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AMTA Paper Archive

A Novel Dual Bridge Near-Field Measurement Facility
Jeff Way,Northrop Grumman Aerospace Systems, November 2012

ABSTRACT Northrop Grumman Aerospace Systems (NGAS), working with Nearfield Systems Inc. (NSI) and others, has installed a state-of-the-art near-field antenna measurement system to test various payload antenna systems. This horizontal planar near-field system was designed to measure antennas with up to 30’ diameter apertures. In addition, a second bridge was included in the design so that the range can operate either as one very large scanner or as two autonomous ranges and double the testing throughput of the range. This near-field system features a large scan plane of nearly 40 ft. x 47 ft. with two smaller scan planes of 17’ x 47’ each. This horizontal near-field measurement system has the capability to operate from 500 MHz to 75 GHz using NSI’s high speed Panther receiver and high speed microwave synthesizers. The system is capable of performing conventional raster scans, as well as directed plane-polar scans tilted to the plane of a specific Antenna Under Test (AUT). The range was completed in December 2011. This paper will describe this near-field range’s design and installation, present test data and plots from its acceptance test including results of a NIST 18­term error assessment.

A 200-500 GHz Bi-Static Scattering System for Material Characterization
David Novotny,National Institute of Standards and Technology, November 2012

We present performance results of a bi-directional scattering measurement system in the 200-500 GHz range. The goal is to provide dense-spectrum, bi­directional reflectance distribution function (BRDF) of sample materials and small objects that can be propagated into detection models and used as standard materials to compare performance of various detection and imaging systems. Our system is built upon a commercial frequency-domain, vector network analyzer system. The system is designed to minimize drift due to movement and temperature changes. The initial data, presented here, of reflectance from a variety of standard targets and sample materials show operation from 200-500 GHz and highlight stability, repeatability, and dynamic range of the system.

A Reference Standard for 18000-6 UHF Tag Testing
David Novotny,NIST, November 2012

We present a method for measuring ultra-high frequency radio-frequency identification (UHF RFID) tag differential RCS that has the potential for being easier and more accurate than current and proposed methods [1-2]. Our method is based on accurately characterizing the reflection states of a modulated load, accounting for transmission losses between the load and an antenna, and using a well-known, low gain antenna. This has the benefit of using a well characterized “golden tag” reference (i.e., repeatability), while being more linear in power response, independent of reader signal, and independent of manufacturer or process changes. Characterizations of the losses in the reference scatterer allow for direct comparisons between tags on different test beds.

Practical Considerations for Determining Polarization Properties from Measured Linear Components
Justin Dobbins,Raytheon Company, November 2012

Polarization properties (e.g. axial ratio, sense, and tilt) of an antenna under test (AUT) are often calculated from measurements with a linear (or dual-linear) polarized range antenna. At first, these calculations appear to be simple and straightforward. However, there are several different conventions used in the literature and some important practical aspects of the measurements are often omitted. Neglect of these small details can easily lead to incorrect results, with the most common error being the reversal of the right-hand-circular and left-hand-circular polarization components. We note the differences in the published polarization conventions and provide practical tips for good polarization measurement practices. We also describe step­by-step procedures for determining AUT polarization properties from two styles of polarization measurements using a linear (or dual-linear) polarized range antenna.

DEVELOPMENT AND VALIDATION OF AN EQUIVALENT CIRCUIT MODEL FOR UHF RFID IC BASED ON WIRELESS TAG MEASUREMENTS
Toni Björninen,Tampere University of Technology, Department of Electronics, Rauma Research Unit, November 2012

We present a method, based on wireless RFID tag measurements and non-linear least-squares model fitting to determine a simple and accurate equivalent circuit model for an RFID IC at the wake-up power of the IC, including the mounting parasitics. This expedites the tag design process greatly and enables judicious design optimization and validation. The obtained circuit model is attested in a practical tag design scenario.

Robotically Controlled mm-Wave Near-Field Pattern Range
Joshua Gordon,NIST, November 2012

The Antenna Metrology Lab at the National Institute of Standards and Technology in Boulder Colorado has developed a robotically controlled near-field pattern range for measuring antennas and quasi-optical components from 50 GHz to 500 GHz. This range is intended to address the need for highly accurate antenna pattern measurements above 100 GHz for a variety of applications including remote sensing, communications and imaging. A new concept in near-field range systems, this system incorporates the positioning repeatability of a precision industrial six-axes robot, six-axes parallel kinematic hexapod, and high precision rotation stage, integrated with a highly accurate laser tracking system. Programmable robot positioning allows the system geometry to be configured for spherical, planar, and cylindrical scans, as well as gain extrapolation measurements. Variable scan volume accommodates different test antenna sizes. Positioning accuracy better than 10 µm is predicted. Specifics of the system design, operating specifications and configurability will be presented.

Modeling and Analysis of Anechoic Chamber using CEM Tools
C.J. Reddy,EM Software & Systems (USA) Inc., November 2012

Advances in computational resources facilitate anechoic chamber modeling and analysis at VHF frequencies using full-wave solvers available in commercial software such as FEKO. The measurement community has a substantial and increasing interest in utilizing computational electromagnetic (CEM) tools to minimize the financial and real estate resources required to design and construct a custom anechoic chamber without sacrificing performance. A full-wave simulation analysis provides a more accurate solution than the approximations inherent to asymptotic ray-tracing techniques, which have traditionally been exploited to overcome computational resource limitations. An anechoic chamber is simulated with a rectangular down-range cross-section to utilize the software’s capability to assess polarization performance. The absorber layout within the anechoic chamber can be optimized using FEKO for minimal reflections and an acceptable axial ratio in the quiet zone. Numerical results of quiet zone disturbances and axial ratios are included for both low- and medium-gain source antennas over a broad frequency range.

Fast spurious emission measurements for product development
Jason Harrigan,Intermec Technologies, November 2012

A simple method of fast radiated spurious emission pretesting using a continuously rotating two-axis antenna positioner and broadband quad-ridge horn is presented. This method is intended to be used iteratively during the development of a product to reduce the turnaround time needed to perform compliance validation. It is most well-suited for small battery powered products. This paper presents the rationale, method, parameters to consider, and an example of a system implemented at Intermec Technologies.

A Novel Approach to RCS Measurements Utilizing Knowledge-Based Information
David Berger,System Planning Corporation, November 2012

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 in 2007 at CEA for indoor near field monostatic RCS assessment. This experimental layout was composed of a 4 meters radius motorized rotating arch (horizontal axis) holding the measurement antennas while the target was located on a polystyrene mast mounted on a rotating positioning system (vertical axis). The combination of the two rotation capabilities allowed full 3D near field monostatic RCS characterization. A new study was conducted in 2011 in order to achieve a more accurate positioning of the measurement antenna. The main objective is to enhance the RCS measurement performances, especially the environment subtraction directly related to the positioning repeatability of the measurement antenna. This new mechanical design has therefore been optimized to allow a +/-100° azimuth range with an angular positioning repeatability of less than 1/1000°. To achieve this level of accuracy, several keys design elements were considered: robust mechanical design, position control system… This paper describes the new experimental layout and the results of a positioning accuracy assessment campaign conducted using a laser tracker.

Precision Motion in Highly Accurate Mechanical Positioning
Tim Schwartz,MI Technologies, November 2012

Numerous applications for antenna, radome and RCS measurements require a very accurate positioning capability to properly characterize the product being tested. Testing of weapons (missiles), guidance systems, and satellites, among other applications, require multi-axis position accuracies of a few thousandths of an inch or degree. For global positioning, spherical error volumes can be extremely small having diameters of .002 inches to .005 inches. This paper addresses the issues that must be resolved when highly accurate mechanical positioning is required. Many factors such as thermal stability, axis configuration, bearing runout and mechanical alignment can adversely affect the overall system accuracy. Additionally, when examined from a global positioning system perspective, the accuracy of the entire system is further degraded as the number of axes increases. Successful system implementation requires carefully examining and addressing the most dominant error factors. The paper will cover current tools and techniques available to characterize and correct the contributing errors in order to achieve the highest possible system level accuracy. A recently delivered 4 ft radius SNF arch scanner, which achieved ± .0043° global positioning accuracy, will provide insight into these methods and show how the dominant factors were addressed.

Computer Reconstructed Holographic Technique for  Phaseless Near-Field Measurements
Zhiping Li,BeiHang University, November 2012

A novel holographic near-field phaseless technique is presented. The measurement system is composed of the antenna under test, the reference antenna, the amplitude scanning measurement system and the holographic reconstructed algorithm. The interference amplitude of the antenna under test with the reference antenna is measured by the amplitude scanning system. The complex near field of the antenna under test is reconstructed by computer, where the measured interference is corrected by the multiplication with the virtual spherical reference wave and then filtered in Fourier Transformation domain (e.g. Plane Wave Angular Spectrum) or the back-projected image space. The reconstruction method is rigorous without traditional Fresnel Approximation. The novel technique requires the amplitude on one measurement surface and the computer reconstructed algorithm, while the previous phaseless technique depends on two measurement surfaces or extra hardware to provide Synthesized­Reference-Wave. The novel holographic measurement method and reconstruction algorithm could be used in many applications as for planar near field measurements for example. Simulated results are presented to demonstrate the complex field retrieval method and near-field to far field transformation.

Achieved mechanical Accuracy of a 3D RCS spherical near field Arch Positioning System
Pierre MASSALOUX,CEA, November 2012

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 in 2007 at CEA for indoor near field monostatic RCS assessment. This experimental layout was composed of a 4 meters radius motorized rotating arch (horizontal axis) holding the measurement antennas while the target was located on a polystyrene mast mounted on a rotating positioning system (vertical axis). The combination of the two rotation capabilities allowed full 3D near field monostatic RCS characterization. A new study was conducted in 2011 in order to achieve a more accurate positioning of the measurement antenna. The main objective is to enhance the RCS measurement performances, especially the environment subtraction directly related to the positioning repeatability of the measurement antenna. This new mechanical design has therefore been optimized to allow a +/-100° azimuth range with an angular positioning repeatability of less than 1/1000°. To achieve this level of accuracy, several keys design elements were considered: robust mechanical design, position control system… This paper describes the new experimental layout and the results of a positioning accuracy assessment campaign conducted using a laser tracker.

“Defects” of Specular Patches in Elongated Anechoic Chambers
John Aubin,ORBIT/FR Inc., November 2012

Specular patches comprising pyramidal absorber components are frequently used in anechoic chambers to suppress potential DUT coupling with the side walls, floor and ceiling of the chamber. However, these specular patches also interact with the incident field radiated by the source antenna, compact range reflector, or tapered chamber feed illuminating the chamber. If the specular patch reflects the incident field in GO fashion, then the reflected field is incident on the absorptive back wall and is sufficiently attenuated there, so that there is no significant degradation of the field uniformity in the Quiet Zone due to the reflected field. If, however, the chamber is long, and the grazing angle of the incident field on the specular patches is relatively low, “non-specular” reflections incident on the Quiet Zone will perturb the field, and accordingly will degrade the field uniformity. If the chamber is operating at high frequencies (e.g., above several GHz) and the distance between the Quiet Zone and side walls is significant in terms of wavelengths, then the “non-specular” reflections will not impact the field uniformity to a noticeable extent, as they are attenuated in free space while propagating from the specular patches to the Quiet Zone. If the chamber is intended for operation at VHF/UHF frequencies, as is prevalent in tapered chambers, then the “non-specular” reflections may be the dominant factor affecting the Quiet Zone uniformity. In this paper the measured reflectivity in a tapered chamber with pyramidal specular patches is presented, illustrating a significant rise of the reflectivity over a portion of the VHF/UHF bands. Thorough investigation has shown the source of the degraded reflectivity to be the specular patch. This effect has been confirmed by simulation, and is analyzed by modeling the specular area as a periodic structure. Replacement of the specular patches by wedges has materially improved the reflectivity in the chamber, as will be shown by comparative reflectivity measurement results. For the application under consideration, the coupling between the DUT and sidewalls was below the specified minimum and, thus, advanced coupling suppression techniques were not required. For more stringent coupling requirements, the use of the ORBIT/FR patented “Two Level GTD” technology (see, for example, [1-4]) is a good choice to minimize reflectivity and DUT/sidewall coupling simultaneously.

Improved Coordinated Motion Control For Antenna Measurement
Charles Pinson,MI Technologies, November 2012

Some antenna measurement applications require the precise positioning of an antenna along a prescribed path which may be realized by a combination of several, independent physical axes. Coordinated motion allows for emulation of a more complex and/or precise positioning system by utilizing axes which are mechanically less complex or precise and are correspondingly more easily realizable. An ideal coordinated motion system should 1) Allow for the description of coordinated paths as parametric mathematical functions and/or interpolated look-up tables 2) Support control variable parameters which affect the trajectory 3) Compute a feasible trajectory within given kinematical constraints 4) Generate measurement trigger signals along the trajectory 5) Minimize control-induced vibration 6) Compensate for multivariate positioning errors. This paper will describe a novel approach to virtual-axis coordinated motion which offers significant improvements over existing motion control systems. This advancement can be applied to many antenna measurement problems such as Helicoidal Near-Field Scanning and Radome Characterization.

An Interface Between A Near-Field Acquisition System And Active Arrays With Digital Beamformers
Scott T. McBride,MI Technologies, November 2012

The increased complexity of an active array's transmit beams by itself elevates the need for an interface between the array and the acquisition system. With the embedded receivers of a DBF, however, standard antenna testing of a DBF becomes nearly impossible without such an interface. MI Technologies has developed a reasonably general interface between its acquisition system and active arrays with digital beamformers. MI has produced minor variations of this interface for multiple customers, and these customers will each use the interface to test multiple types of DBF active arrays. This paper discusses the challenges, capabilities, and architecture of this interface.

An Innovative Technique for Positioner Error Correction
Roger Dygert,MI Technologies, November 2012

Antenna measurement systems employ mechanical positioners to spatially orient antennas, vehicles, and a variety of other test articles. These mechanical devices exhibit native positioning accuracy in varying degrees based on their design and position feedback technology. Even the most precise positioning systems have insufficient native accuracy for some specific applications. As the limits of economical positioning accuracy are approached, a new error correction technique developed by MI Technologies satisfies these higher accuracy requirements without resorting to extreme measures in positioner design. The new technique allows real-time correction of repeatable positioning errors. This is accomplished by (1) performing a finely grained measurement of positioner accuracy, (2) creating a map of the errors in both spatial and spatial frequency domains, (3) separating the errors into their various components, and (4) applying correction filters to algorithmically perform error correction within the positioner control system. The technique may be used to achieve extreme positioning accuracy with positioners of high native accuracy. It may also be applied to conventional (synchro feedback) positioners to achieve impressive results with no modifications at all to the positioner. The following paper discusses the new error correction technique in detail.

Major challenges to wearable and textile antenna measurements in the spherical format
Pawel Kabacik,Wroclaw University of Technology, November 2012

The paper presents in-house developed antenna positioner capable to acquire radiation pattern in the full spherical format for wearable and textile antennas. The positioner features remarkable advantages and mitigates troublesome to measurement accuracy shadowing by the positioner structure. Furthermore, development methodology of the human phantom without heavy liquids is proposed. Since evaluation of wearable and textile antennas must put considerations to major variations in antenna performance during antenna operation, we have found an urgent need to define new engineering measure that will help in quick evaluation of such antennas.

Exploration of the Feasibility of Adaptive Spherical Near-Field Antenna Measurements
Vincent Beaulé,EECS, University of Ottawa, November 2012

The feasibility of using adaptive acquisition techniques to reduce the overall testing time in spherical near-field (SNF) antenna measurements is investigated. The adaptive approach is based on the premise that near-field to far-field (NF-FF) transformation time is small compared to data acquisition time, so that such computations can be done repeatedly while data is being acquired. This allows us to use the transformed FF data to continuously compute and monitor pre-defined decision functions (formed from the antenna specifications most important to the particular AUT) while data is being acquired. We do not proceed with a complete scan of the measurement sphere but effectively allow the probe to follow a directed path under control of an acquisition rule, so that the sampled NF datapoints constitute an acquisition map on the sphere (the geographical allusion being purposeful). SNF data acquisition can be terminated based on decision function values, allowing the smallest amount of data needed to ensure accurate determination of the AUT performance measures. We demonstrate the approach using actual NF data for several decision functions and acquisition rules.

Time space coherence interferometry
Dan Slater,Nearfield Systems Inc., November 2012

Streaming SDR (software defined radio) communication receivers are now high performance, common place and cost effective. Yet these receivers are not easily used for antenna measurements for a variety of reasons including their inability to accept phase reference and measurement trigger information. A new technique called Time Space Coherence Interferometry (TSCI) solves this problem in a simple and elegant manner. TSCI combines the concepts of temporal phase coherence with spatial division multiple access (SDMA) to directly encode phase and spatial information into a single continuous receiver data stream. The stream can be recorded for later analysis or efficiently decoded in real time producing conventional spatially sampled S21 amplitude and phase measurements. Additional data including antenna pulse timing, dynamics and signal quality metrics can be extracted from the TSCI data stream. Several representative TSCI systems are described.

Planar Near-Field Measurement Error Analysis for Multilevel Plane Wave Based Near-Field Far-Field Transformation
M.A. Qureshi,C. Schmidt, E. Thomas, November 2011

This paper describes the behavior of the antenna radiation pattern for different planar near-field measurement errors superimposed on the near-field data. The disturbed radiating near field is processed using multilevel plane wave based near-field far-field transformation to determine the far-field. Errors like scan area truncation, transverse and longitudinal position inaccuracy of measurement points, and irregular sample spacing are analyzed for an electrically large parabolic reflector at 40 GHz. The error behavior is then compared with the standard transformation technique employing 2D Fast Fourier Transform (FFT) using the same near-field data. In order to exclude the effect of any other measurement or environmental error, electric dipoles with appropriate magnitude profile and geometrical arrangement are used to model the test antenna.







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