two spherical surfaces with different radius in the near-field region, and then iteratively recovers the phase of a sampling spherical electric field based on the spherical wave expansion (SWE) method of spherical near-field measurement. Finally, the far-field data is calculated by combining the simulation near-field measurement amplitude and recovered phase. In this paper, we compare the far-field results of near-far field transform algorithm and phaseless spherical algorithm with different sampling intervals and analyze the error of phaseless spherical algorithm. Then, an improved method of reducing SWE order is proposed. The simulation results show that the far-field characteristics of the antenna can be calculated effectively by adjusting the SWE order.

An Adaptive Data Acquisition and Clustering Technique to Enhance the Speed of Spherical Near-Field Antenna Measurements

This letter presents a new approach for adaptive spherical near-field (NF) antenna measurement. The proposed method begins with a number of initial points and sequentially focuses on the areas with a highly dynamic NF pattern. Thereafter, the source reconstruction method is utilized to calculate the equivalent magnetic and electric currents on the surface of an ellipsoid that encompasses an antenna under test. The equivalent sources are used to compute the far-field pattern of the antenna. The comparison of the adaptive algorithm with the uniform sampling indicates that the number of the required samples is decreased significantly using the adaptive method. The adaptive data acquisition can also be used in case of uniform sampling to remove the redundant samples and accelerate the source reconstruction method. Since the newly added point is not necessarily laid on the measurement points, the cubic spline interpolation technique is employed to compute the value of the field. Besides, a machine learning algorithm based on k-means clustering is applied to the uniformly sampled data to determine different clusters of data. Thus, for every new point, the cluster to which the data point belongs can be determined, and only the values of that cluster are used to calculate the value of the new point.

Efficient spherical near‐field antenna measurement using CS method with sparsity estimation

Spherical near-field antenna measurement requires the acquisition of many points, which makes the measurement process time-consuming. Since the antenna radiation pattern has a sparse representation on the basis of vector spherical harmonics, compressive sensing (CS) can be used to reduce the number of sampling points in the near-field antenna measurement. CS requires knowledge of sparsity in advance to determine the number of sampling points. The phase transition diagram is usually used to determine the number of measurements, which is again time-consuming and requires a large number of computing data. In this study, the method for sparsity level estimation is used to estimate the sparsity level of an arbitrary antenna's near field. Using this sparsity estimation method is a novel antenna measurement which has many applied advantages. As a case in point, it estimates the sparsity level of a radiation pattern with a few numbers of measurements, which makes the CS method efficient and affordable in the near-field measurement technique. To confirm the effectiveness of the proposed method, a standard horn antenna is investigated.

Analysis and correction of probe location errors in spherical near-field antenna measurement

Spherical Near-Field Antenna Measurement is a very important method in characterizing an antenna under test (AUT). The antenna far-field radiating pattern can be derived from a spherical wave expansion of the near-field data. Near-field data used for obtaining the spherical mode coefficients depends on the sampling probes. The probe correction, such as probe calibration, probe compensation, is crucial in near-field to far-field transformation procedure. This paper gives analysis of the influence caused by the deviation of the probe location including radial and tangential direction, and proposes Taylor series as a method to modify the radial location errors. The simulation results show that the modified radiating far-field data with the probe location errors in the radial direction fits better with the ideal situation.

Compact First-Order Probe for Spherical Near-Field Antenna Measurements at Low Frequencies

Guidelines for designing compact and lightweight first-order probes for spherical near-field antenna measurements at frequencies below 1 GHz that exploit first-order properties of electrically small self-resonant radiators combined into superdirective endfire arrays are established theoretically, exemplified numerically, and validated experimentally. A prototype of the probe designed to operate at a central frequency of 435 MHz exhibits the impedance bandwidth of 15 MHz with a directivity of more than 9 dBi and parasitic $|\mu |\ne 1$ spherical modes suppressed to at least −42 dB. The probe height is just 343 mm above a 720-mm circular ground plane and weighs about 5 kg.

Mutual Coupling Compensation Matrices for Transmitting and Receiving Arrays

A general method to obtain a matrix which allows the compensation of mutual coupling effects in transmitting arrays for the total field in all directions is introduced. This method is independent of the numerical method used in the analysis and it can include the effect of the antenna platform. The starting point can be the active element patterns or the spherical mode expansion from spherical near-field antenna measurements. Additionally, the spherical mode expansion is also used to find a matrix which allows the compensation of mutual coupling effects in receiving arrays. Through this theory, a simple relation between the compensation matrices of the transmitting and the receiving arrays is found. As a consequence, the scattering matrix of a circuit that allows the simultaneous compensation of mutual coupling effects for the transmission and the reception problem can be easily defined. Finally, it will be shown how the capabilities of the compensation in all directions depend strongly on the array element.

Echo Suppression by Spatial-Filtering Techniques in Advanced Planar and Spherical Near-Field Antenna Measurements [AMTA Corner

This paper presents a comparative investigation of two versatile error-mitigation techniques, applicable to general antenna near-field measurement scenarios with echo signals of unknown origin. Both techniques are based on spatial filtering of the measured field, taking advantage of a priori knowledge of the antenna's size. The first approach takes advantage of the spatial-filtering properties of the spherical-wave expansion of the measured field. The second approach is based on the reconstruction of equivalent currents, and implements the spatial filtering as a direct consequence of the selected size and shape of the reconstruction surface. The investigation was performed using measured data on two different horns in both planar and spherical near-field scanning geometries. The presence and levels of echo pollution in the measurements were controlled by introducing known scattering objects in the anechoic chambers, and comparing with reference situations without disturbance.

Adaptive Sampling in Spherical and Cylindrical Near-Field Antenna Measurements

An adaptive approach for optimized sampling in cylindrical and spherical near-field antenna measurements is described. The presented technique applies higher sampling density in rapidly varying near-field regions, and skips data points in the smoother regions. Abrupt changes in the near field are detected by comparing the extrapolated and the measured near-field values at coarser sampling points during the measurements. A decision function, based on the signal-to-noise ratio of the measured value, is used to determine the threshold difference between the extrapolated and the measured near-field values for skipping the sampling point. The reduced near-field data collected is processed using the fast irregular antenna field transformation algorithm (FIAFTA). FIAFTA is computationally efficient, and capable of handling data on irregular grids with full probe correction. Several test cases are then presented related to the applicability of the given approach. A significant reduction in the number of measurement points was observed, thereby reducing measurement time and the computational effort.

Spherical Wave Scattering Matrix Description of Antenna Coupling in Arbitrary Environments

A framework is presented for investigating antenna coupling in arbitrary environments by way of antenna and environment scattering parameter matrices. The environment scattering parameter matrices are bona fide scattering parameters that describe the environment when all radiation ports are terminated with nonreflecting loads. Simplification of the antenna coupling equations leads to a second formulation that is compatible with classically-defined antenna scattering parameter matrices. A third formulation, based on an auxiliary set of matrices describing the environment's scattering when all unexcited radiation ports are open-circuited, extends the coupling formulation used in spherical near-field antenna measurements to arbitrary environments.

Superdirective Magnetic Dipole Array as a First-Order Probe for Spherical Near-Field Antenna Measurements

The theory as well as numerical and experimental results are presented for a superdirective array composed of closely spaced electrically small resonant magnetic dipole elements. The array operates on a metal ground plane and can exhibit a maximum directivity of 11.5 dBi, 15.2 dBi, and 17.8 dBi (including 3 dB due to the ground plane), for 2, 3, and 4 magnetic dipoles, respectively. The array is self-resonant and is directly excited by a 50-ohm coaxial cable through the ground plane. The array radiates essentially the |μ| = 1 spherical modes, which, despite a narrow bandwidth, makes it an excellent first-order probe for spherical near-field antenna measurements at low frequencies.

Numerical Investigation of the System-Matrix Method for Higher-Order Probe Correction in Spherical Near-Field Antenna Measurements

The system-matrix method for higher-order probe correction in spherical near-field scanning is based on a renormalized least-squares approach in which the normal matrix closely resembles the identity matrix when most of the energy of the probe pattern resides in the first-order modes. This method will be “stressed-tested” in the present paper by employing probes for which up to 49% of the pattern energy resides in the higher-order modes. The condition number of the resulting normal matrix will be computed, and its “distance” from the identity matrix displayed. It is also shown how the condition number of the normal matrix can be further reduced.