AMTA Paper Archive
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Indoor Antenna Measurement Facility: Determination of the phase center position
Indoor antenna measurement facilities are usually dedicated to characterize all the parameters of an antenna. In order to perform phase center position measurements, the CEA has designed a specific experimental layout to characterize this parameter with a very high accuracy. This paper describes this measurement facility and deals with technical decisions made during its design phase. Finally, we will talk about possibilities offered by this specific layout and the advantages of this layout compared to a classical antenna test-bench.
Effective Polarization Filtering Techniques for Ground Penetrating Radar Applications
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.
Evaluation of Software Defined Radio Receiver for Phaseless Near-Field Measurements
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.
Enhanced PNF Probe Positioning in a Thermally-Uncontrolled Environment using Stable AUT Monuments
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.
Application of Complex Image Theory for Nearfield Antenna Measurements over Seawater
The principles of near-field antenna measurements in Cartesian, cylindrical, and spherical coordinates are well established and documented in the literature and in standards used on antenna ranges throughout government, industry, and academia. However the measurement methods used and the mathematics that are applied to compute the gain and radiation of the pattern of the test antenna from the near-field data assume that the antenna is operating in free space. This leaves several questions open when dealing with antennas operating over a lossy ground plane, such as the ocean. In this paper, we shall discuss a possible avenue for addressing this problem : the use of Complex Image Theory (CIT). The CIT approach allows the lossy earth to be removed and an image of each equivalent source point in the space above it to be constructed in the now empty space below it, but where the depth of that image is in general a complex number. While it might appear confusing to define a complex depth, such a depth is merely a mathematical construct that accounts for a magnitude and phase shift that occurs due to the presence of the lossy ground. The depth is computed so that the boundary condition at the surface of the original lossy ground is maintained; in this way, an equivalent problem is formulated. We propose an approach based on CIT that can be applied to the problem of a spherical nearfield antenna measurement taken over seawater. A limiting case of measurements taken over a metal ground plane shall be presented, along with thoughts about some practical concerns involved in the performance of such measurements.
A Modern, Indoor Far-Field Extrapolation Range
Prior literature in the subject area of far-field antenna measurements has demonstrated an extrapolation technique to isolate and correct the errors due to near-zone proximity effects as well as multi-path range reflections, thus allowing data to be collected at distances much less than the conventionally defined far-field criteria. This paper describes a modern, indoor, far-field antenna measurement range specifically designed to support this extrapolation technique. A multi-axis positioning system featuring a mobile horn tower capable of motion along the chamber Z-axis is emphasized. High-speed RF instrumentation and advanced software control support the full automation of the extrapolation method. This contemporary approach is demonstrated, and measurement examples are provided for an X-band slotted waveguide array. The resultant far-field gain calculations are also compared to similar data extracted using near-field scanning techniques.
Uncertainty Analysis Technique for Planar Field-Probing Measurements and Quiet-Zone Simulations of a Compact Antenna Test Range
The performance of a compact antenna test range is evaluated by field-probing measurements of the quiet zone. The comparison between the simulated and measured data, however, is misleading due to the finite measurement accuracy and the limited nature of the numerical model. In order to allow a comparison, the uncertainty terms of the field-probing measurements and the numerical model are identified based on the National Institute of Standards and Technology 18-term uncertainty analysis technique. The individual terms are evaluated with simulations or measurements using the equivalent-stray-signal model. Bearing the differences between the model and the actual measurements in mind, the electrical field can be estimated precisely within the overlapping region of both uncertainty budgets.
A Novel GO-PSO Algorithm for Designing 3D- Printed Optimized Pixelized Inhomogenous and Shaped-Profiled Lens Antennas
In this paper, a novel algorithm for designing 3D-printed shaped inhomogeneous dielectric lens antennas is provided. The synthesis approach is based on a novel combination of Geometrical Optics (GO) and the Particle Swarm Optimization (PSO) method. The GO method can trace rays through inhomogeneous media and calculate the amplitude, phase, and polarization of the electric field. The algorithm is used to design an inhomogeneous lens antenna to produce an electronically scanned revolving conical beam to replace a mechanically scanned parabolic reflector antenna for spaceborne weather radar satellite antenna applications. Two breadboard model on-axis fed lens designs are presented and measured results given to validate the approach. A representative optimum off-axis design is presented which produces the revolving conically scanned beam. Imposition of a Body-of-Revolution restriction allows the optimization to be performed at a single offset feed location. The complex inhomogeneous engineered materials that results from optimization are printed using new 3D printers.
Measurements of the dynamic pattern of an electronically steerable phased antenna array with circular polarization in Ka-band
This paper presents two methods for measuring dynamic antenna patterns of phased arrays in a compensated compact range. The first method uses the turntable of the compact range to counter steer the antenna beam. The dynamic pattern is created by measuring single points of the pattern over time. This method is successfully tested, and the measurement results show the effect of phase jumps during the steering process. The second method extends the range of application to fast steering phased arrays by decoupling the antenna scan angle and the azimuth angle of the turntable.
A Procedure to Characterize and Predict Active Phased Array Antenna Radiation Patterns from Planar Near-Field Measurements
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.
Adding Phase to the Rotating-Source Antenna Polarization Measurement Method
The rotating-source measurement method is usually described as an amplitude only measurement method and the axial ratio is the only characteristic that can be measured. The article illustrates how adding a phase measurement allows to get the sense of polarization and to calculate the circular partial gains over a full cut-plane of the antenna under test. Simulations and a measurement example are shown.
Comparison and contrast of the antenna calibration methods of ANSI and CISPR
This is a brief comparison between the two recently released documents that detail the methods used for the calibration of antennas intended for use in measuring electromagnetic compatibility.
2D RCS Prediction from Multistatic Near-Field Measurements on a Plane by Single-Cut Near-Field Far-Field Transformation and Plane-Wave Synthesis
A near-field far-field transformation (NFFFT) technique with a plane-wave synthesis is presented for predicting two-dimensional (2D) radar cross sections (RCS) from multistatic near-field (NF) measurements. The NFFFT predicts the FF of the OUT illuminated by each single source, then the plane-wave synthesis predicts the FF of the OUT each illuminated by each plane-wave by synthesizing the FFs given in the NFFFT step. The both steps are performed in the similar computational procedure based on a single-cut NFFFT technique that has been proposed previously. The method is performed at low cost computation because the NF and source positions are required only on a single cut plane. The formulation and validation of the method is presented.
A Straightforward Dynamic Range Error Analysis
The significant measurement standards in the antenna measurement community all present suggested error analysis strategies and recommendations. However, many of the factors in these analyses are static in nature in that they do not vary with antenna pattern signal level or they deal with specific points in the pattern, such as realized gain, side lobe magnitude error or a derived metric such as on-axis cross polarization. In addition, many of the constituent factors of the error methods are the result of analyses or special purpose data collections that may not be available for periodic measurement. The objective of this paper is to use only a few significant factors to analyze the error bounds in both magnitude and phase for a given antenna pattern, for all levels of the pattern. Most of the standards metrics are errors of amplitude. However, interest is increasing in determining phase errors and, hence, this methodology includes phase error analysis for all factors.
CATR Quiet Zone Depth Variation
The traditional characterization of the quiet zone for a CATR is to perform field probe scans perpendicular to the range axis at one or more depths of the quiet zone, usually front, middle and back. There is usually no attempt to compare the peak signals across the field probe scans. In recent years, users of CATRs have been using these devices at lower and lower frequencies, sometimes below the lowest frequency that provides the specified performance for the CATR. It is recognized that as a CATR is used at lower and lower frequencies compared to its optics, the quiet zone quality will degrade. The purpose of this study was to create a quiet zone depth variation model to characterize the variation, particularly for low frequencies. The model was to treat the CATR as an antenna aperture and apply a power density versus distance model. It is well known that the extreme near field of an aperture is oscillatory at distances up to approximately 10% of the far-field distance, at which point the power density begins to follow the Fraunhofer approximation. The optics of a CATR place the quiet zone well within the oscillatory zone, indicating that the field will vary through the depth of the quiet zone. This variation will decrease with increasing frequency as the far-field distance for the CATR increases with frequency. The model has been compared to a simulation in GRASP and experimental data collected on a CATR.
Measurement of RF Absorber at Large Angles of Incidence using Spectral Domain Transformations
Pyramidal RF absorber, widely used in indoor antenna ranges, is designed to minimize reflectivity by creating an impedance transform from free space to the impedance of the absorber material. The pyramidal shape provides this transition quite well at normal incidence. It has been shown in  that pyramidal RF absorber performs very well up to angles of incidence of about 45 degrees off-normal, but at wider angles of incidence, the performance degrades significantly. Unfortunately, it is very difficult to perform RF absorber measurements at large oblique incidence angles. In such measurements, the reflected path and the direct path between the antennas are very close in length, making it difficult to use time-domain gating techniques to eliminate the direct coupling. In this paper, a novel approach for performing oblique RF absorber measurements is introduced based on spectral domain transformations. Preliminary measurements using this technique have been compared to RF simulations. Results appear to indicate that this approach is a valid way to perform RF absorber reflectivity measurements at highly oblique incidence angles.
Measurements of Non-Metallic Targets for the Austin RCS Benchmark Suite
A simulation-supported measurement campaign was conducted to collect monostatic radar cross section (RCS) data as part of a larger effort to establish the Austin RCS Benchmark Suite, a publicly available benchmark suite for quantifying the performance of RCS simulations. In order to demonstrate the impact of materials on RCS simulation and measurement, various mixed-material targets were built and measured. The results are reported for three targets: (i) Solid Resin Almond: an almond-shaped low-loss homogeneous target with the characteristic length of ~10-in. (ii) Open Tail-Coated Almond: the surface of the solid resin almond's tail portion was coated with a highly conductive silver, effectively forming a resin-filled open cavity with metallic walls. (iii) Closed Tail-Coated Almond: the resin almond was manufactured in two pieces, the tail piece was coated completely with silver coating (creating a closed metallic surface), and the two pieces were joined. The measured material properties of the resin are reported; the RCS measurement setup, data collection, and post processing are detailed; and the uncertainty in measured data is quantified with the help of simulations.
A Compact Reconfigurable Millimeter-Wave Antenna Measurement System Based Upon an Industrial Robot
Echodyne has recently completed and qualified a new millimeter-wave antenna measurement system for characterization of beam-steering antennas such as our Metamaterial Electronic Steering Arrays (MESAs). Unlike most far-field systems that employ a standard Phi/Theta or Az/El positioner, we use a six-axis industrial robot that can define an arbitrary AUT coordinate system and center of rotation. In different operational modes, the robot is used as an angular AUT positioner (e.g., Az/El) or configured for linear scan areas. This flexible positioning system allows us to characterize the range illumination and quiet zone reflections without modification to the measurement system. With minor modifications, the system could also be used in a planar-near field configuration. Range alignment can be easily performed by redefining the coordinate system of the AUT movement in software. The approximate 5.2-meter range length is within the radiating near-field of many arrays of interest, so we employ spherical near-field (SNF) correction when necessary, using internally-developed code. Specialty tilted absorber was installed in the chamber to improve quiet zone performance, over standard absorber treatment for similar aspect ratio ranges. Narrower ranges often have specular reflections that exceed 60° and benefit from the specialty tilted absorber designed to reduce the angle of incidence. We present an overview of the measurement system and some initial measurement data, along with lessons learned during design and integration. I. MEASUREMENT SYSTEM OVERIVEW A 7.3m x 3.7m x 3.7m footprint was allocated for the new R&D millimeter-wave antenna measurement chamber. After accounting for structural considerations, the final chamber interior dimensions are 7.1m(L) x 3.45m(W) x 3.35m(H) and the final range length (separation between range antenna and quiet zone center) is about 5.2 m. Table 1 lists the high-level goals of the measurement system are listed in. Table 1. Echodyne R&D chamber goals. Parameter Goal Frequency range 12-40 GHz, with provisions up to 80 GHz Polarization Dual-linear switched or simultaneous AUT positioner Azimuth-over-Elevation and linear scanning Quiet zone size 0.4m(L) x 0.4m(W) x 0.4m(H) Side lobe uncertainty +/-1 dB for-20 dB sidelobe Figure 1 shows the dimensions of the rectangular chamber, which is lined with the special absorber design described in Section II. Figure 2 shows an overview of the measurement system. The RF subsystem consists of a 4-port vector network analyzer (VNA), a Gigatronics GT-1050A power amplifier, a directional coupler (placed after the amplifier) to provide the VNA reference signal and a MVG QR18000 dual-polarized closed boundary quad-ridged horn  as the range antenna. This setup provides continuous frequency coverage from 12 to 40 GHz. External frequency converter modules can be used to extend the range further into millimeter wave. Horizontal and vertical polarization are acquired simultaneously by measuring three receiver channels (B, C & R1) and calculating the ratios B/R1 and C/R1 which remove the effects of amplifier drift (such as temperature coefficient). The range antenna is mounted to a rotary stage to allow direct measurement of Ludwig-III polarization if desired (versus polarization synthesis in post-processing). The AUT positioner described in Section III is a six-axis industrial robot that provides both angular azimuth-over-elevation and linear scanning with high-accuracy. Linear scanning allows planar near-field measurements in addition to the quiet zone evaluation shown in Section IV. The 5.2 m range length is within the radiating near-field of many arrays of interest, especially at higher frequencies. For example, even a relatively small (140 mm) AUT would have a 22.5° phase taper across at 40 GHz. We use the spherical near-field measurement correction  described in Section V to obtain true far-field patterns in the Az/El coordinates described by the robot motion. Figure 1. Rectangular chamber dimensions (in inches).
Personal Near-field System
In 1987 the author built the world's first Personal Near-field antenna measurement System (PNS). This led to the formation of Nearfield Systems Inc. (NSI) a company that became a major manufacturer of commercial near-field antenna measurement systems. After leaving NSI in 2015 several new personal antenna measurement tools were built including a modern updated PNS. The new PNS consists of a portable XY scanner, a hand held microwave analyzer and a laptop computer running custom software. The PNS was then further generalized into a modular electromagnetic field imaging tool called "Radio Camera". The Radio Camera measures electromagnetic fields as a n-dimensional function of swept independent parameters. The multidimensional data sets are processed with geometric and spectral transformations and then visualized. This paper provides an overview of the new PNS and Radio Camera, discusses operational considerations, and compares it with the technology of the original 1987 PNS. Today it is practical for companies, schools and individuals to build low-cost personal antenna measurement systems that are fully capable of meeting modern industry measurement standards. These systems can be further enhanced to explore and visualize electromagnetic fields in new and interesting ways.
Experimental Validation of a Non-Redundant NF-FF Transformation for Long AUTs Mounted in Offset Configuration in a Spherical NF Facility
The experimental validation of an accurate and fast near-field-far-field (NF-FF) transformation technique with spherical scan, suitable for long antennas under test (AUTs) mounted in offset configuration, is provided in this work. The main feature of such a NF-FF transformation is to require, unlike the traditional spherical (TS) one, an amount of NF samples, which is minimum and results to be practically the same in both cases of offset and onset mount-ings of the AUT. To this end, an optimal sampling interpolation formula , developed by properly exploiting the non-redundant sampling representations and modeling an offset mounted long AUT by a cylinder ended by two half-spheres, is employed to precisely recover the massive input NF data for the TS NF-FF transformation from the collected non-redundant samples. A considerable measurement time-saving can be so achieved. Experimental results assessing the validity and the practical feasibility of such a technique are shown.
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