AMTA Paper Archive
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Precise Phase Center Localization of Automotive LTE Antennas in the Installed State Through Phaseless LTE Uplink Measurements
With the event of integrated and multi-standard wireless links, phaseless antenna measurements are attracting more and more interest in research. Especially in the context of connected and automated driving, antennas, frontends, and digital signal processing units merge into telematic units and require new methods for performance evaluation in the installed state. The measurement of the phase diagram and the exact absolute positioning of electrically large antennas, i.e., antennas interacting with the car body, present challenges for safety-relevant applications and reliable test methods. This paper describes a way to determine the position of automotive antennas in the installed state with sub-wavelength precision from phaseless measurements. Realistic LTE uplink signals were used as test signals as they would be transmitted by an active device in a real-world scenario. The localization algorithm is based on orthogonal power measurements of the transmitted signal on a cylinder surface and a non-linear optimization. By comparison with a conventional localization based on spherical far-field data, an accuracy of the approach of less than 1 cm was achieved, which is less than λ/16 at the considered frequency of 1870 MHz.
X/Ku/Ka-band high Gain Reflector Antenna Intercomparison Campaign Results
Antenna measurement Intercomparison Campaigns represent a successful activity within the working group on antenna measurement of the European Association on Antennas and Propagation  since the group foundation in 2005. These campaigns, constitute an important resource for participating facilities to demonstrate their measurement proficiency, useful internally but also towards obtaining or maintaining official accreditations. In this paper we present the completion of a campaign involving a high gain X/Ku/Ka-band reflector, MVG SR40 fed by an MVG SH4000 Dual Ridge Horn. Preliminary results were shown in . Results from seven facilities are compared through plots of gain/directivity patterns. The data is used to generate reference patterns and establish accurate gain performance data based on the uncertainty estimates provided by each facility. Statistical analysis of the measured data such as Equivalent Noise Level and Birge ratio of each measurement with respect to the established reference will also be shown.
Errors and Prerequisites of the Short-Time Measurement and Transformation of Continuously Modulated Fields
Near-field far-field transformations (NFFFTs) are usually performed for time-harmonic fields. In cases where insitu antenna measurements are required and the antenna under test (AUT) is not accessible for specifically tailored test signals, the need for handling time-modulated fields arises. The shorttime measurement (STM) approach offers a way to deal with continuously modulated fields while a time-harmonic NFFFT can be employed. We present results of numerical simulations to demonstrate and characterize the STM approach for the case of a cylindrical measurement geometry as found in UAV-based antenna measurements. We further derive guidelines from the simulation results that describe the applicability of the STM for different measurement situations.
Additional Tools for Locating and Quantifying a Range’s Stray Signals
Earlier works have shown the benefits of imaging stray signals in a range with planar-scanner data. This paper discusses some additional tools that can be employed for stray-signal identification. Related range diagnostics are presented that employ Fourier spectral and holographic processing of 1D linear scans through the quiet zone. For the special case of a compact range, the interpretation of arrival angles from the paraboloidal reflector surface is explored. Measured data from multiple facilities are presented that were used to locate, quantify, and remedy the unwanted signals.
Experimental Validation of Full Probe Correction Technique using Wideband and Dual-Polarized Probes in Spherical NF Antenna Measurements
Full Probe Compensation (PC) techniques for Spherical Near Field (SNF) antenna measurements have recently been proposed and validated with success -. Such techniques allow the use of antennas with more than a decade of bandwidth as near field probes in most systems. The clear advantage is that multi-service/frequency measurements campaigns can be performed dramatically reducing the number of probes hence decreasing the downtime between two measurements. This is a highly desirable feature for modern antenna measurement applications such as automotive. The use of a dual-polarized probes further improves the measurement efficiency as two orthogonal field components are measured at the same time. The possible differences between the pattern radiated by the two ports of the probe should sometimes be considered to keep the overall measurement accuracy. The full PC technique objective of this paper accounts for generic dual-polarized probes and is validated for the first time. For this purpose, measurements of three monocone antennas from 450 to 6000 MHz performed with only one wideband (15:1) dual-polarized probe will be considered.
Reduction of Extraneous Signal Effects Using an Enhanced Extrapolation Technique
Prior literature in the subject area of far-field antenna measurements has demonstrated an extrapolation technique to isolate and correct the errors associated with nearzone proximity effects, specifically multiple reflections between the probe and the antenna under test (AUT), thus allowing measurements to be acquired at separation distances much shorter than the conventionally defined far-field criteria. A recent paper on this topic described a modern, indoor, far-field antenna measurement range specifically designed to support the traditional extrapolation technique while also incorporating high-speed RF instrumentation and advanced software control of a mobile probe tower. The automation of the traditional technique was emphasized, and the application focused primarily on X-band performance. Herein presented is an updated and more broadband approach which utilizes both amplitude and phase data to extend the implementation to frequencies in the UHF-, L-, and S-band. Optimized correction factors are generated for additional extraneous signals, most notably the effects of multi-path interference. Using the generalized three antenna measurement approach as highlighted in the original technique, measurement examples are provided for broadband antenna range horns, and the resultant far-field gain calculations are again compared to similar data extracted using traditional near-field scanning techniques.
Nearfield Antenna Measurements over Seawater - Challenges and Prospects
The question of how to perform a nearfield antenna measurement in the presence of the air-sea interface is one that has been raised previously by the author. When discussing spherical near field measurements various approaches have been proposed for addressing this problem, that are also applicable to measurements taken over a conducting ground plane. In this paper we shall discuss some of the practical challenges involved in data collection and measurement methods when performing this type of measurement. Examples shall be taken from both spherical nearfield measurements of simple sources along with single-point at-horizon measurements to examine the challenges associated with these approaches. A notional approach for measuring realized power gain at the horizon will also be discussed.
Reducing phase-measurement errors due to RF-source band breaks
A signal source can introduce phase-measurement errors when its output crosses through internal frequency-band breaks. The source phaselock circuits in this band-break region sometimes report approximate phaselock before complete phaselock occurs. The result of this approximate phaselock is a minor error in the output frequency, which can lead to phase-measurement errors at the system level. The magnitude of the phase errors depends on the amount of frequency offset and the difference in electrical lengths between the measurement system's signal and phase-reference paths. If this behavior were deterministic, then the resulting phase errors might be tolerable. Unfortunately, it was found that the final settling time (measured in many hundreds of milliseconds) was not consistent, depended in part on the two specific frequencies surrounding the band break, became more confused if a second sweep encountered the band break before the first break had settled, and of course changed behavior if the frequencies were sequenced in reverse order or measured one at a time. The design approach described herein reduced to negligible the phase-measurement errors due to frequency errors in two large multioctave test systems. The approach relies on managing range transmission line lengths so that propagation time is sufficiently equal among the various signal and reference paths. Measured data are presented that show the advantage of the optimized system design.
Near-field testing with a 8.9x1.6 m2 planar scanner at Christiaan Huygens Laboratory (CHL)
A near-field scanner has been upgraded, maintaining mechanical hardware more than 65 years old and extending it with suitable computer control to enable an 8.9x1.6m^2 scanplane. Already in 1957 X-band phase accuracies within 3 degrees were reported (ref.1). The facility is computer controlled, with servo's to enable position and polarisation control and a Rohde and Schwartz network analyser in the loop. It is positioned in an area near the main workshop and runs proprietary software for control, acquisition and transformation. An old satellite antenna has been aligned as Antenna Under Test (AUT) and measured near 12 GHz. It was measured before as reported in (ref.2). The antenna is an engineering model of an antenna used on the OTS satellite in mid 80's. It has a few properties which are worthwhile to use for inspection, to enable to get insight into scanner properties and transformation results. Deviation between electrical and mechanical axis, low cross polarisation, orthogonal channels and specific input impedance can be mentioned as points to verify and to control with verification measurements exploiting symmetries and flip-tests, rather than ticking off in an 18-term error budget usually adopted. Direct gain measurements have been established. The probe can be selected, either an open-ended waveguide or a circular waveguide with annular corrugation as probe for instance. It involves related discussion of probe correction. The first results show acceptable information for the facility, with initial comparison to previous results for pattern and absolute gain. It has allowed to survey alignment, assess scanner control properties and assess microwave component properties - with interest into direct gain measurements. A short historical description for the facility (ref.1) and antenna precedes a main discussion of the followed procedures and obtained results for the AUT with related discussion.
Challenges for the Automotive Industry on MIMO OTA Testing
The evolution of cellular communication technologies has been replicated by the automotive industry with modern vehicles being almost universally fitted, as a bare minimum, with a radio system, a cellular communication system and Bluetooth capability. Higher end vehicles have additional capabilities such as WiFi, GNSS, TPMS, smart keyless entry and smart start/stop feature. All these systems are highly integrated as part of the vehicle's infotainment unit and they must operate satisfactorily in a co-existing manner. Automotive wireless testing is currently facing several challenging aspects with one such aspect being MIMO OTA (Multiple-Input-Multiple-Output Over-The-Air) testing of the terrestrial cellular communication system of the vehicle. In this paper, we will examine the current approach for MIMO OTA testing in the 4G and 5G cellular environments and discuss various scenarios on how existing techniques can be adapted to support MIMO OTA testing in the automotive industry. MIMO OTA testing is typically carried out either using conducted testing techniques or using a Multi Probe Anechoic Chamber (MPAC); both these methods have their advantages and limitations and, to a certain extent, a degree of applicability to a very large article under test. This paper covers these two established MIMO OTA testing techniques and considers their applicability to the automotive MIMO OTA testing scene. Following on from this analysis and the challenges exposed herein, additional MIMO OTA test methods are put forward along with an assessment of how well they perform in an automotive test environment.
Influence of the Phase Uncertainty in Spherical Wave Expansion in the Millimeter-Wave Range
Phase uncertainty in antenna measurements introduces significant errors to the amplitude of the transformed pattern in Spherical Wave Expansion (SWE). To get a better understanding of the impact of phase errors, the measured phase error of a Low Noise Amplifier (LNA) is synthesized as a random phase error and subsequently added to the measured antenna patterns of three different antennas during the SWE. The resulting erroneous patterns are compared with the measured reference patterns and the error magnitude and probability distribution are studied. It is proven that the introduced errors to the transformed far-field patterns can be substantial. Furthermore, the relation between the antenna type and the error level and distribution is elaborated. The error level is different for the three antennas and the error level distribution is dependent on the mode spectra of the antennas.
A Straightforward Dynamic Range Error Analysis
The significant measurement standards in the antenna measurement community all present suggested error analysis strategies and recommendations. However, many of the factors in these analyses are static in nature in that they do not vary with antenna pattern signal level or they deal with specific points in the pattern, such as realized gain, side lobe magnitude error or a derived metric such as on-axis cross polarization. In addition, many of the constituent factors of the error methods are the result of analyses or special purpose data collections that may not be available for periodic measurement. The objective of this paper is to use only a few significant factors to analyze the error bounds in both magnitude and phase for a given antenna pattern, for all levels of the pattern. Most of the standards metrics are errors of amplitude. However, interest is increasing in determining phase errors and, hence, this methodology includes phase error analysis for all factors.
Measurement of RF Absorber at Large Angles of Incidence using Spectral Domain Transformations
Pyramidal RF absorber, widely used in indoor antenna ranges, is designed to minimize reflectivity by creating an impedance transform from free space to the impedance of the absorber material. The pyramidal shape provides this transition quite well at normal incidence. It has been shown in  that pyramidal RF absorber performs very well up to angles of incidence of about 45 degrees off-normal, but at wider angles of incidence, the performance degrades significantly. Unfortunately, it is very difficult to perform RF absorber measurements at large oblique incidence angles. In such measurements, the reflected path and the direct path between the antennas are very close in length, making it difficult to use time-domain gating techniques to eliminate the direct coupling. In this paper, a novel approach for performing oblique RF absorber measurements is introduced based on spectral domain transformations. Preliminary measurements using this technique have been compared to RF simulations. Results appear to indicate that this approach is a valid way to perform RF absorber reflectivity measurements at highly oblique incidence angles.
Measurements of Non-Metallic Targets for the Austin RCS Benchmark Suite
A simulation-supported measurement campaign was conducted to collect monostatic radar cross section (RCS) data as part of a larger effort to establish the Austin RCS Benchmark Suite, a publicly available benchmark suite for quantifying the performance of RCS simulations. In order to demonstrate the impact of materials on RCS simulation and measurement, various mixed-material targets were built and measured. The results are reported for three targets: (i) Solid Resin Almond: an almond-shaped low-loss homogeneous target with the characteristic length of ~10-in. (ii) Open Tail-Coated Almond: the surface of the solid resin almond's tail portion was coated with a highly conductive silver, effectively forming a resin-filled open cavity with metallic walls. (iii) Closed Tail-Coated Almond: the resin almond was manufactured in two pieces, the tail piece was coated completely with silver coating (creating a closed metallic surface), and the two pieces were joined. The measured material properties of the resin are reported; the RCS measurement setup, data collection, and post processing are detailed; and the uncertainty in measured data is quantified with the help of simulations.
GSS (Gated-Short-Short) Calibration for Free-space Material Measurements in millimeter-Wave Frequency Band
Electrical properties of materials are requisite to design electromagnetic (EM) devices and systems. Free-space material measurement method, where the measurands are the free-space scattering parameters of MUT (Material Under Test) located at the middle of transmit (Tx)/receive (Rx) antennas, is suitable for non-destructively testing MUT without prior machining and physical contact in high frequencies. In this paper, GSS (Gated-Short-Short) calibration method using a planar offset short is proposed for calibrating a free-space material measurement system and the measurement result is shown in W-band (75-110 GHz).
Experimental Validation of a Non-Redundant NF-FF Transformation for Long AUTs Mounted in Offset Configuration in a Spherical NF Facility
The experimental validation of an accurate and fast near-field-far-field (NF-FF) transformation technique with spherical scan, suitable for long antennas under test (AUTs) mounted in offset configuration, is provided in this work. The main feature of such a NF-FF transformation is to require, unlike the traditional spherical (TS) one, an amount of NF samples, which is minimum and results to be practically the same in both cases of offset and onset mount-ings of the AUT. To this end, an optimal sampling interpolation formula , developed by properly exploiting the non-redundant sampling representations and modeling an offset mounted long AUT by a cylinder ended by two half-spheres, is employed to precisely recover the massive input NF data for the TS NF-FF transformation from the collected non-redundant samples. A considerable measurement time-saving can be so achieved. Experimental results assessing the validity and the practical feasibility of such a technique are shown.
Combination of Spherical and Planar Scanning for Phaseless Near-Field Antenna Measurements
The two scans phaseless technique is a well-known procedure for the characterization of antennas on near-field ranges without need of measuring the phase. Amplitude information over two surfaces compensates for the lack of phase reference. In this paper we propose the combination of spherical and planar surfaces for the application of the two scans technique, together with the application of Wirtinger Flow, a state-of-the art phase retrieval algorithm with high convergence guarantees. The use of different types of surface adds additional information about the field's degrees of freedom, allowing for smaller separation between acquisition surfaces as compared with the 2-sphere techniques. In addition, an initial estimation for the phase is not required. The phase retrieval process is formulated in terms of the Spherical Wave Expansion (SWE) of the antenna under test. The SWE-to-PWE (Plane Wave Expansion) is utilized in order to process the amplitude field on the planar surface. Results for simulated and measured near-field data are shown to demonstrate the potential capabilities of the proposed technique.
Indoor 3D Spherical Near Field RCS Measurement Facility: A new high resolution method for 3D RCS Imaging
Indoor RCS measurement facilities are usually dedicated to the characterization of only one azimuth cut and one elevation cut of the full spherical RCS target pattern. In order to perform more complete characterizations, a spherical experimental layout has been developed at CEA for indoor Near Field monostatic RCS assessment . This experimental layout is composed of a 4 meters radius motorized rotating arch (horizontal axis) holding the measurement antennas while the target is located on a polystyrene mast mounted on a rotating positioning system (vertical axis). The combination of the two rotation capabilities allows full 3D near field monostatic RCS characterization. 3D imaging is a suitable tool to accurately locate and characterize in 3D the main contributors to the RCS. However, this is a non-invertible Fourier synthesis problem because the number of unknowns is larger than the number of data. Conventional methods such as the Polar Format Algorithm (PFA), which consists of data reformatting including zero-padding followed by an inverse fast Fourier transform, provide results of limited quality. We propose a new high resolution method, named SPRITE (for SParse Radar Imaging TEchnique), which considerably increases the quality of the estimated RCS maps. This specific 3D radar imaging method was developed and applied to the fast 3D spherical near field scans. In this paper, this algorithm is tested on measured data from a metallic target, called Mx-14. It is a fully metallic shape of a 2m long missile-like target. This object, composed of several elements is completely versatile, allowing any change in its size, the presence or not of the front and / or rear fins, and the presence or not of mechanical defects, … Results are analyzed and compared in order to study the 3D radar imaging technique performances.
Reduced Aperture Flanged Rectangular Waveguide Probe for Measurement of Conductor Backed Uniaxial Materials
An algorithm is developed for the non-destructive extraction of constitutive parameters from uniaxial anisotropic materials backed by a conductive layer. A method of moments-based approach is used in conjunction with a previously-determined Green function. A dominant-mode analysis is done for rapid comparison of the derived forward model with that of commercially-available software. Finally, laboratory measurements are taken to compare this approach to that of a destructive, high-precision method.
Recent Developments in International Facility Comparison Campaigns
The EurAAP (the European Association on Antennas and Propagation)  Measurements working group (WG5), constitutes a framework for cooperation to advance research and development of antenna measurements. An important ongoing task of this group is to sustain the Antenna Measurement Intercomparisons. The comparison of each facility measurement of the same reference antenna in a standard configuration results in important documentation and validation of laboratory expertise and competence, allowing to validate and document the achieved measurement accuracy and to obtain and maintain accreditations like ISO 17025. An additional outcome is the improvement in antenna measurement procedures and protocols in facilities and contributions to standards, which is one of the long-term objectives of the EurAAP WG5. Several participants among Europe but also USA and ASIA have joined the activity. These campaigns will also serve for a new task, recently approved within the WG5, of self-evaluation from comparison of the measurement results. An important ongoing campaign involves a X/Ku/Ka-band high gain reflector antenna MVI-SR40 fed by SH4000 Dual Ridge Horn. In this paper we report the results here for the first time. The medium gain ridge horn, MVI-SH800, equipped with an absorber plate to enhance the correlation in different facilities has been the reference antenna of another campaign. In  the preliminary results were shown. In this paper we present the final validation. The comparison is reported plotting the gain/directivity patterns and computing the equivalent noise level and the Birge ratio with respect to the reference pattern obtained taking into account the uncertainty declared by each facility.
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