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Calibration

Comparison of Antenna Measurements Obtained Using an Electro-Optical Probe System to Conventional RF Methods
William Dykeman, Benjamin Marshall, Dale Canterbury, Corey Garner, Richard Darragh, Ali Sabet, October 2019

There are certain applications where the use of electro-optical (EO) probes to acquire near-field measurements can provide major advantages as compared to conventional RF measurement techniques. One such application is in the area of high power RF measurements that are required for calibration and test of active electronically scanned arrays (AESA). The family of EO probes presented herein utilizes the Pockels effect to measure the time-varying electric fields of the antenna under test (AUT). The use of a non-invasive, broadband EO probe facilitates measurement of the tangential electric field components very close to the AUT aperture in the reactive near-field region. This close proximity between the AUT and the measurement probe is not possible with conventional metallic probes. In this paper, the far field gain patterns acquired using the EO probe will be compared to the corresponding gain patterns obtained from conventional far-field and near-field methods. The measurement results, along with the advantages and disadvantages of the EO system configuration, will be presented.

Recent Changes to the IEEE std 1502 Recommended Practice for Radar Cross-Section Test Procedures
Eric Mokole, Vince Rodriguez, Jeff Fordham, L J Foged, ,, October 2019

Radar scattering is typically represented as the RCS of the test object. The term RCS evolved from the basic metric for radar scattering: the ratio of the power scattered from an object in units of power per solid angle (steradians) normalized to the plane-wave illumination in units of power per unit area. The RCS is thus given in units of area (or effective cross-sectional area of the target, hence the name). Note that the RCS of the test object is a property of the test object alone; it is neither a function of the radar system nor the distance between the radar and the test object, if the object is in the far field. Because the RCS of a target can have large amplitude variation in frequency and angle, it is expressed in units of decibels with respect to a square meter and is abbreviated as dBsm (sometimes DBSM or dBm2). In terms of this definition, the RCS of a radar target is a scalar ratio of powers. If the effects of polarization and phase are included, the scattering can be expressed as a complex polarimetric scattering (CPS) matrix. The measurement of the RCS of a test object requires the test object to be illuminated by an electromagnetic plane wave and the resultant scattered signal to be observed in the far field. After calibration, this process yields the RCS of the test object in units of area, or the full scattering matrix as a set of complex scattering coefficients. This paper describes the planned upgrades to the old IEEE Std 1502™-2007 IEEE Recommended Practice for Radar Cross-Section Test Procedures [1]. The new standard will reflect the recent improvements in numerical tools, measurement technology and uncertainty estimates in the past decade.

Using Standard Wideband Antennas as Probe in Spherical Near Field Measurements with Full Probe Correction: Experimental Validation
F Saccardi, A Giacomini, L J Foged, L M Tancioni, S Khlif, Martin Kuhn, ,, November 2018

Full probe compensation techniques for Spherical Near Field (SNF) measurements have recently been proposed [1-5]. With such techniques, even antennas with more than decade bandwidth are suitable probes in most systems. The abolition of otherwise frequent probe changes during multi-service campaigns is a highly desirable feature for modern measurement applications such as automotive. In this paper, a standard dual-ridge horn with 15:1 bandwidth is investigated experimentally as probe in a SNF automotive range. The accuracy of the probe compensation technique is validated by comparison to standard single probe measurement.

Near-Field Spherical Scanning Measurement of a 3D Printed Horn at WR-8 Frequencies
Ronald C Wittmann, Michael H Francis, David R Novotny, Joshua A Gordon, Michael S Allman, November 2018

The National Institute of Standards and Technology (NIST) has measured a WR-8, 3D printed horn at 112.25, 118.75, and 125.25 GHz using the near-field spherical scanning method. The data were processed with both the NIST standard software and the probe-position compensation software. We conclude that the positioning capability of the NIST Configurable Robotic Millimeter-wave Antenna System is so accurate that probe-position compensation is negligible at these frequencies.

Low-Cost Pressure/Temperature Measurements of Wideband Antennas
L Boskovic, M Ignatenko, D S Filipovic, November 2018

This paper discusses design and fabrication of a low cost, combined pressure / thermal test-bench engineered for environmental tests of UAV mounted antennas. Both test-beds are mainly made of commercial of-the-shelf (COTS) parts and in-house made frames. They occupy small space and do not require specific professional skills for operation or high maintenance cost. Measurement setup is designed to reliably reproduce temperature and pressure corresponding to altitudes from sea level to 6000 m (20000 ft) with dynamic load equivalent for 200 m/s (400 knots) of air speed. Experimental results of radome enclosed wideband antenna are presented.

Spherical Phaseless Probe-Corrected Near-Field Measurements of the DTU-ESA VAST12 Reflector Antenna
Javier Fernández Álvarez, Jeppe M Bjørstorp, Olav Breinbjerg, November 2018

An experimental case of spherical probe-corrected phaseless near-field measurements with the two-scans technique is presented, based on magnitude measurements at two surfaces of the VAST12 reflector antenna performed at the DTU-ESA Facility. Phase retrieval using strictly the directly measured near-field magnitude was unfeasible in this setup, due to the small sphere separation allowed by the probe positioner, which led to incorrect and excessively slow convergence. Phase retrieval with larger separation between spheres has shown remarkable results. For these tests a measured magnitude was used in combination with calculated near-field magnitudes at different (larger and smaller) spheres with larger separations than allowed by the experimental setup. It has been seen that larger separation between measurement spheres improves accuracy of phase retrieval. A measurement with a backprojected measurement with 3 m sphere separation is of particular interest because it can be potentially replicated in the DTU-ESA Facility assuming such range of movement was allowed, while being accurate down to an error of less than-35dB. Measurements with larger spheres show even better accuracy. These good results were obtained with the normal spatial sampling rate for complex measurements and with a very simple Hertzian dipole initial guess, and show the superior performance of spherical phaseless measurements with the two-scans technique, compared to a planar setup.

Compact Antenna Measurement Range for OTA testing of Active Antenna System Base Stations
L M Tancioni, A Jernberg, P Noren, P Iversen, A Giacomini, A Scannavini, R Braun, M Boumans, H Karlsson, , ,, November 2018

Measurement scenarios for 5G mobile communications are nowadays challenging the industry to define suitable turn-key solutions that allow Over the Air (OTA) testing of non-connectorized devices. In order to respond to the needs of an effective measurement solution, that allow measuring all the required OTA parameters at both sub6GHz and mm-Wave frequencies and that could be deployed in a very short time, the Compact Antenna Test Range (CATR) was chosen. In this paper, we will summarize the performance and the testing capabilities of a short focal-length, corner-fed CATR design, providing a 1.5 m x 1.5 m cylindrical Quiet Zone, operating from 1.7 GHz to 40 GHz and upgradeable to 110 GHz, allowing OTA measurements of Active Antenna System (AAS) Base Stations (BS), installed at Ericsson premises in Gothenburg, Sweden in 2017.

Geometric Effects on Radar Echoes from a Corner Reflector
P S P Wei, November 2018

Radar data on the complete polarimetric responses from a 4" dihedral corner reflector from 4 to 18 GHz have been collected and studied. As a function of the azimuth, the vertically suspended object may present itself to the radar as a dihedral, a flat plate, an edge, a wedge, or combinations of these. A two-dimensional method-of-moment (2-D MOM) code is used to model the perfectly electrical conducting (PEC) body, which allows us to closely simulate the radar responses and to provide insight for the data interpretation. Of particular interest are the frequency and angular dependences of the responses which yield information about the downrange separation of the dominant scattering centers, as well as their respective odd-or even-bounce nature. Use of the corner reflector as a calibration target is discussed.

Rydberg Atom-based RF Power Measurements
Matt T Simons, Marcus D Kautz, Abdulaziz H Haddab, Joshua A Gordon, Christopher L Holloway, Thomas P Crowley, November 2018

The power transmitted through a waveguide was determined using in-situ atom-based electric field measurements. The field distribution in the waveguide was measured using Rydberg atoms to find the maximum field, which was used to determine the power. For a proof-of-concept, the power of radio frequency fields at 17.86, 19.63, 26.53, and 33.03 GHz were measured in a WR42 waveguide. A section of waveguide was sealed and filled with cesium atoms. These atom-based measurements are self-calibrated and independent of typical power measurement methods.

Measurement of Active Reflection Coefficient for Co-located MIMO Radar Using Dual Directional Couplers
N Colon-Diaz, D Janning, T Corigliano, L Wang, J Aberle, November 2018

This paper presents a way to determine mutual coupling effects through analysis of the active voltage standing wave ratio (VSWR) to predict the presence of large reverse power levels in co-located multiple input multiple output (MIMO) radars in transmit mode. The methodology consists of measuring the forward and reverse waves on a dual directional coupler (DDC) to directly obtain the active reflection coefficient on a co-located MIMO radar system. The active VSWR of each individual antenna is computed from measurements of the active reflection coefficient. These results are compared against analytical methodologies.

Fully Probe Corrected Spherical Near Field Offset Measurements with Minimum Sampling Using the Translated-SWE Algorithm
F Saccardi, F Mioc, A Giacomini, L J Foged, P O Iversen, November 2018

The Translated Spherical Wave Expansion (TSWE) has recently been proposed as a very effective Near-Field-to-Far-Field (NF/FF) transformation tool for down-sampled Spherical Near Field (SNF) measurements with offset Antenna Under Test (AUT). In case of electrically small probes and/or small AUT-probe view angles the TSWE can be accurately applied without compensating for the probe effect. Instead, when electrically larger probes and/or larger view angles are considered, the measured signal is affected by an averaging field effect that should be properly compensated to ensure a good accuracy. In this paper the TSWE technique is applied for the first time tacking into account the full effect of the measuring probe. To validate the proposed technique, a standard gain horn intentionally displaced in offset configuration have been measured in SNF geometry with a first order probe and two different wideband higher-order antennas as probe.

Some Advantages of Using Bi-directional S-Parameters in Near-Field Measurements 1
David R Novotny, Alex J Yuffa, Ronald C Wittmann, Michael H Francis, Joshua A Gordon, November 2018

The unknown-thru calibration technique is being used to achieve a system level calibration at millimeter wave frequencies (>50 GHz) on the robotic ranges at NIST. This two-port calibration requires the use of a full bi-directional measurement, instead of a traditional single-direction antenna measurement. We explored the value of the additional data acquired. We find that we can use this information to verify antenna/scan alignment, image the scattering from the positioner/facility, and perform a first order correction to the transmission data for uncertainties due to LO cable flexure.

Spot-Probe Reflectometer Measurements of Geological Core Slab Samples
Jose Oliverio Alvarez, Development, John W Schultz, November 2018

Rock core specimens collected during surveys for oil drilling have, in a standard form, a 4" diameter. Cores are cut in half or in 1/3-2/3 sections to provide core slab. We developed a measurement procedure based on spot probe illumination to characterize geological and/or geochemical properties of core slab specimens via their complex permittivity for frequencies between 2.5 GHz and 20 GHz. Conventional reflectometer methods are based on illumination of a thin slab of air-or metal-backed material. However, in this case only the front surface is flat and the back surface is semicircular. A measurement method was developed based on time-domain gating to separate the back-surface reflection from that of the front. Material inversion is then based on the amplitude and phase of the reflection just from the front surface. This paper presents details of the calibration for this reflectometer measurement method, along with example measurements of core slab materials. Two different inversion methods are applied to these measured data. The first is a more conventional frequency-by-frequency method for inverting complex permittivity from the amplitude and phase of the reflection. The second method applies a physical model, the Debye relaxation model, to the data. This model-based approach minimizes the errors from edge diffraction from the small sample size.

Precision Optical Antenna Alignment System for Tracking Antennas in 6-DOF
Joshua A Gordon, David R Novotny, Michael S Allman, November 2018

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.

Improved Nearfield Gain Measurement of High Gain Antennas Using Directivity and Loss Technique
Brian Park, Amanuel Haile, Paul Werntz, November 2018

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.

Extending the Scan Volume of Planar Near-Field Scanners with AUT Rotation
Dave Neff, November 2018

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.

DTU-ESA Spherical Near-Field Antenna Test Facility -2017/18 Upgrade and Validation Measurements with the DTU-ESA VAST12 Antenna
Jeppe M Bjørstorp, Olav Breinbjerg, November 2018

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.

Power Density Measurement at 5G Millimeter-Wave Using Inverse Source Method
L Scialacqua, F Saccardi, A Scannavini, L J Foged, November 2018

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.

Coupling Suppression and Measurements on a Millimeter Wave Cylindrical Repeater
M Ignatenko, B Allen, S Sanghai, L Boskovic, D Filipovic, November 2018

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.

Reference Chip Antenna for 5G Measurement Facilities at mm-Wave
A Giacomini, F Scattone, L J Foged, E Szpindor, W Zhang, P O Iversen, Jean-Marc Baracco, November 2018

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.







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