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Anechoic ranges require constant temperature and humidity, proper lighting to be able to work inside the range and closed-circuit television (CCTV) cameras to monitor the system while the measurement is being done. In addition, anechoic chambers require fire detection and suppression. Traditionally these penetrations are minimized and placed in non-critical areas. But the true effect of them has not been fully investigated. In this paper, antenna measurements as simulated in an indoor far field range. The approach to model the measurement is like the one the author presented in [1] and [2]. Thus, a range antenna (or near- field probe) and an antenna under test (AUT) are placed in free space and the AUT is rotated at discrete angles as it was done in [1]. Then a second model includes CCTV cameras, HVAC vents, light fixtures and both air sampling tubes and fire suppression nozzles and placed around. The simulation with these disruptions is repeated at the given discrete angles. The model does not include the absorber on the range. The model assumes a perfect absorber and the results of the simulated antenna measurement are compared to an ideal case with no disruptions. The results, while being approximations, provide a worst-case error for those disruptions of the RF-absorber layout. The results can be used to estimate the potential uncertainty on the measurement caused by the different systems that must be part of the anechoic enclosure. The technique is applied here to indoor far field measurements, and for near-field systems. Results show that for your typical roll over azimuth positioner, the effects of the penetrations on the ceiling are very small with differences in the -35 to -40 dB levels.
The first mention of a Robot for near-field measurements of antennas appears is by Jeff Snow in [1]. This was a simple robotic arm to do planar measurements. About 7 years later, the use of off-the-shelf industrial robotic arms for doing antenna measurements is introduced [2]. Since then, industrial- robot-arm based antenna measurement systems have become increasingly popular due to their flexibility to measure over different surfaces allowing the system to do planar, spherical and cylindrical. The use of other methods to perform the transform, by numerically compute the currents on an arbitrary surface from the measured fields has helped in the growing popularity of robotic systems. This is related that the measurement surface does no longer have to be a canonical surface but can be any shape. However, the flexibility of the robots may be limited by the RF absorber coverage used in treating them. In this paper, the authors explore the potential scattering from the robotic arm in different positions and its effect on the probe illuminations. This is an area of research on the use of absorber that has not been explored until recently [3]. Numerical experiments are conducted to explore the effects of RF absorbers in the 300 MHz to 3 GHz range. Open ended waveguides (OEWG) as well as dual ridged horns (see Figure 1) are used as the probes. The results suggest that some areas of the arm need to be treated while others can be left bare. The analyses performed suggest that optimized treatment of robotic arms to maintain the flexibility of the technique while also reducing effects on the probe illuminations are possible.
The wall-reflectivity technique is a proven method to validate the reflectivity reduction of large metallic walls covered with electromagnetic absorbing materials. While for a theoretical perspective it is sufficient to perform only two quasi-monostatic measurements, one for a known reference target and one of the wall, it has been shown that system dynamic range as well as clutter reduction can be achieved by performing spatial averaging using a linear slide, taking several measurements along the perpendicular direction against the wall. In this work the effect of improvements of the accuracy and positioning repeatability of the linear track is investigated. Measurement results are shown indicating the improved capabilities of the measurement setup.
Hypersonic platforms develop skin temperatures exceeding 1500°C. Materials applied to these skins often require specific electric and magnetic properties which must be validated at temperature, requiring tailored furnaces, refractory metals, specialized ceramics, and procedures to protect RF hardware and personnel. Free space measurement configurations maximize the distance between high temperature components and measurement hardware, thus limiting errors and damage due to radiant heat. Free space material property inversions assume planewave illumination on an infinite sample, which, in practice, is approximated by a finite sample with attenuated fields at its boundaries. Far field generation using Gaussian shaping dielectric lenses, Hogg horn reflectometers, and lens covered antennas have all been previously employed. Given the need to maximize sample- to-hardware distance, an alternative room temperature measurement approach has been developed for the 8-18 GHz band as a baseline architecture for testing up to 1500°C and beyond. This baseline system utilizes two opposing ellipsoidal reflectors with a shared focus to create a localized sample plane. First published in 1971 for use with high temperature dielectric measurements, the simplicity of the elliptical shape, frequency scalability, low-cost material, and straightforward fabrication, makes this approach a good candidate for high temperature sample illumination. The baseline system extends the initial work to include magnetic materials and a discussion of primary error sources, including incident field performance versus feed displacement, reflector conductivity, feed-reflector interactions, and off-normal illumination. System performance is demonstrated through complex permittivity and permeability estimates for Neoprene, reticulated absorber, one-pound density Polystyrene, and commercial magnetic absorber. Planned modifications to the baseline system for high temperature applications are also presented.
An international comparison of antenna calibration results was conducted between two internationally recognized laboratories—NICT (Japan) and RRA (Korea)—in the 18–40 GHz frequency band. Due to the high cable loss in the band, antenna calibrations were performed inside SACs using two different configurations. In the first configuration, absorbers were lined on the floor, and the antenna height was set to 1.5 m. In the second configuration, no floor absorber was used, and the antenna height was set to 2.0 m. Results obtained using the extrapolation method were used as a reference to assess the accuracy of each method. The results indicate that the antenna gains of horn antennas and double-ridged horn antennas obtained using the configuration with floor absorbers showed good agreement with the extrapolation method, with differences within 0.5 dB for both NICT and RRA. These findings are expected to contribute to the ongoing development of CISPR standards, particularly by supporting the establishment of harmonized calibration procedures for high-frequency EMC measurements.
The primary role of an anechoic chamber is to provide a reflection free environment that can be used for electro-magnetic measurements, with antenna pattern being one of the most prominent measurements utilizing anechoic chambers. Real world anechoic chambers, however, rarely provide a reflection free environment. Reflections in an anechoic chamber can arise from a mismatch between the absorber size and the frequency used, the angle of incidence between the absorbers and the wave front, various metallic objects inside the chamber and more. As reflections can introduce impairments in the measurements, it is highly desirable to measure an anechoic chamber for reflections and reduce these reflections as much as possible (especially in the designated “quiet zone”). This paper introduces an innovative reflection evaluation method that harnesses both communication processing and radar processing to localize reflection sources in an anechoic chamber. The chamber setup consists of a probe and an antenna under test (AUT). The probe emits a signal, which is directly received by the AUT along with reflections within the anechoic chamber. Employing either a frequency modulated continuous-wave (FMCW) or stepped-frequency signal, the indirect path length is estimated, resulting in a ellipsoid representing potential reflection points. By intersecting multiple ellipsoids generated through relocating the probe and projecting the intersection onto the chamber, the reflection location is determined. The method’s efficiency has been demonstrated through implementation and validation in an anechoic chamber, with the paper presenting real measurement results for validation purposes.
This paper presents an extended uncertainty analysis of a multiprobe antenna measurement system developed for large platform testing across the 64 MHz to 6 GHz frequency range. Installed at the Pulsaart by AGC facility in Belgium, the system enables fast and accurate characterization of complex structures integrating multiple antennas. Building on previous studies, the analysis expands the uncertainty budget by including a broader set of antennas, such as monocones operating down to 50 MHz, and evaluating key figures of merit including radiation pattern, gain, efficiency, and cross-polarization. Particular emphasis is placed on reflectivity-related uncertainty, which is a dominant factor at lower frequencies due to chamber electrical size and absorber limitations. The methodology incorporates modal filtering and spatial displacement of antennas to isolate the environmental effects. The results offer detailed insights into antenna-dependent uncertainties and, for the first time, provide complete uncertainty estimations for the aforementioned metrics across the full operating frequency range.
There are many antennas and microwave analysis and modeling software packages available, each with its preferred computational approach. Sometimes some of the available packages can use different numerical techniques. It is always gratifying if the solutions are consistent. Conventionally at NSI-MI compact range (CR) performance is evaluated with a proprietary software tool that drives two different approaches depending on the type of edge treatment. Serrated edge reflectors are handled using a well-known commercial package based on Asymptotic methods such as Geometrical (GO), Physical Optics (PO) and Geometrical Theory of Diffraction (GTD). For rolled blended-edge reflectors, the tool calls on a GO and modified unified theory of diffraction (UTD) introduced by Ellingson, Gupta and Burnside [1]. UTD used the method introduced by. Recently, NSI-MI has been using a commercial package based on the Method of Moments (MoM) using higher order basis functions. This tool showed correlation with the GO and m-UTD approach introduced in [1]. The results were presented in [2]. While the Asymptotic methods are faster and can be used for quick optimization of the design, they are not suited for analysis of the feed fence interaction, the reflector absorber skirt that hides the support structure or the interaction with the antenna under test (AUT) positioner. The MoM based approach allows for these types of analysis [3,4]. The MoM package leverages techniques like high- order basis functions (HOBFs), and sophisticated reduction methods. In this software a CR dish is modeled though the import of a CAD file that is used in the manufacture of the CR dish or is modeled within the software package GUI using its native CAD functionality. In this paper the quiet zone (QZ) performances predicted by the commercial package using asymptotic techniques and those predicted by the MoM-HOBF package are compared for a typical serrated CR dish. The QZ performance is determined by a set of metrics driven by amplitude and phase flatness along one- dimensional cuts across two lateral and orthogonal axes centered at the center of the QZ as recommended in [5]. The results show that with the proper meshing constraints the performances modeled by the asymptotic approach and the MoM-HOBF are consistent and comparable for the cases presented in this. The long history of predictions that match the measured results upon implementation on the field of reflector designed by the asymptotic technique means that the MoM results can be used to accurately predict the performance of ranges while analyzing the effects of fences, skirts and the absorber on the AUT positioners that the MoM tool allows.
Metamaterial-based blackbodies are playing an increasingly important role in spaceborne remote sensing due to their tailored terahertz emissivity profiles. Accurate characterization of their angular-dependent reflectivity is essential for establishing reliable radiometric baselines, particularly given their structured surfaces. In this work, we present a focused-beam, angular-resolved measurement method using a monostatic configuration that integrates a polymethylpentene (TPX) lens with a standard gain horn antenna. The setup, implemented on the Configurable Robotic Millimeter-wave Antenna (CROMMA) system at NIST, enables micron-level spatial precision through a robotic arm and hexapod stage. For calibration, a reflecting mirror is employed to extract system parameters, thereby eliminating the need for near-field lens antenna characterization. A forward model based on plane- wave scattering matrix decomposition is developed to extract the target’s angular reflectivity, with time-gating and spectral filtering applied to isolate temporal reflections and mitigate modal interference. Using this calibrated system and signal processing workflow, we measure the angular reflectivity of a metamaterial-based blackbody designed for the G-band (140–220 GHz). The results reveal elevation-angle-dependent reflectivity about the boresight normal, demonstrating the method’s ability to resolve angular variations with high accuracy. This approach offers a practical and precise framework for characterizing structured terahertz absorbers and supports the advancement of calibration standards for microwave and terahertz applications.
This paper presents an overview of the induced ripples observed in the far-field antenna patterns of the Antenna Under Test (AUT) when measured with an open-ended waveguide antenna probe in a near-field planar system. The author hypothesized that induced ripples in far-field patterns are primarily originated by diffracted fields on the ground plane that supports the collar absorber. This study systematically evaluates the effects of absorber size and quality. Numerical simulations and experimental measurements are employed to validate the author’s hypothesis, providing insights into the relationships between these parameters and their influence on the induced ripples in far-field patterns. Results indicate that collar absorbers with reflectivity better than -30 dB are optimal for achieving accurate element characterization of phased array antennas.
Tapered chambers use the reflections from the surfaces adjacent to the range antenna to illuminate the quiet zone (QZ). Polyurethane substrate is the preferred and most widely used radio frequency (RF) absorber in these chambers, due to its ability to be cut into complex shapes to conform to the tapered sections. Unfortunately, this type of absorber always presents slight differences in permittivity related to the manufacturing process. To analyze the effects of the permittivity of the lossy foam on the QZ illumination in a tapered chamber, a series of numerical experiments using a full wave analysis technique are executed. The results are mainly obtained for frequencies under 1 GHz. The upper frequency of the simulation is limited by the electrical size of the problem and by the available information on the material permittivity. However, frequencies below 1 GHz is where the tapered chambers are superior to other methods for indoor antenna measurements. Magnitude and phase are recorded over a 1.82m diameter spherical QZ to show the effects of the different absorber on the illumination. Results show that a variation on the absorber around the range antenna will deviate the illumination and skew the amplitude taper across the QZ. The amplitude distribution peak can be shifted by as much as 3.5 degrees from boresight. The effect on the phase taper is smaller with a negligible change in phase.
In anechoic chambers, the level of spurious reflections is determined by the reflectivity of the installed absorbers and is usually estimated using ray-tracing methods. But since the basic assumption of a purely specular reflection in most of these ray-tracing methods can lead to insufficient results, the reflectivity of the absorbers must be analyzed for oblique incidence and over a broad range of observation angles. In this paper, a bi-static measurement setup is proposed, which overcomes angular limits of the NRL-arch method and allows to analyze the scattering behavior of absorbers in an extended angular range. Using this setup, and applying the radar cross-section method, the reflectivity of two types of pyramidal absorbers was analyzed with respect to different illumination and observation angles for parallel and perpendicular polarization between 2 and 18 GHz. While the measurement results for normal incidence agree well with the specifications, additional non-specular reflections of similar strength were detected in the time-domain at different observation angles. Especially for the case of oblique wave incidence, it becomes apparent that the highest reflectivity does not necessarily occur for specular reflection. These findings help to improve the understanding of the scattering behavior of absorbers in general, as more comprehensive analyses become possible with this method. Index Terms—bi-static scattering, electromagnetic wave absorption, reflectivity, RF absorber, time-domain analysis.
Motivation and background: With the increasing abundance and functionality of wireless communication systems, the requirements for virtual electromagnetic environments like shielded anechoic chambers, and the complexity of the test procedures increase accordingly. The scattering behavior of microwave absorbers is an essential indicator of their quality and suitability for use in such anechoic chambers. Current research activities deal with the revision of the IEEE standard 1128 on recommended practice for absorber characterization and give room for improved test procedures. Objectives and methods: In this paper, the angle-dependent backscattering of microwave absorbers was studied experimentally with respect to their different geometric shapes and material parameters. The dielectric permittivity of pyramidal and flat absorbers was measured between 1 GHz and 10 GHz, followed by systematic monostatic reflectivity measurements. Signal post-processing, including phase-coherent background subtraction and time-domain gating, were applied to minimize unwanted reflections and extract the wanted scattered signals. The radar cross-section (RCS) method was applied to derive the reflectivity with respect to different illumination angles for parallel and perpendicular polarizations. The results were compared to supplier specifications, electromagnetic simulations of the reflectivity, and the scattering pattern of a metal plate. Results and conclusions: The measurement results agree well with the numerical simulations. The data reveal that the reflectivity patterns of microwave absorbers are governed by their geometric shape, while the material properties do not modify the angular dependences qualitatively but result in a quantitative offset. Our findings help to improve the accuracy of monostatic RCS and absorber reflectivity measurements even further and lead to a better understanding of the physical origin of the scattering phenomena of microwave absorbers in general. Future work will extend our studies towards bi-static angle-dependent reflectivity measurements, in order to establish a consistent and comprehensive method for characterizing different types of microwave absorbers with respect to type, frequency, angle of illumination, angle of observation, and polarization. This research has been funded by the German Research Foundation (Deutsche Forschungsgemeinschaft, DFG) under the grants HE3642/14-1 and BO4990/1-1 (Electromagnetic modeling of microwave absorbers - EMMA).
There have been a number of numerical analyses of RF absorber presented in the literature. These analyses, however, tend to focus on the reflectivity of the material and not on the radar cross section (RCS) that it presents. Brumley studied the RCS of RF absorbers for the purpose of estimating the background RCS of anechoic ranges [1]. The study was done empirically; obtaining measurements of the RF absorber and looking at the RCS of different pyramids and wedges, with and without paint. Brumley presents some potential methods to improving the RCS signature of the range, thus reducing the background RCS of the site. In this paper, the suggestions presented by Brumley are revisited. Specifically, his recommendation for the twisted pyramid configuration which he was unable to measure due to the lack of absorber samples available for use in the test. In addition to the twisted pyramid, Brumley's approach of inserting smaller pyramids in the valleys of a larger pyramidal arrangement to reduce the edges parallel to the incoming wave are also presented. Different carbon loadings are modeled for the inserted pyramids. One is the standard loading of the inserted pyramid, and the other is the same loading as the main larger pyramidal arrangement such that all the absorber on the wall has the same material properties. Numerical studies are performed using time domain techniques as well as frequency domain techniques. The model is validated by comparing the RCS of a flat square plate with the theoretical solution. The results validate the data and the suggestions presented in [1] and present ways of improving some of the solutions by adjusting the material properties of the absorber.
Financial impacts often drive decisions to repurpose existing ranges instead of procuring new measurement facilities. These existing ranges have fixed geometries (height, width and length) that were set when the range was originally constructed and often are designed for a different purpose. The inability to set the geometry precludes the range designer from using the range geometry to improve measurement performance. Thus, the performance of the range is mostly dependent on the RF absorber and the range antenna directivity. In rectangular-shaped ranges for example, the lateral surfaces, side walls, ceiling and floor, are the critical surfaces to address in RF absorber arrangement. In this paper, numerical analyses of Chebyshev arrangements as well as dragon tail or tilted absorber are studied. This paper also analyzes the performance of Chebyshev absorber for normal incidence and for oblique incidence along with the proper arrangement of the Chebyshev period. While certainly these have been discussed previously in the literature, this paper consolidates the previous information and illustrates it with numerical examples to help the reader understand the best approach to use when repurposing a range.
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.
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 [1] 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.
Electrical properties of materials are requisite to design electromagnetic (EM) devices and systems. Free-space material measurement method, where the measurands are the free-space scattering parameters of MUT (Material Under Test) located at the middle of transmit (Tx)/receive (Rx) antennas, is suitable for non-destructively testing MUT without prior machining and physical contact in high frequencies. In this paper, GSS (Gated-Short-Short) calibration method using a planar offset short is proposed for calibrating a free-space material measurement system and the measurement result is shown in W-band (75-110 GHz).
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.
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 [1] 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 [2] 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).