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Antennas are used in virtually all wireless, communications and radar systems. As key elements in these applications, antennas play a crucial role in determining the system’s overall performance. This makes accurate antenna characterization essential for any wireless application. Traditionally, electrically large antenna ranges are not equipped to perform return loss measurements and thus a separate benchtop vector network analyzer (VNA) setup is required for measuring reflection coefficient or VSWR of an antenna under test (AUT). In this paper, we demonstrate the two-port S-parameter measurement capability of the NSI-MI Vector Field Analyzer™ (VFA) and how it can be used to integrate return loss measurements in an antenna range. The VFA’s ability to perform multi-channel vector (amplitude and phase) electrical measurements and its long cable support using remote mixer interface (RMI) modules makes it well suited for antenna characterization, especially in electrically large measurement systems. For this experiment, we selected three known millimeter-wave components as devices-under-test (DUT) and measured the S-parameter matrix for each. These WR-10 band measurements were made using the VFA with Virginia Diodes VNAX frequency extension modules. Results are compared with Keysight’s N5225A performance network analyzer (PNA) using the same set of frequency extension modules for verification. Millimeter-wave S-parameter measurements taken on VFA and PNA setups for all DUTs are compared based on three factors: Repeatability, reproducibility, and measurement comparison. The variations between successive measurements are presented in graphical form to compare repeatability of both instrument setups. Reproducibility results are compared to show the difference between independent repeat measurements taken on both instrument setups. Error distribution comparison is presented for reproducibility test data to compare the measurement variations contributed by random sources for both instrument setups. Measurement comparison result shows the total difference between independent VFA and PNA measurements taken for each DUT. Keywords — Millimeter wave measurements, Scattering parameters, Calibration, Network Analyzers, Antenna measurements
The IEEE Antennas and Propagation Standards Committee (APS/SC), sponsored by the IEEE Antennas and Propagation Society (AP-S), is currently developing a recommended practice tailored for the modeling and simulation of antennas (IEEE P2816). Different numerical modeling techniques such as the finite element method (FEM), integral methods e.g. method of moments (MoM), the finite difference time domain (FDTD) method and the transmission line matrix (TLM) method are described. In addition, benchmark problems are also considered to ease the use of the recommended practice. For example, the biconical antenna is proposed as a benchmark problem for the numerical simulations using the above methods. Several international laboratories have already performed the numerical simulations of this biconical antenna. To confront the theoretical and numerical results with measurements, the same biconical antenna was proposed for fabrication and inter-laboratory measurement campaign. Herein, the fabrication and initial measurements of the prototype are discussed. An important difference between the theoretical, simulated and fabricated antenna is the feeding point. In theory, it is considered infinitely small whereas in the proposed numerical model, the gap in-between the two cones is assumed to be 0.3 mm. In practice, such a small gap cannot be enforced during the fabrication process. Since the off-the-shelf available SMA connector has a minimum diameter of 0.7 mm, the minimum diameters of the central and outer conductors of the 50W feed-line (Teflon filled) section were kept at 1 mm and 3.34 mm, respectively. In the actual fabrication, only a gap of 4 mm could be achieved in-between the two cones. An external quarter-wave skirt is further used as a balun for reasonable impedance matching. A Rohacell section is used to hold the arms of the antenna which are heavier. The realized prototype, therefore, differs significantly at the feeding point. The relevant simulation and experimental results are presented.
Over the past decades, near-field (NF) measurements have been established as a reliable alternative to direct far-field (FF) or compact-range measurements for the verification of radiation properties of antennas. Quantities of interest typically include the FF characteristic obtained by means of an NF FF transformation (NFFFT), which is a computational post-processing step applied to the NF data. Common NFFFTs work with time-harmonic data and require the acquisition of magnitudes and phases of the NF samples with respect to a common reference signal. In other words, classical NFFFTs require the observed magnitude and the observed phase data to be drift-free during the time span of the complete measurement. With increasing measurement frequency and exposed or complex measurement setups in uncontrollable environments, e.g., as encountered in outdoor NF antenna measurements with unmanned aerial vehicles (UAVs), phase stability quickly becomes a limiting factor. Therefore, nonlinear phaseless NFFFTs have been developed that do not require any phase information, which, however, heavily rely on accurate and globally consistent magnitude information and are notorious for their unreliable behavior. Recently, a linearized and reliable NFFFT operating on locally consistent, i.e., relative, phase information has been reported. By using local phase differences within the NF data, the method becomes immune to phase drifts of the reference signal. However, in real-world measurements in uncontrolled environments, drifts in the measured power or magnitude may occur as well, e.g., caused by temperature variations during UAV flights, which render common phaseless NFFFTs useless. In particular, this prevents the use of the otherwise reliable linearized transformation. We investigate NFFFTs requiring a variable degree of global synchronization. In particular, a linearized transformation utilizing only relative, i.e., locally synchronized, information, both in magnitude and phase, is presented. It is shown that the transformation yields similarly accurate results as transformations employing global data, while being immune to magnitude and phase drifts. Furthermore, we compare the overall benignancy of a complete set of retrieval problems: completely phaseless, magnitudeless and mixed relative/global magnitudes and phases. Transformation results for simulation data illustrate the accuracy and suitability of the transformations with relative data, even for electrically large problems.
Motivation and background: The increasing sophistication of wireless communication systems necessitates accurately designed test environments such as anechoic chambers. The minimum achievable level of noise and interference in such test environments is essentially determined by the reflectivity of the absorbers installed, emphasizing the importance of characterizing their scattering behavior under realistic test conditions. In order to improve the modeling of absorber-lined anechoic chambers e.g., based on ray-tracing methods, a profound understanding of the relationships between the geometrical (e.g., pyramidal or convoluted shapes) and material properties (complex-valued dielectric permittivity) and the frequency- and angle-dependent reflectivity of the absorbers is needed. Objectives and methods: The angle-dependent scattering off convoluted microwave absorbers at normal and oblique incidence was investigated at frequencies between 2 GHz and 18 GHz. Based on measured permittivity values, a unit-cell model was constructed to compute the angle-dependent reflectivity of absorbers of different shapes. To verify the model, the scattering off such absorbers was measured in a bi-static setup at different angles-of-incidence up to 60 degrees, and compared to the numerical results. In addition to the convoluted absorber geometry, pyramidal and wedge-shaped absorbers were studied, in order to analyze the influence of the absorber geometry on the reflectivity while maintaining the same material properties. Results and conclusions: The numerical results of the convoluted absorbers agreed well with the measured reflectivity, thus validating the numerical model. The results revealed an increase of the reflectivity at angles-of-incidence above 45 degrees, in accordance with expectation. Compared to the convoluted geometry, the pyramidal and wedge absorber shapes showed reflectivity values about 10 dB lower, for frequencies at which the electrical size of the absorbers exceeded unity. Together with the results of previous studies, these findings provide important ingredients for a comprehensive database of the angle- and frequency-dependent absorber reflectivity, from which a consistent ray-tracing modelling of anechoic test environments can be derived. This research has been funded by the German Research Foundation (Deutsche Forschungsgemeinschaft, DFG) under the grants HE3642/14-1 and BO4990/1-1 (Electromagnetic modeling of microwave absorbers - EMMA; Project-No. 418894892).
Base station antennas for mobile communications (BTS) emit high levels of electromagnetic radiation in their vicinity. These antennas are usually located on the top of a building, and it is critical to determine those areas where the total power density surpasses the levels dictated by the regulators of the corresponding country. This estimation allows mobile operators to optimize the performance of the cellular network while keeping safe EM emission levels in occupational and public areas. The power density on a given region depends not only on the total radiated power but also the radiation pattern of the antenna and the influence of the environment. As a result, antenna measurements become useful to perform these calculations. This paper presents a simulation tool which computes EMF exposure values of BTS antennas considering the influence of the building roof. The tool uses analytical calculations to obtain a fast evaluation of the fields radiated by all the antennas of a given cell site. The calculations are performed considering the radiation of the antenna as a contribution of three different propagation phenomena: a free space direct radiation component, a reflected component due to the presence of the ground and a diffracted field due to the roof corners. Both direct and reflected rays are computed using the Spherical Wave Expansion (SWE) of the BS antenna assuming PEC boundary. The diffracted ray is computed using ITU 526-8 recommendation. The proposed software requires a measurement of the BTS antenna radiation pattern in anechoic chamber. Spherical near-field measurements are proposed to retrieve all antenna parameters needed for the calculations (SWE, efficiency, electrical steering configurations). Full details of all performed calculations will be disclosed on the paper, as well as some simulation examples with measurement data of real antennas to demonstrate its capability and computational efficiency.
Nowadays, 5G new radio and commercial networks have been widely developed and deployed by many communication service providers around the world. State-of-the-art techniques such as massive multiple input multiple output systems, 3D beamforming technologies, etc. are utilized to enhance spectral efficiencies and system capacities within cellular coverage areas. Additionally, 5G base stations with active antenna systems (AAS) are comprised of the passive antenna array, transceiver frontend and base band units, all integrated into one module, so that the traditional antenna RF ports are replaced with ethernet-based interfaces. Consequently, different from conventional 4G antenna system verification processes, to ensure the optimal cellular signal coverages of the 5G AAS base stations, new measurement methods to verify the radiation properties at RF carrier frequencies of the AAS need to be employed. The radiation pattern test method of the AAS by using a vector network analyzer (VNA) based spherical near field antenna measurement system through over-the-air (OTA) to obtain the magnitude and phase distributions of the electromagnetic fields from the antenna under test (AUT) with single-tone transmitting will be presented in this paper. In order to verify the above mentioned concepts, a signal generator is used to provide the single-tone source into a commercial passive base station antenna. Also, two received channel connections to the VNA are included, where a reference antenna placed adjacent to the back of the AUT for phase recovery is added together with the existing probe of the spherical near field antenna measurement system for reconstructing the near-field amplitude and phase of the fields during scanning. Thus, the near-field to far-field transformations and back projections can be performed for active antenna performance verifications. Radiation patterns obtained by the above OTA near field measurement method demonstrate good agreements with those from conventional near field tests at 3.5 GHz for 5G FR1 AAS performance verifications.
Plane wave generator (PWG) for Over The Air (OTA) characterization of beamforming millimeter wave devices, provides an attractive solution comparing to conventional measurement techniques (Compact Antenna test Ranges (CATR) and Far-field chambers). MVG’s Plane wave generator for 5G NR FR2 applications ([1]-[4]) is an innovative tool which permits the user to measure the radiating elements with low to medium directivity radiation characteristics with excellent precision. Conventional CATR systems are not suited for stationary DUT (with / without person) measurement scenario. In this paper, experimental results are presented for a dual-polarized PWG system, covering the 3GPP bands n257, n258 and n261 (24.25-29.5 GHz). System measurement results show good comparison with simulations and measurements of the PWG alone. Another advantage of PWG presented here, is that we can modify the size of the QZ. Results from a pre-production unit for a 15cm QZ shows amplitude variation of less than ±1 dB and achieve more precision for smaller DUT. Measurement results from the pre-production unit with a quiet zone of up to 38cm sphere diameter, show amplitude variations of less than ±2dB. This variation is compatible with the DUT + phantom or human measurement application. Pattern results for Antenna Under Test (AUT) with low to medium directivity (6dBi up to 17dBi) compare well with simulations and measurements from other systems. For a given AUT, the impact of different positioning mast is also evaluated. Excellent stability of patterns, when the AUT is placed at different positions inside the QZ, is observed. These results confirm that the dual-polarized PWG system presents an attractive solution for FR2 characterization of low to medium directivity radiating elements.
Progressing towards highly automated vehicles, radar systems have been developed into reliable assistance systems of environmental perception for a wide spectrum of mobile applications in air, on sea and rails, and especially on roads. This success as well as further improvements necessitate the precise characterization of radar objects with radar cross-section (RCS) measurements throughout the manifold parameter space, including frequency, aspect angles or even illumination and observation angles in bi-static radar constellations. According to the IEEE RCS standard 1502-2020, parasitic effects from ground reflections have to be taken into account in the test setup in almost all RCS measurement systems. In ground-plane ranges, multipath signal propagation is even considered intentionally. In this paper, the influence of ground reflections on bi-static radar measurements has been investigated both analytically and experimentally. A geometry-based analytical model was applied to calculate interference and the resulting small-scale fading of the electric field strength at the receiver. The geometry consists of independently arranged transmitter, receiver, and scatterer. The model includes antenna crosstalk, ground reflections for different relative heights of transmitter and receiver, and the scattered signal contributions. As a result, six paths are considered in terms of their time delays and phases, resulting from the classical two-way propagation model in a point-to-point link plus the four-way radar propagation model including ground reflections. The model yields interference gains between 0 and 4, where the maximum value is uniquely obtained at equal distances between transmitter and scatterer, and between scatterer and receiver, respectively. The variations of the bi-static angle lead to further fluctuations, which confirm to expectation and will be described in the full paper. The analytical model was validated with bi-static radar measurements in a semi-anechoic chamber with a metallic ground floor. As a canonical radar target, a metal sphere was measured in the frequency range between 1GHz and 10GHz at different heights and distances. The measurement results confirm the analytical model and provide the basis for further extensions of practical relevance, e.g., the reflectivity parameters of the ground (e.g., dry and wet road surfaces). Funded by: Federal Ministry of Education and Research (BMBF), grant number 16ME0164K
This paper presents a selection guide for metal mesh based on shielding effectiveness and optical visibility requirements. Concepts of mesh sizing, wire diameter, metal type, opening size, metal color, and mesh patterning are discussed. The guide provides a detailed explanation of factors that contribute to the shielding performance and optical transparency of various mesh options. Metal mesh has a range of applications in the microwave, antenna, and EMC industries as they are particularly suited for protecting chamber viewing windows. Shielding effectiveness performance is dependent on the mesh sizing, wire diameter, and opening size, where the dimensions of the apertures directly influence the suitability of the mesh for a given frequency range. Finer mesh yields superior shielding, but with limited optical visibility. For this reason, the necessity for finding an optimal tradeoff between shielding performance and optical properties arises, where the selection guide in this paper can be used to make an informed decision. For ideal optical properties, the mesh sizing is critical since it determines the visibility through the material. Mesh layering and alternative optically transparent shielding solutions like RF film are also compared. Although layering of metal mesh offers additional shielding, the layering is associated with a reduction in visibility due to the mesh density and patterning from the Moiré effect. Alternatives like RF film can offer highly transparent solutions, but with inferior shielding effectiveness than metal mesh. This paper provides a quantitative analysis of shielding effectiveness results based on mesh parameters and aperture dimensions. The appropriate frequency ranges for the various metal meshes are also calculated. Using the selection guide presented in this paper, the user is enabled to make an educated technical decision on the metal mesh best suited to satisfy the shielding effectiveness and optical visibility requirements for the application.
We demonstrate cylindrical near-field measurement and far-field characterization of 300-GHz band antennas using photonics-based technologies. The measurement system is based on a self-heterodyne technique and non-polarimetric frequency down-conversion technique in which an electrooptic (EO) sensor was used. An optical beat signal generated using two 1550 nm laser diodes with the difference frequency of 300 GHz was EO converted by a uni-traveling-carrier photodiode (UTC-PD) to generate 300 GHz RF signal. The material of the EO crystal was a DAST (4-N,N-dimethylamino-4-N-methyl stilbazolium tosylate) and the dimension was approximately 0.5 mm x 0.5 mm x 1 mm. The EO probe was composed entirely of dielectric material, thus the scattering by the probe was minimized. In addition, the use of optical fiber significantly reduces scattering compared to conventional probe antennas that use metal cables or waveguides. As an optical local oscillator signal (LO signal), a coherently frequency-shifted optical beat signal generated based on the self-heterodyne technique was used. By tuning the frequency of one laser diode, the frequency of the RF signal can be swept. The system bandwidth is limited by the bandwidth of the UTC-PD, which covers WR-3.4 band (220-330 GHz). The RF signal up-converted to the optical domain in the EO crystal was coherently detected by a low-speed photodiode to generate intermediate (IF) frequency signal. The amplitude and phase of the IF signal, which are copies of those of the RF signal, were detected with a lock-in amplifier. The antenna under test (AUT) was a rectangular-type horn antenna (WR-3.4) with an antenna gain of approximately 25 dBi. The AUT was rotated by ±45° horizontally and the EO probe mounted on a 4-axis robotic arm was linearly moved vertically by ±15 mm to measure the cylindrical near-field distribution. The far-field distribution was estimated by transforming the measured cylindrical near-field distribution without probe correction. The simulation results and measured results were agreed very well. In E-plane, the first to third sidelobe levels agreed within 1 dB. The 3dB beamwidths of the measured result were 10.3° and 9.5°, whereas those of the simulated results were 9.5° and 9.7°, in the E- and H- plane, respectively.
Motivation and background: Wireless communications are key for connected and automated driving. Beyond automation levels that require the presence of a driver, tele-operated driving has been receiving more attention recently. For such applications, gapless wireless coverage and stable connectivity are required, enabling a reliable exchange of data like control information or high-definition maps even under poor radio wave propagation conditions. Therefore, extensive testing of the link stability of mobile wireless communication systems is necessary, especially in challenging scenarios that are susceptible to link failure. Objectives and methods: We propose the emulation of relevant corner-case scenarios for virtual-drive testing, consistently and reproducibly derived from field-operational tests on public roads. The available data rate of a LTE link near mobile cell edges is considered a relevant test metric, since the link is expected to be particularly susceptible to failure under such conditions. We performed field-operational tests on two different test tracks, in order to prove the reproducibility and consistency of the proposed case. We have emulated the scenarios in a wired setup under realistic conditions using a communication tester and an interference generator. Power-related key-performance indicators like RSRP and SINR as well as the achievable throughput were systematically studied under laboratory conditions. Results and conclusions: The region around cell edges could undoubtedly be identified as a challenging scenario for automotive LTE communications, leading to a reduction of the data throughput by a factor of 5, on average, compared to the maximum data rate during a test run. This effect could be consistently observed on both test tracks. The emulation of wireless link parameters in such corner-cases reproduced the physical parameters of the field-operational test results very well. Changes of the data rate could be associated with the channel quality indicator. Approaches to improve the emulation of the drive tests is in the focus of future work. However, given the simplicity of the test setup, it represents a sound basis for refined over-the-air tests.
The ESA HERA-JUVENTAS mission relies on 50-70MHz dipole antennas mounted on a cubesat [1]. The mission requires an accurate verification of the radiation properties of the whole antenna system including the matching and amplification boards. The performance verification of low gain antenna systems below 400MHz is a challenging task because of the reflectivity of the measurement environments. Spherical Near-Field (SNF) measurements are the most suitable approach for such Devices Under Test (DUT) [2] but require a sufficiently large anechoic chamber equipped with absorbers able to provide low reflectivity. Meeting these requirements at low frequencies is often too expensive, as for the case of the HERA-JUVENTAS antenna system verification. An outdoor testing solution could be an alternative but at the expenses of measurement accuracy and repeatability. The SNF system installed in the HERTZ testing facility at ESA-ESTEC has been selected as cost-effective solution for the verification of the HERA-JUVENTAS cubesat. The HERTZ anechoic chamber was originally designed for measurements down to 400 MHz, hence, due to limited electrical size (~5λx2λx2.5λ) and poor absorption provided by the treatment of the chamber walls (~2dB), a high reflective environment is expected at 60MHz. The so-called Synthetic Probe Array (SPA) technique is a very effective solution to significantly improve the measurement accuracy in case of reflective environments. With the SPA technique each sample point on the NF sphere is measured with several probe positions generating a “virtual” array able to properly shape the equivalent probe radiation pattern, minimizing the illumination of the chamber walls. Validation of the SPA technique, combined with the λ/4-averaging technique to also minimize the effect of the backwall of the DUT, have been recently performed by means of simulations and scaled measurements as presented in [3]-[4]. In this paper the actual measurement results of the HERA-JUVENTAS cubesat performed in HERTZ with the SPA and λ/4-averaging techniques will be presented for the first time. Comparisons in terms of radiation pattern and gain, with the conventional single probe SNF approach will be shown to highlight the effect of the measurement environment at 60MHz and the improvements obtained with the considered techniques.
Near-field (NF) measurements is considered as a very accurate and versatile antenna testing technique. It became widely used as a preferred measurement technology in antenna measurement systems about four decades ago. Today, hundreds of near-field antenna test facilities are installed worldwide. The IEEE Std 1720™ “Recommended Practice for Near Field Antenna Measurements” is specifically dedicated to near-field antenna measurements. It therefore complements the IEEE Std 149-1979™ “Standard Test Procedures for Antennas” which describes general antenna measurement procedures. The Std 1720™ was originally approved in 2012 as a completely new standard by the IEEE Standards Association Standards Board. It is highly relevant for users performing NF antenna measurements but also the design and evaluation of NF antenna measurement facilities. After 10 successful years, the standard expires this year and will no longer be an active standard under the IEEE. A revision is required to update the document with new developments and technologies that have matured since the first edition. This is the scope of project P1720 that was approved by IEEE-SA in 2019 to undertake “minor revision” of the current standard. A Working Group (WG) of the Antennas and Propagation Society Standards Committee (APS/SC) has been formed for this task. The WG is transversal, with users and experts of the near field measurement field and consist of approximately fifty dedicated volunteers from industry, academia, and government. This paper gives an update on the running activities and discusses the suggested changes to the standard.
The near-to-far-field transformation (NTFFT) technique adopting the plane-rectangular (PR) scanning is the most simple one from the analytical and computational points of view and can be suitably employed when characterizing antennas which exhibit pencil beam radiation patterns. In recent years, NTFFTs using the nonconventional planar wide-mesh scanning (PWMS) have been developed. They allow a remarkable measurement time saving with respect to that adopting the classical PR scanning, since their raster grid is characterized by meshes which become larger and larger as their distance from the scanning plane center increases. These NTFFTs have been obtained by applying the non-redundant sampling representations of the electromagnetic fields to the voltage detected by the scanning probe and adopting suitable AUT modellings for volumetric and quasi-planar AUTs. The evaluation of the AUT far field is then got by applying the PR NTFFT, whose input data are accurately recovered through optimal sampling interpolation expansions from the collected PWMS samples. In both the conventional and non-conventional scannings, the sampling points are reached through an x-y scanner. However, the finite resolution of the probe positioners and/or their imprecise control can prevent to exactly collect the near-field samples at the prescribed sampling points and imperfections in the mechanical rails driving the motion of the probe can cause a deviation from the considered measurement plane. Accordingly, 3-D positioning errors, which can be revealed via laser interferometric techniques, affect the acquisition. The aim of this work is to develop an effective NTFFT with PWMS from 3-D probe positioning error affected near-field measurements. To this end, the so named k-correction (Joy and Wilson, AMTA Proceedings 1982) will be used to compensate the positioning error related to the deviation from the considered measurement plane. Then, an iterative procedure (D’Agostino et al., International Journal of Electronics and Communications 2020) will be applied to retrieve the near-field samples at the points specified by the non-redundant sampling representation from those obtained at the previous step and affected by 2-D positioning errors. Numerical tests will show the capability of the procedure to fully compensate the 3-D positioning errors affecting the acquisition of the PWMS samples.
With the growing applications of wireless systems in different aspects of everyday life, from the consumer-electronic devices to internet-of-thing applications to wireless health-monitoring systems, there is an expanding need for reliable measurement of the radiating performance of these devices. The electromagnetic compatibility (EMC) tests, including immunity and interference tests, are normally done in shielded rooms the walls of which are covered by the so-called hybrid absorbers. These absorbers are made of a magnetic lossy layer giving absorption at low frequencies and a dielectric lossy geometrical absorber which is mainly responsible for the electromagnetic absorption at higher frequency range. The heart of hybrid-absorber design, which can be reduced to a wide-band matching problem, is to match the dielectric lossy part to the magnetic lossy layer. The magnetic layer which is mostly made up of a ferrite material, is a relatively thin layer, i. e. less than 1 cm thick, which is supposedly homogeneous. The dielectric geometrical absorber part has a relatively large thickness, e.g. 30 inches, and incorporates geometries like a pyramid to make a tapering against the wave which is going to be absorbed by the absorber. Because of the relatively large thickness of the dielectric lossy part and the production-process techniques used to make these parts, there are inhomogeneities in this part. In the current paper, the impact of the inhomogeneity of the dielectric part on the matching performance of the hybrid absorber is investigated in details. In the 1st stage, a 30-ich absorber is chosen and sliced in 15 different layers the permittivity is of each measured separately. Using these permittivity values, a complex model is made and simulated to get an expected reflectivity value on the ferrite layer. In the 3rd step, the absorbers of the same production batch are chosen to be measure in real-scale measurement setup and the reflectivity values are measured. Finally, the measurement and simulation results are compared and the impact of inhomogeneity of the dielectric absorber on the hybrid-absorber performance in concluded.
For millimeter-wave wireless devices used in the close proximity to the human head or body, the compliance evaluation to regulatory exposure limits is determined from near-field free-space power density measurements. For mobile phones and other portable equipment, the standard assessment technique involves the characterization of the power density on planes, as close as 2 mm from each facet of the device under test (DUT), as well as on anthropomorphic surfaces. These measurements are typically realized by means of a pseudo-vector diode-detected probe, acquiring the electric field magnitude and polarization ellipse at multiple locations over the scanning area. Phaseless techniques have been employed to deduce the required phase and magnetic field information for the calculation the Poynting vector. Although accurate, this technique presents some limitations: prohibitive test times; the inability to distinguish between various frequency contributions of the electric field due to the detection process; or the necessity to implement specific test modes in the DUT to fix the radiated beam in a given state. A recent paper proposed an alternative method which overcomes these listed limitations. The approach relies on the use of spherical antenna / over-the-air (OTA) phasor electric field acquisitions in an anechoic chamber environment, performed in the radiative field region and combined with near-field processing through equivalent currents reconstruction. This paper proposes an extensive validation of this method based on simulations and measurements of reference antennas, as defined in the IEC 63195. The reference measurements are realized with a standard-compliant 6-axis robot assessment system. The uncertainty contribution coming from probing at distances where reactive field components cannot be captured is investigated, demonstrating a negligible influence on reconstructed field distributions down to a third of the wavelength from the reference antenna, for which a theoretical interpretation is provided. It is also shown that characterizing the detailed changes in the reactive field is not necessary to obtain an accurate peak spatial-average power density value, which is the relevant metric for compliance assessment. A systematic analysis of the error affecting this specific quantity is also provided.
We propose a new Fresnel-field to far-field transformation to measure the absolute gain patterns of an antenna on a strip region between some elevation angles when the long axis of the antenna is placed in the horizontal plane. The measurements are done on multi circles of the same radius on the multi-cut planes (parallel to each other) at the Fresnel distance. The number of the multi circles depends on the antenna height and the radius of the circles. The number reduces to one, that is, the single-cut near-field to far-field transformation if the measurement radius is larger than the far-field distance determined by the height of the antenna. In our method, the number of the circles or the cut planes is proportional to the square root of the antenna height, whereas the conventional cylindrical scanning needs the number proportional to the antenna height because the height interval is a constant less than the half wavelength. Therefore, the measurement time by our method can be much less than the one by the cylindrical scanning. The proposed transformation is an extension of a single-cut near-field to far-field transformation combined with the Fresnel approximation along the z (height) direction. In our method, we can obtain the absolute gain pattern in the stirp region within the elevation angles spanned by the cut planes where the measurements are done. Whereas the elevation angles are limited by the angles where the Fresnel approximation holds, the azimuth angle range is only limited by the measured one and can be 360 degrees. In the presentation at the Symposium, we will show the simulation results and demonstrate the measurement results for a standard horn antenna at 70 GHz band using the new type of a photonic sensor. Our method has a possibility to extend the measurements on the circles to arbitrary curves on the multi-cut planes. This means that our method is most suitable to the measurement system using a robotic arm and a RoF (Radio on optical Fiber) technique.
Large deployable reflectors are critical for future Earth observation missions, science missions and in telecommunication, where an enhanced footprint and increased resolution are required and ensured by electrically very large reflector antennas. To accurately correlate simulations and measurements of such large and complicated antenna structures is a crucial step in improving the technology readiness level of these innovative antenna designs. A useful tool in this process is equivalent current reconstruction methods for antenna diagnostics, to allow comparisons between expected and realized performance. By finding the equivalent currents in the extreme near-field region that radiate a given/measured electromagnetic field, the user can accurately characterize the electromagnetic behaviour of the antenna under test. In this work, we present an antenna diagnostics investigation of an electrically large reflector antenna from the European Large Deployable Reflector project [1]. The antenna consists of a 5.1 m diameter deployable offset reflector in lightweight mesh technology. The antenna is an offset parabolic reflector with f/D equal to one and it has been measured at 10.65 GHz and 18.7 GHz. At such electrical sizes, an equivalent current investigation has previously been out-of-scope for the computational solvers in the market. In a recent ESA study, an accelerated equivalent current reconstruction solver based on [2] has been carefully implemented and then applied [3] to perform source reconstruction of the full reflector antenna based on measured and simulated data. Comparing the two sets of reconstructed currents gives the possibility to highlight potential deviations and pinpoint problematic aspects of the antenna design. [1] C. Cappellin, M. Lori, A. Geise, C. Hunscher, and L. Datashvili, “Predicted and Measured Antenna Patterns of the European Large Deployable Reflector,” Proceedings of EuCAP, 2022. [2] J. Kornprobst, R. A. M. Mauermayer, E. Kılıç and T. F. Eibert, "An Inverse Equivalent Surface Current Solver with Zero-Field Enforcement by Left-Hand Side Calderón Projection," Proceedings of EuCAP, 2019. [3] O. Borries, M. H. Gaede, P. Meincke, A. Ericsson, E. Jørgensen, D. Schobert, and E. Gandini, “A Fast Source Reconstruction Method for Radiating Structures on Large Scattering Platforms,” Proceedings of AMTA 2021.
The characterization of antennas is a time-consuming task. Its acceleration leads often to large and sensitive numerical problems. Therefore, special care must be taken of the choice of the parameters, the optimization, and the stability of the employed resolution methods. Based on Huygens’ principle, the radiation operator can be defined from an equivalent surface enclosing the Antenna Under Test (AUT). The discretization of this operator leads to the so-called radiation matrix. An expansion basis of the fields radiated from the equivalent to the measurement surface is constructed by the Singular Value Decomposition (SVD) of that matrix. The Reduced-Order Model (ROM) is the compressed representation of this basis obtained by truncating the SVD. The truncation order, T, is computed by inspection of the singular value distribution and is strongly linked to the number of degrees of freedom of the radiated fields. Several practical and technical aspects are studied in this article to provide a systematic, efficient and reliable procedure for the characterization of the radiated fields using the ROM. Analytical criteria are used to define the dimensions of the radiation matrix enabling a stable determination of the compressed basis. The truncation order, T, is the key-point of this method as it determines the size of this basis. Therefore, its variation is studied with respect to the discretization step and the geometry of both equivalent and measurement surface. Finally, the Randomized SVD (RSVD) is used in order to significantly reduce the computation time with negligible impact on the accuracy. To illustrate our procedure, it is applied to various scenarios and experimental results of spherical measurements. Estimations of the time savings by using the RSVD are also provided.
With the growing of vehicular communication technologies, the need for performing radiated measurements accounting for the full vehicle is becoming increasingly important. In modern cars, antennas are an integral part of the vehicle which is too complex to be represented by a simple ground plane during the tests. 5GAA test report [1] unifies measurement procedures for vehicle mounted antennas for both passive (at the antenna level) and Over-the-Air (OTA - at the modem level) measurements. Both Far-Field (FF) and Near-Field (NF) measurement systems are considered for testing of the full vehicle. In NF systems the radiated signals are measured on a closed surface at reduced distance, and a NF-to-FF transformation is applied. Since the transformation requires phase coherence between the transmitter and the receiver, NF systems are suited for passive tests. On the other hand, FF ranges are more suited for OTA tests, but requires large measurements distances, and hence expensive testing environments. OTA testing at reduced distances offers several advantage including the possibility to consider smaller and cost-effective anechoic chambers and the reduction of the system path losses (improved dynamic range). Moreover, the use of multi-probe systems dramatically reduces the testing time. The possibility of performing automotive OTA tests in spherical multiprobe systems at a reduced distance will be investigated in this paper. Simulations of a realistic vehicle with antennas installed in different locations will be considered to assess the uncertainty introduced by the reduced measurement distance. Figure of merits like different partial radiated powers [1] will be considered. Experimental automotive OTA measurements of a monopole-like antenna, installed on the roof of a vehicle will also be presented. These measurements have been performed at LTE bands in a spherical multiprobe system with a 4m radius. The measurements will be analysed comparing direct OTA measurement with a two-stage method, where passive antenna measurements and a few sampled OTA measurement points are combined. The outcome of the simulated analysis and experimental tests will be used to preliminary assess the uncertainty of automotive OTA measurements at reduced distance considering metrics relevant to automotive technologies [1].