Spherical Near-field Measurements 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.

Correction of antenna far-field image error data based on spherical scanning

The antenna pattern is an important graph for measuring antenna performance, and various parameters of the antenna can be observed from the antenna pattern. The method of obtaining antenna far-field pattern through near-field measurement technology and near-to-far-field transformation algorithm has the advantages of being free from environmental interference, low cost, and fast speed, making it an effective research method. In the process of spherical near-field measurement, it is necessary to ensure the stable operation of the probe used for measurement. If one of the probe channels is abnormal, it will have a serious impact on the measurement of the antenna pattern. This article proposes an algorithm for repairing near-field measurement data through Newton interpolation based on spherical scanning. The feasibility of the proposed method is verified by using the HFSS simulation data and the measured data of the microwave anechoic chamber. It is proved that the algorithm proposed in this paper has obvious effect on improving the accuracy of antenna pattern.

Estimation of Absorbed Power Density Based on Spherical Wave Expansion to Plane Wave Expansion for Exposure Assessment at 30 GHz

The international guidelines and standard specified the area-averaged absorbed/epithelial power density (APD/EPD) as the basic restriction at frequencies above 6 GHz. Assessment methods for APD are current under the consideration of standardization organizations. In this study, we first showed the enhancement of the APD due to the antenna/body interaction using full-wave simulations. We then proposed a hybrid method combining the spherical near-field measurement and numerical simulation of the exposure dose in human body, using an equivalent source reconstructed on the total field/scattered field boundary using the spherical wave expansion to plane wave expansion. By comparing with the full-wave simulated APD at 30 GHz for various antenna arrays, the proposed method gives a rather accurate estimation of the upper bound of the area-averaged APD (with relative differences < 28%). It can be utilized as an efficient method for electromagnetic safety assessment as the need for specific measurement equipment is eliminated.

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).

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.

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.

Deformation measurement by single spherical near-field intensity measurement for large reflector antenna

This paper presents a new method to obtain the deformation distribution on the main reflector of an antenna only by measuring the electric intensity on a spherical surface with the focal point as the center of the sphere, regardless of phase. Combining the differential geometry theory with geometric optics method, this paper has derived a deformation-intensity equation to relate the surface deformation to the intensity distribution of a spherical near-field directly. Based on the finite difference method (FDM) and Gauss-Seidel iteration, deformation has been calculated from intensity simulated by geometrical optics (GO) and physical optics (PO) methods, respectively, with relatively small errors, which prove the effectiveness of the equation proposed in this paper. By means of this method, it is possible to measure the deformation only by scanning the electric intensity of a single hemispherical near-field whose area is only about 1/15 of the aperture. The measurement only needs a plane wave at any frequency as the incident wave, which means that both the signals from the outer space satellite and the far-field artificial beacon could be used as the sources. The scanning can be realized no matter what attitude and elevation angle the antenna is in because the size and angle of the hemisphere are changeable.

Exposure Assessment of Array Antennas at 28 GHz Using Hybrid Spherical Near-Field Transformation and FDTD Method

To protect humans from overexposure to electromagnetic (EM) fields above 6 GHz, recently revised International Commission on Non-Ionizing Radiation Protection guidelines and IEEE standard specify the basic restrictions in terms of absorbed power density (APD) and epithelial power density, respectively. To assess the EM exposure resulting from the 5G mmWave array antenna, we adopted a hybrid method that combines spherical near-field measurements and numerical dosimetry with spherical near-field transformation. Four array antennas working at 28 GHz were used as the radiation sources. The incident power density (IPD), APD, and temperature rise in a stratified body model and anatomical partial body models corresponding to different body locations were evaluated for various antenna-body distances. The results were validated by comparison with the full-wave simulations. The differences in peak spatial-average IPD and APD between two methods are below 0.08 and 1.05 dB, respectively, for the simulated cases and generally decrease with the increasing antenna-body distance. The highest heating factor for the peak spatial-average APD was approximately 0.025°C/(W/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ). The results also show that the differences in APD and temperature rise caused by various exposed body locations are marginal.

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.

Study on System Design and Variation of Vehicle Antenna Eccentricity test

This paper takes the vehicle antenna as the research object, and carries out a comparative study on vehicle antenna testing by improving the NF-FF transformation algorithm and combining the special application scenarios of vehicle testing. Research has shown that when the vehicle is used as the DUT, there is a significant difference in the near-field test data when the vehicle antenna is in the center and the geometric center of the vehicle is in the test center; however, through correct NF-FF transformation processing, both far-field radiation patterns are consistent. It shows that the far-field radiation information of the whole vehicle can be obtained under the processing of spherical near-field measurement and near-far-field transformation. This conclusion provides a basis for the promotion of wireless performance testing of the vehicle.

Single-cut Far-Field Antenna Radiation Pattern Reconstruction Accuracy Analysis in Compact Anechoic Chamber Setup

In production testing, it is of importance to measure the key radiation parameters of an antenna under test (AUT), e.g., main beam peak and direction, sidelobes, and null depth and direction in a cost-effective setup with a short measurement time. As a result, practical measurement setups are often compact and equipped with only a few probes (or probe locations). However, these system limitations would introduce errors for antenna testing. This problem has become even more pronounced for 5G radios due to utilization of large-scale antenna configurations and high frequency bands. Spherical near-field measurements are nowadays an accurate and mature technique for characterizing AUTs, which however, necessitates a full spherical acquisition, leading to a long measurement time. Singlecut near-to-far-field transformation is a promising strategy since most of the key AUT parameters are available in the single-cut pattern and it requires much reduced measurement time. In this work, a simple and flexible scheme is proposed to evaluate errors introduced by limitations in practical setups for single-cut far-field (FF) antenna radiation pattern reconstruction, where the near-field data can be easily generated and modified according to the limitations introduced in practical multi-probe anechoic chamber setups, e.g., measurement distance, truncation range, and sampling interval. The reconstructed FF pattern is obtained using a commercial near-field to far-field transformation tool, SNIFT. The proposed scheme is numerically validated via comparing the reference FF pattern of a 4 × 8 uniform planar array composed of ideal Hertzian dipoles and reconstructed FF pattern. With the proposed scheme, the impact of practical system limitations on single-cut reconstruction accuracy can be easily analyzed.

Assessment of mmWave Exposure From Antenna Based on Transformation of Spherical Wave Expansion to Plane Wave Expansion

To protect human beings from overexposure to mmWave electromagnetic fields, international guidelines and standard specified the (exposure) reference level in terms of the peak spatial-average incident power density ( ps PD). In this study, we proposed a method to effectively assess the ps PD from a mmWave antenna using its spherical near-field measurement data. This method employed the transformation of spherical wave expansion to plane wave expansion to reconstruct the electric and magnetic field components on an evaluation plane in front of the antenna aperture. The ps PD was subsequently obtained based on the time-average Poynting vector. The accuracy of the proposed method was validated by comparing it with full-wave simulation results using 12 array antennas at frequencies ranging from 10 to 90 GHz. It was revealed that at 30 GHz, the reconstruction errors for ps PD4cm2 were less than 0.8 dB and 0.2 dB on evaluation planes 2 and 5 mm, respectively, in front of the antennas. The robustness of the proposed method against the noise, scan range, and number of spherical modes were evaluated. The proposal may eliminate conventional compliance assessment procedures once antenna performance is evaluated in spherical near-field measurement.

Fast Antenna Characterization Improvement by Pattern Rotation

A postprocessing procedure is proposed to improve the characterization of antenna patterns from a small number of field samples. By appropriately rotating the measured pattern of the antenna under test, the sparsity of its spherical harmonic spectrum can be enhanced. Our procedure enables extracting as much information as possible from a given measurement data set and leads to a notable improvement of the antenna radiation pattern reconstruction accuracy. The best orientation of the antenna is found by testing a reduced set of angles and using Gaussian processes as interpolation approach. This postprocessing requires no modification of the measurement procedure and, therefore, no extra measurement time. The improved fast antenna characterization accuracy is validated on both the spherical near-field and far-field simulations and measurements for various types of antennas and compared with the state-of-the-art approach.

Fast Spherical Near-Field to Far-Field Transformation for Offset-Mounted Antenna Measurements

In spherical near-field measurements, the number of measured samples grows with the radius of the minimum sphere enclosing the antenna under test (AUT). This leads to unnecessary long acquisition times for antennas mounted in offset positions with respect to the measurement sphere. In this letter, a general technique is proposed to reduce the sampling rate requirements of offset mounted antennas. The approach is based on centering the spherical wave expansion (SWE) over the AUT by a shift in the coordinate system to compensate for the offset. The purpose of this shift is to reduce the number of significant spherical waves required to represent the AUT field. In the subsequent steps of the algorithm, the coordinate system is brought to its original location to preserve the efficiency and probe-correction capabilities of the traditional spherical near-field to far-field transformation technique. This is performed by a proper translation of the SWE coefficients between both coordinate systems. Higher order probe correction is also supported intrinsically. The proposed algorithm is numerically tested to assess its performance. Electromagnetic simulation and anechoic chamber tests are used to validate it, showing reductions in measurement times with low transformation errors.

Range Extension in Partial Spherical Near-Field Measurement Using Machine Learning Algorithm

Often due to physical limitations, there is a gap in the very-near-field and near-field (NF) measurements of antennas. However, to compute the complete far-field (FF) pattern, near-field data over the whole measurement sphere are required. In this letter, an iterative extrapolation-based machine learning algorithm is presented to expand the region over which the calculated far-field is more accurate. In each iteration, the well-known analysis of variance test is used to check the overall feasibility of the regression model and derive the coefficients of the extrapolation function. To validate the method, three examples with a folded dipole antenna at 1 GHz, a vivaldi antenna at 5 GHz and a dual-frequency planar antenna are presented using both simulated and measured full and truncated near-field data.

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.

A Non-Redundant Sampling Representation Requiring the Same Number of Spherical Near-Field Measurements for Both Onset and Offset Mountings of a Quasi-Planar Antenna

An effective non-redundant near-field to far-field transformation (NFTFFT) with spherical scan, which accounts for the offset mounting of a quasi-planar antenna and employs the same number of NF data as in the case of onset mounting, is developed in this paper. The interest for this NFTFFT arises from the fact that the mounting where the antenna and the measurement sphere centers coincide could result to be impracticable. In this case, the amount of NF data required by the traditional spherical NFTFFT can significantly increase. In the proposed NFTFFT, this drawback has been overcome by accurately recovering these NF data from the collected non-redundant ones via a sampling expansion, obtained by exploiting the non-redundant sampling representations and assuming the antenna as enclosed in an oblate spheroid. Numerical and experimental results assessing the effectiveness of the developed technique are reported.

Research on dual‐polarisation printed dipole antenna for spherical near‐field measurements

Here a dual-polarisation printed dipole antenna for spherical near-field measurements is proposed. The printed dipole employs double-layered structure to obtain low cross-polarisation and uses irregular patch to get wide bandwidth. Two such dipoles are cross arranged to form a dual-polarisation dipole antenna. Simulated results show that the bandwidth is over 72% when VSWR is less than 3, the cross-polarisation ratio is better than 30 dB, the port isolation between two dipoles is better than −26 dB, while good radiation performance is also obtained. All the results above prove that the crossed arrangement structure will not affect the performance of the printed dipole, and the strength of mutual coupling between probes can be neglected.

Full Hemisphere Aperture-Antenna Far-Field Patterns From Planar Near-Field Measurements

A simple geometrical estimate resulting from rigorous asymptotic evaluation of the Fourier transform integral leads one to conclude that the angular range of the far-field obtainable using planar near-field measurement techniques is limited by the extent of the scan plane and the separation of this plane from the antenna under test. Here, it is shown that this angular range can be extended to a full hemisphere through proper selection of the measurement parameters and careful treatment of the transform integral. In particular, it is noted that the transverse spectrum of the field can possess singularities. These singularities are spectrally filtered by the finite size of the scan plane resulting in a “muting” effect that must be accounted for in the data processing. This is accomplished through the analytic approximation of the bandlimited singularities. Three hardware examples are shown that demonstrate this more detailed approach comparing the results with corresponding patterns obtained using spherical near-field measurement.

A Wideband, Single-Layer Reflectarray Exploiting a Polarization Rotating Unit Cell

We present a new technique for designing wideband, single-layer reflectarray antennas. The proposed reflectarray exploits a wideband, polarization rotating unit cell that rotates the polarization of the reflected wave with respect to that of the incident wave by 90°. This unit cell and its mirror image were used as 1 bit spatial phase shifters capable of providing a 0°/180° phase shift over a wide bandwidth of 5.4–12.7 GHz. Using this, a reflectarray antenna with 1 bit phase quantization was designed to operate at ${X}$ -band with aperture dimensions of $28.8\,\,\text {cm}\times 28.8$ cm and a focal length of 30 cm. A prototype of the proposed reflectarray was fabricated and characterized using a spherical near-field measurement system. We determined the antenna’s 3 dB gain bandwidth to be approximately 37.4% with a maximum realized gain of 24.2 dBi. Over its entire band of operation, the polarization purity of the antenna remains better than 13 dB and sidelobe levels of better than 12 dB were achieved.