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Calibration
A New Dielectric Analyzer for Rapid Measurement of Microwave Substrates up to 6 GHz
John W Schultz, November 2018
This paper presents a new measurement method based on the parallel plate capacitor concept, which determines complex permittivity of dielectric sheets and films with thicknesses up to about 3.5 mm. Unlike the conventional devices, this new method uses a greatly simplified calibration procedure and is capable of measuring at frequencies from 10 MHz to 2 GHz, and in some cases up to 6 GHz. It solves the parasitic impedance limitations in conventional capacitor methods by explicitly modeling the fixture with a full-wave computational electromagnetic code. Specifically, a finite difference time domain (FDTD) code was used to not only design the fixture, but to create a database-based inversion algorithm. The inversion algorithm converts measured fixture reflection (S11) into dielectric properties of the specimen under test. This paper provides details of the fixture design and inversion method. Finally, example measurements are shown to demonstrate the utility of the method on typical microwave substrates.
A Procedure to Characterize and Predict Active Phased Array Antenna Radiation Patterns from Planar Near-Field Measurements
Rodrigo Lebrón, José D Díaz, Jorge L Salazar-Cerreno, November 2018
This contribution details a procedure to collect and process necessary data to describe the antenna patterns of PAAs using a planar near-field (NF) range. It is proposed that a complete characterization methodology involves not only capturing beam-steered antenna patterns, but also measuring embedded element patterns, exhaustive testing of the excitation hardware of the antenna under test (AUT), and performing a phased array calibration technique. Moreover, to demonstrate the feasibility of the proposed approach, the methodology is applied onto a 2x8 microstrip patch PAA, proving its utility and effectiveness. Finally, by means of the collected data, any array pattern could be predicted by post-processing, as proven by the great agreement found between a measured pattern and its computed predicted version.
Highly accurate fully-polarimetric radar cross section facility for mono- and bistatic measurements at W-band frequencies
Andreas Olk, Kais-Ben Khadhra, Thiemo Spielmann, October 2017
New requirements in the field of autonomous driving and large bandwidth telecommunication are currently driving the research in millimeter-wave technologies, which resulted in many novel applications such as automotive radar sensing, vital signs monitoring and security scanners. Experimental data on scattering phenomena is however only scarcely available in this frequency domain. In this work, a new mono- and bistatic radar cross section (RCS) measurement facility is detailed, addressing in particular angular dependent reflection and transmission characterization of special RF material, e.g. radome or absorbing material and complex functional material (frequency selective surfaces, metamaterials), RCS measurements for the system design of novel radar devices and functions or for the benchmark of novel computational electromagnetics methods. This versatile measurement system is fully polarimetric and operates at W-band frequencies (75 to 110 GHz) in an anechoic chamber. Moreover, the mechanical assembly is capable of 360° target rotation and a large variation of the bistatic angle (25° to 335°). The system uses two identical horn lens antennas with an opening angle of 3° placed at a distance of 1 m from the target. The static transceiver is fed through an orthomode transducer (OMT) combining horizontal and vertical polarized waves from standard VNA frequency extenders. A compact and lightweight receiving unit rotating around the target was built from an equal OMT and a pair of frequency down-converters connected to low noise amplifiers increasing the dynamic range. The cross-polarization isolation of the OMTs is better than 23 dB and the signal to noise ratio in the anechoic chamber is 60 dB. In this paper, the facility including the mm-wave system is deeply studied along with exemplary measurements such as the permittivity determination of a thin polyester film through Brewster angle determination. A polarimetric calibration is adapted, relying on canonical targets complemented by a novel highly cross-polarizing wire mesh fabricated in screen printing with highly conductive inks. Using a double slit experiment, the accuracy of the mechanical positioning system was determined to be better than 0.1°. The presented RCS measurements are in good agreement with analytical and numerical simulation.
Assessment of a 3D-Printed Aluminum Corrugated Feed Horn at 118.7503 GHz
Joshua Gordon, Lavanya Periasami, Albin Gasiewski, David Novotny, Michael Francis, Ronald Wittmann, Jeffrey Guerrieri, October 2017
We investigate all-metal 3D printing as a viable option for millimeter wave applications. 3D printing is finding applications across many areas and may be a useful technology for antenna fabrication. The ability to rapidly fabricate custom antenna geometries may also help improve cub satellite prototyping and development time. However, the quality of an antenna produced using 3D printing must be considered if this technology can be relied upon. Here we investigate a corrugated feed horn that is fabricated using the powder bead fusion process for use in the PolarCube cube satellite radiometer. AlSi10Mg alloy is laser fused to build up the feed horn, including the corrugated structure on the inner surface of the horn. The intricate corrugations, and tilted waveguide feed transition of this horn made 3D printing a compelling and interesting process to explore. We will discuss the fabrication process and present measurement data at 118.7503 GHz. Gain extrapolation and far-field pattern results obtained with the NIST robotic antenna range CROMMA are presented. Far-field pattern data were obtained from a spherical near-field scan over the front hemisphere of the feed horn. The quasi-Gaussian HE11 hybrid mode supported by this antenna results in very low side lobe levels which poses challenges for obtaining good SNR at large zenith angle during spherical near field measurements. This was addressed through using a single alignment and electrical calibration while autonomously changing between extrapolation and near-field measurements using the robotic arm in CROMMA. The consistency in parameters between extrapolation and near-field measurements allowed the extrapolation data to be used in-situ as a diagnostic. Optimal near-field scan radius was determined by observing the reflection coefficient S11 during the extrapolation measurement. The feed horn-to-probe antenna separation for which |S11| was reduced to 0.1 dB peak-to-peak was taken as the optimal near-field scan radius for the highest measurement SNR. A comparison of these measurements to theoretical predictions is presented which provides an assessment of the performance of the feed horn.
Additive manufacturing metallic sphere as a RCS measurement standard
Pierre Massaloux, October 2017
RCS measurements are usually performed in 3 steps in an anechoic chamber. First, the reflectivity of the target is measured. Then a reference measurement (generally without the target) is performed. Finally, a calibration standard of known RCS is used as a reference target. The main goal of the calibration phase is to transform raw measurements of reflectivity (S11 parameter in dB) into RCS (in dBsm) through the determination of the inverse transfer function of the entire RCS measurement layout. This calibration process indirectly converts the received electric field into a complex scattering coefficient. Moreover, it establishes a phase reference relatively to the rotation center of the target positioning system. The most frequently used standards are metallic spheres which have advantageous characteristics: monostatic RCS is well known by Mie-Series and independent of azimuth and elevation. However, manufacturing a perfect metallic lightweight sphere using conventional techniques include many issues that can generate defects in the spherical shape. The purpose of this paper is to evaluate the geometric and RCS performances of metallic spheres obtained from metal additive manufacturing systems using Selective Laser Melting (SLM) solutions. SLM is a fast prototyping technique designed to melt and fuse metallic powders together. On the one hand, these metallic spheres were checked by a 3D scanner in order to quantify the potential shape defects and on the other hand, RCS measurements were performed in an anechoic chamber. All these results will be presented in the final paper and compared with theoretical RCS data.
Serial-Robotic-Arm-Joint Characterization Measurements for Antenna Metrology
Michael Allman, David Novotny, Scott Sandwith, Alexandra Curtin, Josh Gordon, October 2017
The accurate alignment of antennas and field probes is a critical aspect of modern antenna metrology systems, particularly in the millimeter-wave region of the spectrum.Commercial off-the-shelf robotic arms provide a sufficient level of positional accuracy for many industrial applications.The Antenna Metrology Project in the Communications Technology Laboratory at the National Institute of Standards and Technology has shown that path-corrected commercial robotic arms, both in hardware and software analysis, can be used to achieve sufficient positioning and alignment accuracies (positioning error ~ /50) for antenna characterization measurements such as gain extrapolation and near-field pattern out to 183 GHz [1]. Position correction is achieved using a laser tracker with a 6 degree of freedom sensor attached to the robot end effector.The end effector’s actual position, measured using the laser tracker, is compared to its commanded position and a path correction is iteratively applied to the robot until the desired level of accuracy is achieved in the frequency range of interest.At lower frequency ranges (< 40 GHz), sufficient positional accuracy can be achieved, without path correction, using a using a calibrated kinematic model of the robot alone [2].This kinematic model is based on knowledge of the link frame transformations between adjacent links and captures deviations due to gravitational loading on the joints and small mechanical offsets between the joints.Additionally, the calibration procedure locates the robot’s base frame in the coordinate system of the robot’s end effector.Each link frame is described by four physical quantities, known as Denavit-Hartenberg (DH) parameters [3]. We performed calibration measurements of our CROMMA system’s DH parameters over a working volume of ~1 m3.We then use the laser tracker to compare the robot’s positional accuracy over this working volume with and without the calibrated kinematic model applied.The path errors for the calibrated case set an upper frequency limit for uncorrected antenna characterization measurements. [1]D. R. Novotny, J.A. Gordon, J.R. Guerrieri, “Antenna Alignment and Positional Validation of a mm Wave Antenna System Using 6D Coordinate Metrology, ” Proceedings of the Antenna Measurements Techniques Association, pp 247-252, 2014 [2]R.Swanson, G. Balandran, S. Sandwith, “50-micron Hole Position Drilling Using Laser Tracker Controlled Robots, ” Journal of the CMSC, Vol 9, No 1, Spring 2014 [3].J.J. Craig, “Introduction to Robotics: Mechanics and Control, 3rd ed.,” New Jersey, Prentice Hall, 2004, pp. 62-69
Filtering Antenna-to-Antenna Reflections in Antenna Extrapolation Measurements
Robert Horansky, Mohit Mujumdar, Dylan Williams, Kate Remley, Joshua Gordon, David Novotny, Michael Francis, October 2017
At NIST, we have developed a precision, wide-band, mmWave modulated-signal source with traceability to primary standards. We are now extending the traceability path for this modulated-signal source into free space to be used for verifying over-the-air measurements in 5G, wireless receivers. However, to obtain a traceable modulated signal in free space, the full scattering matrix of the radiating antenna must be measured. We have extended the extrapolation methods used at NIST, based on the work of Newell, et al. [1]. The extrapolation measurement provides a very accurate, far-field, on-axis, scattering matrix between two antennas. When combined with scattering-matrix measurements made with permutations of pairs of three antennas, far-field scattering, and, thus, gain, is obtained for each antenna. This allows an accurate extrapolation of the antenna’s near-field pattern. We have incorporated the extrapolation fitting algorithms into a Monte Carlo uncertainty engine called the NIST Microwave Uncertainty Framework (MUF) [2]. The MUF provides a framework to cascade scattering matrices from various elements, while propagating uncertainties and maintaining any associated correlations. By incorporating the extrapolation measurements, and the three-antenna method into the MUF, we may provide traceability of all measurement associated with the gain, including the scattering parameters. In this process, we studied several aspects of the gain determination. In this work, we show simulations determining the efficacy of filtering to reduce the effect of multiple reflection on the extrapolation fits. We also show comparisons of using only amplitude (as is traditionally done) to using the full complex data to determine gain. Finally, we compare uncertainties associated with choices in the number of expansion terms, systematic alignment errors, uncertainties in vector network analyzer calibrations and measurements, and phase error introduced by cable movement. With these error mechanisms and their respective correlations, we illustrate the NIST MUF analysis of the antenna scattering-matrix with data at 118 GHz. [1] A. C. Newell, R. C. Baird, and P. Wacker “Accurate Measurement of Antenna Gain and Polarization at reduced distances by an extrapolation technique” IEEE Transactions on Antennas and Propagation. Vol. 21, No 4, July 1973 pp. 418-431. [2] D. F. Williams, NIST Microwave Uncertainty Framework, Beta Version. NIST, Boulder, CO, USA, Jun. 2014. [Online]. Available: http://www.nist.gov/pml/electromagnetics/related-software.cfm
Group Delay Measurement For Satellite Payload Testing
Daniel Janse Van Rensburg, Allen Newell, Stuart Gregson, Pat Pelland, October 2017
Equivalent Isotropically Radiated Power (EIRP), Saturating Flux Density (SFD) and Group Delay (GD) are three system level parameters often measured during the characterization of spacecraft systems. EIRP is of interest for transmitters, SFD for receivers and GD for the entire up/down link. A test methodology for EIRP and SFD was first presented in [1] and [2] and a detailed procedure presented in [3]. To date GD has only been measured under far-field (or simulated far-field) conditions. In [4], a concept for measuring GD in a planar near-field (PNF) range is described, but no methodology is presented. In this paper, we present a method for measuring GD in a planar near-field range. The technique is based on a set of three antenna pairs, measured sequentially, from which the insertion phase of the measurement system and the near-field probe [5] can be resolved. Once these parameters are known, insertion phase for the device under test (i.e. a Tx or Rx antenna) can be measured and GD calculated as the negative frequency derivative of the insertion phase. An added complexity in the case of a near-field measurement is the near-field probe is in close proximity to the device under test (not far-field condition) for which compensation is needed. We show through simulation and measurement, that the plane wave expansion allows us to compute a correction factor for the proximity of the probe to the device under test; thus allowing correction of the measured insertion phase. The final step in measuring payload GD through both uplink and downlink channels is to set up a fixed Tx probe in close proximity to the Rx antenna and an equivalent Rx probe in close proximity to the Tx antenna and performing a through measurement as one would do on a far-field range. Correction factors for compensating for the proximity of both probes are then applied, based on independent a-priori Rx and Tx case measurements performed on the antennas. Simulated and measured data will be presented to demonstrate the process and to illuminate some of the finer nuances of the correction being applied. Index Terms— Group Delay, Planar Near-Field, Antenna Measurements, Three Antenna Method. [1] A. C. Newell, R. D. Ward and E. J. McFarlane, “Gain and power parameter measurements using planar near-field techniques”, IEEE Trans. Antennas &Propagat, Vol 36, No. 6, June 1988 [2] A. C. Newell, “Planar near-field antenna measurements”, NIST EM Fields Division Report, Boulder, CO, March 1994. [3] D. Janse van Rensburg and K. Haner, “EIRP & SFD Measurement methodology for planar near-field antenna ranges”, Antenna Measurement Techniques Association Conference, October 2014. [4] C. H. Schmidt, J. Migl, A. Geise and H. Steiner, “Comparison of payload applications in near field and compact range facilities”, Antenna Measurement Techniques Association Conference, October 2015. [5] A. Frandsen, D. W. Hess, S. Pivnenko and O. Breinbjerg, “An augmented three-antenna probe calibration technique for measuring probe insertion phase”, Antenna Measurement Techniques Association Conference, October 2003.
Automating RCS Measurements for High Speed Production Line In-Process Verification
Roger Richardson, Brett Haisty, October 2017
In June of this year, DSC completed the installation of a turnkey RCS measurement system that is used for in-process verification (IPV) and final component validation using standard near field QC techniques in an echoic chamber. The delivered system included a radar, antennas, shroud, ogive pylon, foam column, elevators for each – column and pylon, automated pit covers, test bodies, target transport carts, and calibration targets. The system automatically loads test objects on the correct target support system, requiring no action by the operator to connect a target onto the azimuth over elevation “tophat” positioner – it is all automatic. The user interface is designed to be operated by production line workers, greatly reducing the need for experienced RCS test engineers. Simple pass/fail indicators are shown to the test technicians, while a full detailed data set is stored for engineering review and analysis. A wall display guides users through a test sequence for target handling and starting the radar. Radar data collection of all azimuth and elevation angles and target motion are initiated from a single button push. This is followed by all data processing necessary to conduct the ATP on the parts providing a pass/fail report on dozens of parameters. The application of production line quality automation to RCS measurements improves the repeatability of the measurements, greatly reduces both measurement time as well as overhead time, and allows systems operators to become more interchangeable. This highly successful project, which was completed on-time and on-budget, will be discussed. This discussion will include radar performance, antenna and shroud design, target handling, data processing and analysis software, and the control system that automates all the functions that are required for RCS measurements.
A 60 GHz Dual-Polarized Probe for Spherical Near-Field Measurements
Paula Popa, Olav Breinbjerg, October 2017
In millimeter wave near-field measurements dual polarized probe system can be used with some of the advantages: the two electric field components are simultaneously measured within a single scan, amplitude and phase drift affects the two polarization components in the same way and there is no need of mechanical rotation of the probe. Today at DTU-ESA Facility we have dual-polarized probes in range 400MHz-40GHz and this study is part of extending the operational frequency range of the DTU-ESA Facility up to 60GHz. First order µ = ± 1 rotationally symmetric probes are desired because they employ an efficient data-processing and measurement scheme. In this work we design and test at DTU-ESA Facility a dual polarized first order probe system at 60GHz - a conical horn, including the elements: a pin diode SPDT (single pole double throw) switch up to 67GHz from Ducommun an OMT (ortho-mode transducer) from Sage Millimeter in 50-75GHz band with square waveguide antenna port (3.75mm) a square to circular transition (3.75mm to 3.58mm) from Sage Millimeter which is integrated between the OMT and conical horn 1.85mm connector cables up to 75GHz and two coaxial to waveguide adapters to connect the switch to the OMT from Flann Microwave To ensure accurate measurements at 60GHz, the hardware components were selected to provide a low cross polarization of the probe, the switch and the OMT having 40dB isolation between ports. The path loss at 60GHz is 83dB for a 6m distance and to compensate for such a loss, a 26dB gain is desired for the conical horn, which is simulated using WIPL-D software and in-house manufactured. The 60GHz dual-polarized probe is currently being assembled and will be tested in both planar and spherical near-field setups. In the full version of the paper calibration results will be shown but also results from using the probe as a probe for the measurement of a 60GHz AUT.
Channel De-embedding and Measurement System Characterization for MIMO at 75 GHz
Alexandra Curtin, David Novotny, Alex Yuffa, Selena Leitner, October 2017
As modern antenna array systems for MIMO and 5G applications are deployed, there is increased demand for measurement techniques for timely calibration, at both research and commercial sites.[1] The desired measurement method must allow for the de-embedding of information about the closed digital signal chain and element alignment, and must be performed in the near-field. Current means of measuring large arrays cover a variety of methods. Single-element gain and pattern calibration must cover the parameter space of element weightings and is extremely time-consuming, to the point where the measurement may take longer than the duration over which the array response is stable.[4] Two other popular methods are the transmission of orthogonal codes and the use of holography to reconstruct a full-array pattern. The first of these methods again requires extremely long measurement time. For an array of N elements and weightings per element W_n, the matrix of orthogonal codes must be of an order greater than NW_n.[4][3]. This number varies with the form of W_n depending on whether the array is analog or digital, but in both cases for every desired beam configuration, an order-N encoding matrix must be used. The second method relies on illuminating subsets of elements within an array and reconstructing the full pattern.[2] Each illuminated subset, however, neglects some amount of coupling information inherent to the complete system, making this an imperfect method. In this work we explore the development of a sparse set of measurements for array calibration, relying on coherent multi-channel data acquisition of wideband signals at 75 GHz, and the hardware characterization and post-processing necessary to perform channel de-embedding at an elemental level for a 4x1 system. By characterizing the complete RF chain of our array and the differential skew and phase response of our measurement hardware, we identify crucial quantities for measuring closed commercial systems. Additionally, by combining these responses with precise elemental location information, we consider means of de-embedding elemental response and coupling effects that may be compared to conventional single-element calibration information and full-pattern array measurements. [1] C. Fulton, M. Yeary, D. Thompson, L. Lake, and A. Mitchell. Digital phased arrays Challenges and opportunities. Proceedings of the IEEE, 104(3):487–503, 2016. [2] E. N. Grossman, A. Luukanen, and A. J. Miller. Holographic microantenna array metrology. Proceedings of SPIE, Passive Millimeter-Wave Imaging Technology VIII, 5789(44), 2005. [3] E. Lier and M. Zemlyansky. Phased array calibration and characterization based on orthogonal coding Theory and experimental validation. 2010 IEEE International Symposium on Phased Array Systems and Technology (ARRAY), pages 271–278, 2010. [4] S. D. Silverstein. Application of orthogonal codes to the calibration of active phased array antennas for communication satellites. IEEE Transactions on Signal Processing, 45(1):206–218, 1997.
An Experimental and Computational Investigation of High-Accuracy Calibration Techniques for Gain Reference Antennas
Olav Breinbjerg, Kyriakos Kaslis, Jeppe Nielsen, October 2017
Gain is a principal property of antennas; it is essential in establishing the link budget for communication and sensing systems through its presence in Friis’ transmission formula and the radar range equation. The experimental determination of antenna gain is most often based on a gain-transfer technique involving a reference antenna for which the gain has been calibrated to high accuracy; this is typically a pyramidal horn antenna [1]. The required accuracy of antenna gain obviously depend on the application; in some cases it can very high, ±0.1 dB or less, and this implies an even higher accuracy, of the order of ±0.01dB, for the gain reference antenna. This work investigates the accuracy to which a gain reference antenna can be calibrated; the investigation is based on experimental spherical near-field antenna measurements [2] and computational integral equation / method of moments simulations [3]. While calibration of gain reference antennas has been studied in many previous works, even works from early 1950s [4]-[6], this work is novel in systematically supporting measurements with full-wave simulations. Such simulations facilitate the study of e.g. the effect of multiple reflections between antennas at short distances. We study two absolute calibration techniques for the gain of pyramidal horn antennas. The first technique determines gain as the product of directivity and radiation efficiency; this technique has been referred to as the pattern integration technique [7] (which is not an entirely adequate designation since gain cannot be determined from the radiation pattern). The second technique determines the gain from Friis’ transmission formula [8] for two identical antennas; this technique is generally referred to as the two-antenna technique [1]. These two calibration techniques involve very different steps and contain very different sources of error; for both techniques our investigation involves measurements as well as simulations. For the pattern integration technique we compare experimental and computational results for the directivity and demonstrate agreement within one-hundredth of a dB. The radiation efficiency is calculated by different techniques based on the surface impedance boundary condition for the metallic walls of the pyramidal horn. This technique is not influenced by proximity effects or by impedance mismatch between the measurement system and the gain reference antenna. For the two-antenna techniques we compare experimental and computational results for the gain and we compare the calculated distance-dependence with that of the extrapolation technique [9]. It is demonstrated how the use of the phase center distance in Friis’ transmission formula notably decreases the necessary separation between the antennas for a required accuracy, but that multiple reflections may then become a limiting factor. This technique is highly influenced by the impedance mismatch that must be accurately accounted for. We compare the gain values resulting from the pattern integration technique and the two-antenna technique, including their very different uncertainty estimates, for a C-band standard gain horn. The work is related to an on-going ESA project at the DTU-ESA Spherical Near-Field Antenna Test Facility for the on-ground calibration of the scatterometer antennas of the EUMETSAT MetOp Second Generation B-series satellites. IEEE Standard – Test Procedures for Antennas, Std 149-1979, IEEE & John Wiley & Sons, 1979. J.E. Hansen, “Spherical Near-Field Antenna Measurements”, Peter Perigrinus Ltd., London 1987. www.wipl-d.com W.C. Jakes, “Gain of Electromagnetic Horns”, Proceedings of the IRE, pp. 160-162, February 1951. E.H. Braun, “Gain of Electromagnetic Horns”, Proceedings of the IRE, pp. 109-115, January 1953. W.T. Slayton, “Design and Calibration of Microwave Antenna Gain Standards”, Naval Research Laboratory, Washington D.C., November 1954. A. Ludwig, J. Hardy, and R. Norman, “Gain Calibration of a Horn Antenna Using Pattern Integration”, Technical Report 32-1572, Jet Propulsion Laboratory, California Institute of Technology, October 1972. H.T. Friis, “A Note on a Simple Transmission Formula”, Proceedings of the I.R.E. and Waves and Electrons, pp. 254-256, May 1946. A.C. Newell, R.C. Baird, P.F. Wacker, “Accurate Measurement of Antenna Gain and Polarization at Reduced Distances by an Extrapolation Technique”, IEEE Transactions on Antenna and Propagation, vol. 21, no. 4, pp. 418-431, July 1973.
A Multi-Robot Large Antenna Positioning System for Over-The-Air Testing at the National Institute of Standards and Technology
David Novotny, Joshua Gordon, Alexendra Curtin, Michael Allman, Jeffrey Guerrieri, Kim Hassett, Quang Tom, George McAdams, October 2017
NIST has been using coordinated robotics in the Configurable Robotic Milli-Meter wave Antenna (CROMMA) system to assess antenna performance and radiative emissions since 2010. The focus to date has been using coordinated motion to arbitrarily position and correct antenna alignment for high frequency (>60 GHz) applications. Coordinated robotic motion was originally chosen to overcome the systematic alignment and range configurability limitations inherent in legacy stacked-stage ranges. A limitation with CROMMA is the relatively small spatial reach of the robotic arm (2.5 m), which limits antenna size and the number of wavelengths in separation achievable for lower frequencies. To overcome these limitations and address other dynamic testing requirements, NIST proposed the Large Antenna Positioning System (LAPS). The LAPS consists of two kinematically linked robotic systems, one of which is integrated with an 8 m linear rail system. The stationary robot is a 6 axes, 2.5m horizontal/4.4m vertical reach robotic arm, while the robot integrated with the linear rail system is a 6 axes, 3m horizontal/5.5m vertical reach robot arm. This configuration allows antennas to be positioned within a 5m x 6m x 10m volume. The motion system can operate in either a coordinated or independent motion control state, allowing independent or dynamic dual-robot motion. The coordinated capabilities of the system are designed to support not only traditional antenna measurement geometries (i.e. spherical, cylindrical, planar, gain-extrapolation), but also intended to be used to dynamically interact with changing RF conditions. The robots can independently scan or interrogate multiple bearings of a device under test, or trace out complex 6D paths during system testing. Initial data on performance of the system, including comparison of robot kinematics, RF acquired data, and physical locations verified by a laser tracker, will be presented.
Phase Error Characterization of a Space-Fed Array
Brian Holman, Jacob Houck, Philip Brady, November 2016
GTRI has been developing a method for insertion phase calibration, as discussed in the paper “Insertion Phase Calibration of Space-Fed Arrays,” which was presented at AMTA in 2015 [1]. This method has been implemented to characterize the phase response of phase shifters in a system currently under fabrication at GTRI. One of the primary requirements for the phased-array antenna of this system is a maximum RMS phase error. The RMS phase error for this array is influenced by a variety of error sources, including phase shifter quantization, beam steering computer (BSC) algorithmic error, phase shifter unpredictability error, test fixture induced error, phase shifter thermal drift, and phase shifter frequency dependency. Each of these error sources has been categorized as either a non-deterministic error, whose behavior can be statistically characterized but not calibrated out, or as a deterministic error, whose behavior can be characterized and potentially calibrated out. The non-deterministic errors include element unpredictability, which is induced by the inability of an individual phase shifter to precisely repeat a given phase command, and errors induced by the calibration test fixture itself. The deterministic errors include phase shifter quantization error, which is a function of the phase state bit precision, BSC algorithmic error, which is driven by the numerical preciseness of calculation of the commanded phase states for each element, thermal driven phase drift, and phase shifter frequency dependency across the band of operation. To calibrate the insertion phase and phase-state response curves for all phase shifters used in the system, a custom-built calibration fixture was constructed into a septum wall that separates two semi-anechoic chambers. The realized phase-error budget of the system under fabrication was affected directly by the accuracy of both the calibration method and this fixture. We will present our analysis of all phase-error sources as they contribute to the overall phase-error design goal of the system. We have shown how the design and implementation of both the calibration fixture and methodology meet that goal.
Radar Echoes from Metal Spheres Large and Small
Pax Wei, November 2016
Wave scattering from a perfectly conducting sphere provides an important example for theoretical studies as well as RCS calibrations [1, 2].  At the Boeing 9-77 Range and the Millimeter Wave Range in Seattle, we measured spheres of large and small diameters, supported by strings or a foam tower, and through a wide range of frequencies.  In addition to co-polarized calibration, the emphasis was also on uncertainty analysis in order to verify that the experiments carried out under different conditions were mutually consistent [3].  Aside from the well-defined conditions for an indoor range, metal spheres may be dropped from the air free fall while being measured [4].  A news article on January 5, 2016, reported that three metal spheres were picked up in three provinces in northern Vietnam [5].  Though details of the experiments were obscure, from the pictures they happened to correspond to spheres of sizes from large to small.  Based on our experiences, some speculation will be discussed.  References [1]. E. F. Knott, "Radar Cross Section Measurements," (Van Nostrand Reinhold,  New York, 1993), pp. 176-180, (on spheres and the Mie series).   [2]. E. F. Knott, E. F. Shaeffer, and M. T. Tuley, "Radar Cross Section," (Artech House,      2nd ed, 1993), pp. 86 & 234-235, (on creeping waves).  [3]. P. S. P. Wei, A. W. Reed, C. N. Ericksen, and J. P. Rupp, “Uncertainty Analysis and      Inter-Range Comparison on RCS Measurements from Spheres,” Proc. 26th AMTA,      pp. 294-299 (2004).   [4]. “Mysterious silver balls fall down on town; can the black helicopters be far behind?”   By Steve Vogel, The Seattle Times, August 7, 2000, (from the Washington Post).  [5]. “3 mysterious spheres fall onto 3 Vietnam provinces,”  Tuoi Tre,  Tue, 05 Jan 2016.  http://www.sott.net/article/309800-3-mysterious-spheres-fall-onto-3-Vietnam-provinces
Compact First-Order Probe for Spherical Near-Field Antenna Measurements at P-band
Oleksiy Kim, November 2016
A number of European Space Agency's (ESA) initiatives planned for the current decade require metrology level accuracy antenna measurements at frequencies extending from L-band to as low as 400 MHz. The BIOMASS radar, the Galileo navigation and search and rescue services could be mentioned among others. To address the needs, the Technical University of Denmark (DTU), who operates ESA’s external reference laboratory “DTU-ESA Spherical Near-Field (SNF) Antenna Test Facility”, in years 2009-2011 developed a 0.4-1.2 GHz wide-band higher-order probe. Even though the probe was manufactured of light-weight materials -- aluminium and carbon-fibre-reinforced polymer (CFRP) -- it still weighs 22.5 kg and cannot be handled by a single person without proper lifting tools. Besides that, a higher-order probe correction technique necessary to process the measurement data obtained with such a probe is by far more demanding in terms of the computational complexity as well as in terms of calibration and post- processing time than the first-order probe correction. On the other hand, conventional first-order probes for SNF antenna measurements utilizing open-ended cylindrical waveguides or conical horns fed by cylindrical waveguides operating in the fundamental TE11-mode regime also become excessively bulky and heavy as frequency decreases, and already at 1 GHz an open-ended cylindrical waveguide probe is challengingly large. For example, the largest first-order probe at the DTU-ESA SNF Antenna Test Facility operates in the frequency band 1.4–1.65 GHz and weighs 12 kg. At 400 MHz, a classical first-order probe can easily exceed 1 cubic meter in size and reach 25-30 kg in weight. In this contribution, a compact P-band dual-polarized first-order probe is presented. The probe is based on a concept of a superdirective linear array of electrically small resonant magnetic dipole radiators. The height of the probe is just 365 mm over a 720-mm circular ground plane and it weighs less than 5 kg. The probe covers the bandwidth 421-444 MHz with more than 9 dBi directivity and |µ| ? 1 modes suppressed below -35 dB. The probe design, fabrication, and test results will be discussed.
Characterizing Multiple Coherent Signals Near 60 GHz Using Standard RF Hardware for MIMO and 5G Applications
Alexandra Curtin, David Novotny, Joshua Gordon, November 2016
In wireless communication technology, the growth of 5G and MIMO (multiple- input and multiple-output) systems has revealed a gap in the methods to characterize and calibrate hardware for high frequency and coherent MIMO applications. For coherent array configurations and ad hoc systems we need to measure transmission loss and phase/delay over every element. We demonstrate the use of standard RF hardware to generate and receive multiple signals in a system that is a tabletop analogy for an ad hoc system. The initial test system consists of using a single WR-15 VNA extender to detect two separate modulated signals. As our sources, we individually modulate WR-15 VNA extenders to generate continuous waveform, modulated signals around 60 GHz. On the receive side, our IF signal is first measured with a high-dynamic- range spectrum analyzer and then later collected in a digital oscilloscope. All the signal generators for the receiver LO and transmitter(s) RF IN are tied together with a common 10 MHz reference. Characterizing this initial 2x1 system is then extensible to multiple-receiver applications. We will use these coherent sources to get full complex waveform characterization element-by-element in a receiving array. We report on measurement and calibration methods to characterize the response of these systems for continuous waveforms, modulated signals, and multi-frequency applications needed for next generation coherent MIMO systems.
Uniaxial Anisotropic Material Measurement using a Single Port Waveguide Probe
Alexander Knisely, Milo Hyde, Michael Havrilla, Peter Collins, November 2016
Anisotropic material characterization requires versatile sample fixtures in order to provide sufficient measurement diversity for material parameter extraction.  However, extensive sample preparation is often required prior to making a measurement, especially for anisotropic materials.  An alternative nondestructive material measurement approach using a Single Port Waveguide Probe (SPWP) is proposed to simplify measurement of uniaxial anisotropic media.  Instead of cutting a material sample to fit into a given fixture, nondestructively interrogating a sheet of material via the SPWP greatly simplifies sample preparation and measurement.  The SPWP system measures a metal-backed sample of a known thickness.  A flange with a waveguide aperture cut in the center is placed on the metal backed sample (thus forming a parallel plate region) and a length of rectangular waveguide is connected to the flange aperture. A Vector Network Analyzer port is connected to the end of the rectangular waveguide to collect calibration and sample data.  Measurements of two different thicknesses of a sample are performed to provide sufficient data for extracting the sample permittivity tensor.  The sample permittivity tensor is computed via comparison of the measured and theoretical S-parameters using a least squares minimization algorithm.  The theoretical S-parameters are derived using a magnetic field integral equation which utilizes a uniaxial parallel plate Green’s function to constitute the fields in the parallel plate region.  Love’s Equivalence Principle is used to relate the fields in the parallel plate flange region to the fields in the waveguide (assumed to be the dominant TE10 mode only).  In this paper, the SPWP theoretical development, measurement and material parameter extraction are discussed.  Measurements and simulations of isotropic and uniaxial samples are made to assess the SPWP performance.
60 GHz Reference Chip Antenna for Gain Verification of Test Chambers
William McKinzie, Per Iverson, Edward Szpindor, Michael Smith, Bradley Thrasher, November 2016
We have developed a 60 GHz chip antenna designed for use as a gain and pattern verification tool in the calibration process of a millimeter wave antenna test chamber. The antenna is designed to interface with ground-signal-ground (GSG) micro-probes that have a probe pitch of 150 um to 250 um.  This low temperature cofired ceramic (LTCC) chip antenna is fabricated using DuPont’s 9K7 GreenTapeTM material system with gold conductors.  Features include a wafer-probe transition, a shielded stripline corporate feed network, aperture coupled patch elements, and an integrated Sievenpiper electromagnetic bandgap (EBG) structure for surface wave mode suppression.  The use of the EBG structure enables main beam gain enhancement and side lobe level suppression.  This 2x2 antenna array is directive such that it offers a nominal gain of 12 dBi at broadside over 58-62 GHz with an antenna efficiency of at least 60%.  The entire antenna package has a nominal size of only 10.9 mm x 12.2 mm x 0.71 mm.  Since this antenna package material is hermetic, it has stable performance under varying humidity and temperature which is highly desirable as a reference antenna.
Multiple Target, Dynamic RF Scene Generator
David Wayne, John McKenna, Scott McBride, November 2016
The evaluation of RF Sensors often requires a test capability where various RF scenes are presented to the Unit Under Test (UUT). These scenes may need to be dynamic, represent multiple targets and/or decoys, emulate dynamic motion, and simulate real world RF environmental conditions. An RF Scene Generator can be employed to perform these functions and is the focus of this paper. The total test system is usually called Hardware in the Loop (HITL) involving the sensor mounted on a Flight Motion Simulator (FMS), the RF Scene Generator presenting the RF Scene, and a Simulation Computer that dynamically controls everything in real time. This paper describes the system concept for an RF Scene Generator that simultaneously represents 4 targets, in highly dynamic motion, with no occlusion, over a wide range of power, frequency, and Field of View (FOV). It presents the test results from a prototype that was built and tested over a limited FOV, while being scalable to the total FOV and full system capability. The RF Scene Generator employs a wall populated with an array of emitters that enables virtually unlimited velocity and acceleration of targets and employs beam steering to provide high angular resolution and accuracy of the presented target positions across the FOV.   Key words: RF Target Simulator, RF Scene Generator, Multiple Targets, Beam Steering Wall of Emitters, Steering Array Calibration, Plane-Wave Generator, Radar Environment Simulator.


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