AMTA Paper Archive
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Synthesis of Van Atta Array Retrodirective Patterns Using Conventional Array Characterization
Van Atta Arrays are antennas with uniquely configured beamforming networks (BFNs) that allow for innate retrodirection of incident signals. While useful for a range of applications, their characterization has typically necessitated the use of radar crosssection (RCS) ranges. Our work proposes an alternate method that uses conventional array characterization, specifically element patterns and scattering matrix measurements, to synthesize both bistatic and monostatic RCS patterns for Van Atta arrays. This method is demonstrated theoretically and experimentally first with a cross-polarized dipole array followed by a counterwound octafilar helix antenna array. The benefits of the proposed synthesis method include fast design studies and trades of the Van Atta BFN enabling retrodirective operation. Among other things, this allows for broader access to experimental research on this topic. The significance of the structural radar cross-section is also discussed.
Reduced-Order Model for Antenna Pattern Characterization from a Small Number of Samples
The characterization of the radiation performances is a necessary step in the conception of any wireless system. These systems require always more demanding radiation performances that calls for time consuming characterizations. This duration can be reduced by the decrease of the number of field samples. By enclosing the antenna in a Huygens’ surface, we can build a radiation matrix that maps equivalent surface currents to the radiated field. A singular value decomposition of this matrix enables to build a compressed representation of the antenna measurement and more specifically a reduced basis of the radiated fields. By harnessing the outer dimensions of the antenna, the number of field samples can be reduced as compared to spherical wave expansion techniques. This number is shown to be connected to the area of the convex equivalent surface enclosing the AUT, as hinted by previous analytical works for canonical enclosing surfaces. The whole antenna characterization procedure is validated by simulations and experiments.
Aircraft Antenna Placement Investigation Utilizing Measuered Sources in Simulation Model
Antenna placement or antenna in-situ performance analysis on large and complex platforms such as ships, airplanes, satellites, space shuttles, or cars has become even more and more important over the years. We present a systematic investigation of different antenna types for space applications in G- and S-band on an experimental aircraft. In this process, the individual antennas are measured with the help of a dual reflector compact antenna test range (CATR) under far-field conditions in various configurations. These results are validated and compared utilizing a finite element method (FEM) based solver simulation model. At first, the antennas are simulated and measured alone without any supporting or mounting structure. Subsequently, the effect of mounting structures on the overall radiation performance is added by analyzing the antennas over a large conducting ground plane, on top and the side of winglets, and on top of a cylinder body with dimensions on the order of the actual aircraft. For the detailed in-situ investigations, a second method of moments (MoM) based simulation tool is employed which works on measured sources. These measured sources are obtained from the CATR measurements of the isolated antennas. By means of a spherical wave expansion (SWE), they are transformed into a near-field source for the simulation model. These measured data based results are again compared and validated with the full FEM simulation for the complete aircraft setup and the simplified cylinder body. By this means, the expensive design and measurement of a full-scale electromagnetically equivalent mock-up of the aircraft could be saved. Furthermore, the pure simulation of the installed antenna performance often suffers from the limited availability of exact antenna design parameters. In some cases, the antenna design data remains undisclosed deliberately due to IP reasons. The presented results reveal the influence of physical structure on the radiation characteristics and demonstrate the benefits of working with measured data in simulation tools.
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.
Adding Phase to the Rotating-Source Antenna Polarization Measurement Method
The rotating-source measurement method is usually described as an amplitude only measurement method and the axial ratio is the only characteristic that can be measured. The article illustrates how adding a phase measurement allows to get the sense of polarization and to calculate the circular partial gains over a full cut-plane of the antenna under test. Simulations and a measurement example are shown.
Measurement Traceability in the CISPR 16-1-6 pattern measurements for CISPR 16-1-4 Site Validations
The publication of CISPR 16-1-6  in 2107 marked a significant change in the CISPR documents, for the first time the consideration of how to perform antenna pattern measurements in and determine the associated estimate of the uncertainty of those measurement. This is a look at that technique and presentation of how that helps and relates to measurement traceability.
Comparison and contrast of the antenna calibration methods of ANSI and CISPR
This is a brief comparison between the two recently released documents that detail the methods used for the calibration of antennas intended for use in measuring electromagnetic compatibility.
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.
2D RCS Prediction from Multistatic Near-Field Measurements on a Plane by Single-Cut Near-Field Far-Field Transformation and Plane-Wave Synthesis
A near-field far-field transformation (NFFFT) technique with a plane-wave synthesis is presented for predicting two-dimensional (2D) radar cross sections (RCS) from multistatic near-field (NF) measurements. The NFFFT predicts the FF of the OUT illuminated by each single source, then the plane-wave synthesis predicts the FF of the OUT each illuminated by each plane-wave by synthesizing the FFs given in the NFFFT step. The both steps are performed in the similar computational procedure based on a single-cut NFFFT technique that has been proposed previously. The method is performed at low cost computation because the NF and source positions are required only on a single cut plane. The formulation and validation of the method is presented.
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.
Robust Automotive Satellite Navigation Achieved with Distributed Groups of Sub-arrays
Ambiguous direction-of-arrival estimation is a key problem for uniformly distributed antenna arrays with inter-element spacing exceeding half of the carrier wavelength. The primary reason behind such ambiguity are the grating lobes generated in the radiation patterns due to insufficient spatial sampling. An L-shaped orthogonal arrangement of radiating elements in distributed sub-arrays is an approach that removes grating lobes and consequent ambiguity to a great extent. The reduction of footprint area by distributing the elements across a car also makes it a suitable approach for conformal integration into automotive exterior parts. In order to realize the feasibility of its application in passenger cars, we investigate and evaluate this concept through measurements and digital array signal processing. This paper presents a comparison of L-shaped antenna element arrangements for different spacings between two sub-arrays, as well as a verification of the concept when mounted on a passenger car. For each scenario, the radiation patterns are analyzed and the robustness of the system against a static interferer is verified.
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.
A Compact Reconfigurable Millimeter-Wave Antenna Measurement System Based Upon an Industrial Robot
Echodyne has recently completed and qualified a new millimeter-wave antenna measurement system for characterization of beam-steering antennas such as our Metamaterial Electronic Steering Arrays (MESAs). Unlike most far-field systems that employ a standard Phi/Theta or Az/El positioner, we use a six-axis industrial robot that can define an arbitrary AUT coordinate system and center of rotation. In different operational modes, the robot is used as an angular AUT positioner (e.g., Az/El) or configured for linear scan areas. This flexible positioning system allows us to characterize the range illumination and quiet zone reflections without modification to the measurement system. With minor modifications, the system could also be used in a planar-near field configuration. Range alignment can be easily performed by redefining the coordinate system of the AUT movement in software. The approximate 5.2-meter range length is within the radiating near-field of many arrays of interest, so we employ spherical near-field (SNF) correction when necessary, using internally-developed code. Specialty tilted absorber was installed in the chamber to improve quiet zone performance, over standard absorber treatment for similar aspect ratio ranges. Narrower ranges often have specular reflections that exceed 60° and benefit from the specialty tilted absorber designed to reduce the angle of incidence. We present an overview of the measurement system and some initial measurement data, along with lessons learned during design and integration. I. MEASUREMENT SYSTEM OVERIVEW A 7.3m x 3.7m x 3.7m footprint was allocated for the new R&D millimeter-wave antenna measurement chamber. After accounting for structural considerations, the final chamber interior dimensions are 7.1m(L) x 3.45m(W) x 3.35m(H) and the final range length (separation between range antenna and quiet zone center) is about 5.2 m. Table 1 lists the high-level goals of the measurement system are listed in. Table 1. Echodyne R&D chamber goals. Parameter Goal Frequency range 12-40 GHz, with provisions up to 80 GHz Polarization Dual-linear switched or simultaneous AUT positioner Azimuth-over-Elevation and linear scanning Quiet zone size 0.4m(L) x 0.4m(W) x 0.4m(H) Side lobe uncertainty +/-1 dB for-20 dB sidelobe Figure 1 shows the dimensions of the rectangular chamber, which is lined with the special absorber design described in Section II. Figure 2 shows an overview of the measurement system. The RF subsystem consists of a 4-port vector network analyzer (VNA), a Gigatronics GT-1050A power amplifier, a directional coupler (placed after the amplifier) to provide the VNA reference signal and a MVG QR18000 dual-polarized closed boundary quad-ridged horn  as the range antenna. This setup provides continuous frequency coverage from 12 to 40 GHz. External frequency converter modules can be used to extend the range further into millimeter wave. Horizontal and vertical polarization are acquired simultaneously by measuring three receiver channels (B, C & R1) and calculating the ratios B/R1 and C/R1 which remove the effects of amplifier drift (such as temperature coefficient). The range antenna is mounted to a rotary stage to allow direct measurement of Ludwig-III polarization if desired (versus polarization synthesis in post-processing). The AUT positioner described in Section III is a six-axis industrial robot that provides both angular azimuth-over-elevation and linear scanning with high-accuracy. Linear scanning allows planar near-field measurements in addition to the quiet zone evaluation shown in Section IV. The 5.2 m range length is within the radiating near-field of many arrays of interest, especially at higher frequencies. For example, even a relatively small (140 mm) AUT would have a 22.5° phase taper across at 40 GHz. We use the spherical near-field measurement correction  described in Section V to obtain true far-field patterns in the Az/El coordinates described by the robot motion. Figure 1. Rectangular chamber dimensions (in inches).
Personal Near-field System
In 1987 the author built the world's first Personal Near-field antenna measurement System (PNS). This led to the formation of Nearfield Systems Inc. (NSI) a company that became a major manufacturer of commercial near-field antenna measurement systems. After leaving NSI in 2015 several new personal antenna measurement tools were built including a modern updated PNS. The new PNS consists of a portable XY scanner, a hand held microwave analyzer and a laptop computer running custom software. The PNS was then further generalized into a modular electromagnetic field imaging tool called "Radio Camera". The Radio Camera measures electromagnetic fields as a n-dimensional function of swept independent parameters. The multidimensional data sets are processed with geometric and spectral transformations and then visualized. This paper provides an overview of the new PNS and Radio Camera, discusses operational considerations, and compares it with the technology of the original 1987 PNS. Today it is practical for companies, schools and individuals to build low-cost personal antenna measurement systems that are fully capable of meeting modern industry measurement standards. These systems can be further enhanced to explore and visualize electromagnetic fields in new and interesting ways.
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.
Topology for Maintaining Symmetry in Hybrid LPDA-Broadband-Dipole Antennas
Topologies for hybrid LPDA-broadband-dipole antennas (hybrid antennas) are systematically presented and evaluated regarding their ability to provide symmetric response as defined and required in recent standards. The symmetry property of the hybrid antenna is fundamentally related to the intrinsic infinite balun, the choke structure, and the matching transformer for the broadband dipole, if one is employed. In general, hybrid antennas incorporating matching transformers are more symmetric if the transformer is effectively center-tapped. More specifically, in a hybrid antenna employing an impedance matching transformer derived from an equal-delay hybrid, the sum port can be advantageously connected via a low-impedance load to the center of a symmetric choke arrangement. A specific topology for a hybrid LPDA-broadband-dipole antenna is given here which employs a 1:4 impedance transforming balun between the LPDA and broadband dipole but at the same time provides symmetry such that the antenna satisfies the requirements given in recent standards. Thus, the advantages of the impedance transforming balun are realized, but the symmetry of the antenna is maintained. Finally, it is shown that a hybrid antenna satisfies the symmetry requirements if a 180 • rotation about the bore sight axis is equivalent to a 180 • electrical phase shift in the source and that this behavior is obtained with a combination of 2-fold rotational symmetry in the radiating structure and electrical symmetry in the intrinsic balun structure.
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.
Experimental Validation of Minimum Redundancy Scanning Schemes in PNF Measurements at V band
The planar wide-mesh scanning (PWMS) methodology is based on a non-redundant sampling scheme ,  and is thus without loss of accuracy. It has the potential to enable much faster measurements than standard Planar Near Field (PNF) scanning that is based on denser, regular, equally spaced NF sampling fulfilling Nyquist criteria. In , the non-redundant methodology has been validated numerically by simulated measurements on a highly shaped reflector antenna and with actual measurements on a pencil beam antenna in Ku-band and on a navigation antenna in L-band. In this paper, we present the experimental verification of the PWMS methodology, at V band using dedicated PNF measurements of a Standard Gain Horn antenna MVG SGH4000. The results accuracy of the non-redundant methodology has been investigated against Far-Field patterns, implemented by standard scanning methods, by visual comparison, and by computation of the Equivalent Noise Level (ENL). The achieved under-sampling factor is equal to 12, corresponding to similar time reduction in the stepped measurement system employed for the presented validation.
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