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We present on an all-optical spatial metrology system , the PiCMM, that aids in the alignment and tracking of antennas with accuracies on the order of 25 microns and 0.01 deg. This system speeds up millimeter-wave antenna alignment, does not require contact, and links spatial measurements to a laser tracker world coordinate frame. An automated Pixel Probe and dark-field imaging are used to directly measure the aperture geometry and its pose. These measurements are absolute in the world-frame of a laser tracker and associated coordinate metrol-ogy space of the antenna scanner. Thus, aperture geometries can be linked directly to any laser tracker target (i.e. 6DOF, 3DOF) and data such as that used to calibrate positioner kinematics. For example, the links and joints defining the Denavit-Hartenberg kinematic model of a robotic arm scanner. The new automated aspect of the system reduces alignment time to under an hour. The synergy with laser tracker targets allows for a high level of repeatability. Furthermore, antennas can be exchanged or realigned in the antenna scanner autonomously because antenna geometry and kinematic models reside in the same laser tracker coordinate metrology space.
Antenna gain is the product of directivity and antenna loss. Antenna gain is typically measured by comparing the antenna under test (AUT) to a standard gain horn (SGH) or direct gain measurement using a calibrated probe. This requires an accurate account of power into the AUT and SGH, the loss of all test cables and switches must be measured to obtain an accurate AUT gain. Additionally, SGH calibration uncertainty reduces the quality of the measurement. The gain measurement technique describe here exploits the near-field range capability of accurately producing the pattern of high gain antennas. The near-field range allows the full wave capture of antenna aperture fields and transformation to the far-field with high resolution. The new technique uses the directivity obtained by integrating the far-field pattern, accounts for the spill-over energy not measured by the near-field range, and uses measured network losses of the AUT. It does not require measured losses of test cables and switches. Since AUT losses are typically measured as part of antenna integration the technique reduces overall measurement burden. Accurate calculation of spill-over energy is the key to success. The technique has been shown to yield better accuracy than the typical gain calibration method for multi-beam high gain antennas.
Planar near-field ranges are popular facilities to evaluate far-field antenna patterns. These ranges typically have the scanner plane parallel to the Antenna Under Test (AUT). Having the scanner plane parallel to the AUT can limit the maximum far-field angles that can be properly measured due to the mechanical extents over which the range can accommodate. This paper summarizes a test approach where the AUT is rotated in the near-field such that sufficient energy is concentrated within the range extents, ultimately resulting in an accurate far-field pattern. Measured results will be shown which demonstrate the limitations of the current testing approach, as well as the benefits of the near-field rotation approach.
This paper documents the various elements of the 2017/18 upgrade and presents results from the performance validation measurements with the DTU-ESA 12 GHz Validation Standard antenna conducted before and after the upgrade. The upgrade concerned several major improvements to the building infrastructure, the ventilation system, the antenna positioner, and the probe positioner. The validation measurements involved the averaging of measurements at different distances between the antenna under test and the probe to compensate the multiple reflections between these. This in turn necessitated the investigation of the compensation of the system drift between the measurements and of the sensitivity of the probe calibration to the position of the probe on the probe positioner.
5G user equipment, such as mobile terminals and tablets shall comply with radio frequency (RF) safety guidelines. At millimeter-wave frequencies, human exposure to RF electromagnetic fields (EMFs) is evaluated in terms of incident power density, i.e., free space Poynting vector. As, the electrical size of mobile terminals increase with higher frequencies, longer testing times are needed to determine power densities in the proximity of devices by traditional scanning. A new approach to accurately determine the power densities in close vicinity to a device is based on standard near-field measurements and processing by the inverse source method. This method is faster, simpler, and thus desirable compared to dedicated scan on a device dependent surface. The new method uses the capabilities of the equivalent current expansion to determine the power density by NF/NF transformation on any surface and at any distances according to the safety normative. The method was presented on a device in [7] by comparison to simulated power densities. Although correlations were promising, some discrepancies were likely due to the approximated numerical model of the device. In this paper, the SH4000, Dual Ridge Horn, with well-known full wave model is investigated to validate the power density determination against accurate simulations.
This paper discusses some aspects of isolation improvement and associated measurements on a cylindrical millimeter-wave repeater operating over K, Ka and V bands. The isolation between the transmitting and receiving antennas is improved by means of reactive impedance surface implemented as tapered depth corrugations. The designed tapered depth profile broadens bandwidth of the surface compared to the traditional quarter wavelength corrugations. Required isolation of 80 dB and large electrical size of the platform make numerical analysis and actual measurements challenging. Details of the analysis and measurements are summarized. Along with external coupling, the coupling due to leakages from waveguide components and antennas is also discussed. Measurements confirm that the design goal isolation is accomplished.
In this paper, we present a chip antenna in the 27GHz band, targeting 5G measurements. This antenna can be used as reference in mm-wave measurement systems, such as the MVG µ-Lab, feeding the antenna under test through a micro-probe station. The reference antenna is employed to calibrate in gain through the substitution method. The antenna shown in this paper is an array of four patches, fed through a strip-line beam forming network. A transition strip-line to coplanar waveguide allows the antenna be fed by the micro-probe.
This paper discusses about the design, fabrication and testing of a compact P-band (370 MHz) dual circular polarization (CP) patch antenna. The antenna is intended for reflectometry applications by measuring both direct and ground reflected 370 MHz signals transmitted from a satellite or airborne source. This design adopts quadrature-phase hybrid feeding network for achieving excellent polarization purity and supporting simultaneously LHCP and RHCP measurements. Another novel design aspect is placing the feeding network on top of the patch so that the antenna can be mounted directly on a ground plane. Therefore, the resonant modes inside the patch is excited from the top instead of from ground plane as in conventional designs. High dielectric material (ECCOSTOCK®HiK) with a dielectric constant of 9 and loss tangent of 0.002 was used as the substrate to reduce the antenna size. The final antenna has a dimension of 5.9" x 5.9" x 1.3" (excluding ground plane) and weight of 1620 gram. The measured performance on a 1-foot diameter circular ground plane showed 4.5 dBic gain and 23 dB co-polarization to cross-polarization isolation at the center frequency for both LHCP and RHCP. The 1-dB gain bandwidth is approximately 3.7%.
Compliance with regulatory exposure requirements of power density for 5G systems will need accurate measurements. In this work a near field measurement technique for electromagnetic exposure of 5G communication devices is presented. The technique requires two measurements, one of a device under test and one of a small aperture as a calibration measurement. The method uses method of moments and involves reconstructing equivalent currents on a predefined surface. These currents are then used to generate and propagate the electromagnetic fields to an arbitrary plane and further compute the power density. The measurement data are obtained through a planar scan of a device under test using a probe and probe calibration using a small aperture to obtain an accurate field with absolute positioning. Measurement data is presented and compared with simulations for several distances and two antennas, operating at 28 GHz and 60 GHz. The computed power density agrees well with simulations.
The effect of different decomposition techniques on the imaging and detection accuracy for polarimet-ric surface penetrating data is studied. We derive the general expressions for coherent polarimetric decomposition using the Stokes parameters and model based polarimetric decomposition using the Yamaguchi technique. These techniques are applied to multi-frequency (0.4-4.8GHz) full polarimetric near-field radar measurements of scattering from surface laid calibration objects and shallow buried landmine types and show in detail how the decomposition results provide effective surface and sub-surface clutter reduction and guide the interpretation of scattering from subsurface objects. Data processing methods assume cross-polar symmetry and a novel bistatic calibration procedure was developed to enforce this condition. The Yamaguchi polarimetric decomposition provides significant clutter reduction and image contrast with some loss in signal power; while Stokes parameters also provide imagery localising targets, complementary information on the scattering mechanism is also obtained. Finally a third novel polarimetric filter was formulated based on differential interferometric polarimetric decomposition. The three combined techniques contribute to a significant improvement of subsurface radar performance and detection image contrast.
This paper presents a time domain antenna measurement technique by using a low cost software defined radio receiver. The technique aims to resolve measurement challenges derived from antennas where the reference signal is not accessible. The phase reconstruction implemented in this work is based on calculating the Fast Fourier Transform of the time domain signal to estimate the power spectrum and the relative phase between measurement points. In order to do that a reference antenna is used to retrieve the phase, providing a full characterization in amplitude and phase of the electric field and allowing source reconstruction. The results demonstrate the potential of this technique for new antenna measurement systems and reveal some of the limitations of the technique to be optimized, like the undesired reflections due to the interactions between the probe and the reference antenna.
The Low frequency Coaxial Reflectometer is the recommended procedure to measure the absorbers' reflectivity as per the IEEE 1128-1998 standard. The standard recommends the operable frequency range up to 500 MHz with a permissible error of 2 dB and higher error beyond 600 MHz. This paper studies and discusses the error on different types of absorber. Each of the absorber type is simulated in the square section of the reflectometer setup to compute the absorber's reflectivity using Ansys HFSS. An effective time gating technique is applied to reduce the effect of edge effects. These results are compared to the unit cell simulation results with a plane wave excitation and periodic boundary conditions. The absorbers are then simulated in the complete reflectometer setup to include the mismatch associated with the transition and compared to the unit cell model results. The errors associated with the comparison of the absorbers' simulation results for these different models are analyzed. The combination of these different absorbers is simulated in unit cell model. The absorbers are placed in different regions and orientations inside the reflectometer. The comparison between the unit cell results of the combination of the absorbers and the results of the absorbers inside the reflectometer in different orientations give the effect of the non-uniform field distribution inside the reflectometer.
The need for thermal stability in a test chamber is a well-established requirement to maintain the accuracy and repeatability sought for high frequency planar near-field (PNF) scanner measurements. When whole chamber thermal control is impractical or unreliable, there are few established methods for maintaining necessary precision over a wide temperature range. Often the antenna under test (AUT) itself will require a closed-loop thermal control system for maintaining stable performance due to combined effects from transmission heat dissipation and the environment. In this paper, we propose a new approach for near-field system design that leverages this AUT stability, while relaxing the requirement of strict whole chamber thermal control. Fixed reference monuments strategically placed around the AUT aperture perimeter, when measured periodically with a sensing probe on the scanner, allow for the modeling and correction of the scanner positioning errors. This process takes advantage of the assumed stability of the reference monuments and attributes all apparent monument position changes to distortions in the scanner structure. When this monument measurement process is coupled with a scanner structure that can tolerate wide thermal variations, using expansion joints and kinematic connections, a robust structural error correction model can be generated using a bilinear mapping function. Application of such a structure correction technique can achieve probe positioning performance similar to scanners that require tightly controlled environments. Preliminary results as well as a discussion on potential design variations are presented.
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.
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.
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.
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.
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.
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
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