AMTA Paper Archive
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Geometry- and angle-dependent monostatic scattering of microwave absorbers
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).
Numerical Study of the RCS of Pyramidal Absorber Geometries
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 . 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  and present ways of improving some of the solutions by adjusting the material properties of the absorber.
Numerical Analysis of Techniques to Improve Oblique Incidence of 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.
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.
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.
GSS (Gated-Short-Short) Calibration for Free-space Material Measurements in millimeter-Wave Frequency Band
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).
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.
Multiphysics Analysis of RF Pyramidal Absorbers
RF absorbers dissipate the incident electromagnetic wave by converting the RF energy into heat. In many applications, the absorbers can be subjected to high power incident fields. It is imperative to characterize and analyze the thermal behaviors for these high power applications. In this paper, a multi-physics (of EM and thermal) study has been conducted. The absorbers are first simulated in the ANSYS HFSS for electromagnetics. The absorbers are placed under plane wave incident field as well as from a pyramidal horn antenna in the near field. The output of the HFSS model is then imported to the thermal and computational fluid dynamics (CFD) tool, ANSYS ICEPAK. In order to obtain accurate thermal properties of the material, an experimental setup was designed. The simulation results are validated against measured data. Several effects are shown to affect the absorber internal temperatures for the same incident field level at the front of the absorbers, such as the antenna test distance to the absorber, the shape of the pyramid, and the measurement frequency. These simulation data provide greater insights into the heat dissipation and temperature distribution inside the absorbers.
Recent Developments in International Facility Comparison Campaigns
The EurAAP (the European Association on Antennas and Propagation)  Measurements working group (WG5), constitutes a framework for cooperation to advance research and development of antenna measurements. An important ongoing task of this group is to sustain the Antenna Measurement Intercomparisons. The comparison of each facility measurement of the same reference antenna in a standard configuration results in important documentation and validation of laboratory expertise and competence, allowing to validate and document the achieved measurement accuracy and to obtain and maintain accreditations like ISO 17025. An additional outcome is the improvement in antenna measurement procedures and protocols in facilities and contributions to standards, which is one of the long-term objectives of the EurAAP WG5. Several participants among Europe but also USA and ASIA have joined the activity. These campaigns will also serve for a new task, recently approved within the WG5, of self-evaluation from comparison of the measurement results. An important ongoing campaign involves a X/Ku/Ka-band high gain reflector antenna MVI-SR40 fed by SH4000 Dual Ridge Horn. In this paper we report the results here for the first time. The medium gain ridge horn, MVI-SH800, equipped with an absorber plate to enhance the correlation in different facilities has been the reference antenna of another campaign. In  the preliminary results were shown. In this paper we present the final validation. The comparison is reported plotting the gain/directivity patterns and computing the equivalent noise level and the Birge ratio with respect to the reference pattern obtained taking into account the uncertainty declared by each facility.
A Review of the CW-Ambient Technique for Measuring G/T in a Planar Near-Field Antenna Range
Techniques for measuring G/T have been previously presented at AMTA; however, there are very few papers that discuss how to measure G/T in a near-field antenna range. One recent paper discussed such a method and gave a brief description within the larger context of satellite payload testing . The paper's treatment of G/T was necessarily brief and gives rise to several questions in relation to the proposed method. Other papers that treated this topic required the antenna aperture to be separable from the back-end electronics, which may not be possible in all cases [2-3]. In this paper, we discuss in great detail a slightly modified version of the G/T measurement method presented in . A signal and noise power diagram is presented that can be useful for understanding how system signal-to-noise ratio (SNR) relates to G/T, and a few common misconceptions concerning the topic of G/T are addressed. The CW-Ambient technique for computing G/T of a Unit Under Test (UUT) from measurements in a planar near-field system is described in detail, and a list of assumptions inherent to the CW-Ambient technique is presented. Finally, the validity of the CW-Ambient technique is assessed by analyzing measured data collected from a separable UUT.
Method-of-Moments Modeling of Conducting Objects within the Fast Irregular Antenna Field Transformation Algorithm
Reducing near-field measurement times is an important challenge for future antenna measurement systems. We propose to incorporate knowledge about material parameters of the antenna measurement environment within the simulation model. To do so, a method-of-moments code with surface discretization is implemented as a side constraint to the near-field far-field transformation problem performed with the fast irregular antenna field transformation algorithm. Transformation and source reconstruction results of synthetic measurement data demonstrate the effectiveness of the proposed method.
Extraction of Magneto-Dielectric Properties from Metal-Backed Free-Space Reflectivity
Intrinsic magnetic and dielectric properties of magneto-dielectric composites are typically determined at microwave frequencies with both transmission and reflection data. An iterative method, such as root-finding, is often used to extract the properties in a frequency-by-frequency basis. In some situations, materials may be manufactured on a metal substrate that prevents transmission data from being obtained. This happens when the materials are too fragile or too strongly bonded to the substrate for removal and must be characterized with the metal substrate in place. This paper compares two different free-space extraction algorithms, developed for the simultaneous extraction of complex permittivity and permeability from metal-backed reflection. One of the algorithms relies on reflection measurements of the same material with two known thicknesses. The second method takes advantage of wide bandwidth measurements to fit the reflection to analytical models (e.g. Debye). The accuracy of these methods are evaluated and the stability criteria for the techniques will be discussed, as well as the tolerance of the techniques to various measurement errors.
Waveguide-integrated Rydberg Atom-based RF Field Detector for Near-field Antenna Measurements
We demonstrate simultaneous amplitude and phase measurements of a radio-frequency (RF) field through the use of a Rydberg atom-based sensor embedded inside a waveguiding structure. This measurement uses the Rydberg atom-based sensor in a mixer configuration, which requires the presence of a local oscillator (LO) RF field. The waveguiding structure supplies the LO field. The combined waveguide and Rydberg atom system is used to measure phase and amplitude in the near-field of a horn antenna to extract the far-field pattern.
Update of IEEE Std 1720-2012 Recommended Practice for Near-Field Antenna Measurements
The IEEE Standards Association Standards Board (IEEE-SASB) approved the IEEE Std 1720™ "Recommended Practice for Near-Field Antenna Measurements" in 2012 . More than forty dedicated people from industry, academia and other institutions contributed to the creation of this new document. The main motivation for a new standard dedicated to near-field measurements was to complement the existing IEEE Std 149-1979™ "Test Procedures for Antennas" . According to the IEEE-SA policies, the existing standard IEEE Std 1720-2012™ is approaching expiration in 2022. A working group of the APS Standard Committee has been formed to review the current document. Most of the current standard is still relevant and useful for individuals designing and evaluating near-field antenna measurement facilities and other people involved in antenna measurements. However, the standard needs update and renewal in areas in which new developments and technologies have matured. This paper gives an overview of the current standards and discusses the suggested potential changes.
Proposed Changes and Updates on IEEE Std 1128 - Recommended Practice on Absorber Evaluation
The last published version of the IEEE Std 1128 is the 1998 edition. It is titled "Recommended Practice for RF Absorber Evaluation in the Range of 30 MHz to 5 GHz". Over the years, the document has been used widely for absorber evaluations in electromagnetic compatibility (EMC) applications as well as in antenna and microwave measurement applications. Besides the obvious frequency range which needs to be expanded to satisfy today's applications, several areas are in need of an update. The proposed document will change the upper frequency limit to 40 GHz (with provisions in the document to potentially extend above 40 GHz based on test methods). Measurement uncertainties were not discussed in the IEEE Std. 1128-1998. In the new edition, measurement instrumentation and test methods are expected to be updated with guidance on estimating measurement uncertainties. In the proposed document, a section on absorber evaluations for high power applications is planned, and fire properties and test methods will be included.
Accurate Calibration of Truncated Spherical Near Field Systems with Different Ground Floors using the Substitution Technique
The calibration of the antenna measurements system is a fundamental step which directly influences the accuracy of any power-related quantity of the device under test. In some types of systems, the calibration can be more challenging than in others, and the selection of a proper calibration method is critical. In this paper, the calibration of the truncated spherical near-field ranges typically used for automotive tests is investigated, considering both absorbing and conductive floors. The analyses are carried out in a 12:1 scaled multi-probe system, allowing access to the "true", full-sphere calibration which is used as reference. It will be demonstrated that the substitution (or transfer) method is an excellent calibration technique for these types of systems, if applied considering the efficiency of the reference antenna.
Small Antenna Testing in a Compact Antenna Test Range
The Compact Antenna Test Range (CATR) was initially conceived as an efficient way of testing electrically large antennas at very much reduced, fixed, range lengths than would otherwise be the case. However, when testing lower gain, physically smaller antennas, the measurements can become susceptible to inhomogeneities within the CATR QZ including phenomena associated with edge diffraction effects, feed spill-over, chamber multipath etc. Whilst it has been demonstrated experimentally that many of these measurement artefacts may be effectively mitigated using standard and modern more sophisticated post-processing techniques. This paper supports those findings through simulation of the direct and indirect far field ranges and by careful examination of the data processing chain. Results are presented, the relative success of the various techniques examined and the utility of this is set, and expounded, in the context of modern, i.e. 5G, communications systems.
Comparative Investigation of Spatial Filtering Techniques for Ground Plane Removal in PEC-Based Automotive Measurements
Radiating performances of vehicle-installed antennas are typically performed in large spherical near-field systems able to accommodate the entire car. Due to the size and weight of the vehicle to be tested, such spherical systems are often nearly hemispherical, and the floor is conductive or covered with absorbers. The main advantage of the first is the ease of the accommodation of the vehicle under test. Conversely, the latter is more time consuming in the setup of the measurements because the absorbers need to be moved in order to be placed around the vehicle. On the other hand, the absorber-covered floors emulate a free-space environment which is a key enabling factor in performing accurate measurements at low frequencies (down to 70 MHz). Moreover, the availability of the free-space response allows easy emulation of the cars' behaviors over realistic automotive environments (e.g. roads, urban areas etc.) with commercially available tools. Such emulations are instead much more challenging when a conductive floor is considered. Furthermore, the raw measurements over conductive floors are a good approximation of realistic grounds (such as asphalts) only in a limited number of situations. For these reasons, when PEC-based automotive measurements are performed, it is often required to retrieve the free-space response, or equivalently, to remove the effect of the conductive ground. In this paper two spatial-filtering techniques (the spherical modal filtering and the equivalent currents) will be experimentally analyzed and compared to verify their effectiveness in removing the effect of the conductive floor. For this purpose, a scaled automotive PEC-based measurement setup has been implemented considering a small spherical multi-probe system and a 1:12 scaled car model. The two techniques will be analyzed considering two different heights of the scaled car model with respect to the conductive floor.
Validation of Electromagnetic Compatibility Chambers with a Spherical Wave Expansion Approach
Electromagnetic Compatibility (EMC) radiated emissions measurements above 1 GHz are performed in a nominal free space environment as required by international standards, typically in an anechoic chamber. In an EMC chamber, the test zone consists of a circular region defined by a turn table, where an equipment under test is rotated and measured. The test zone is commonly referred to as quiet zone (QZ). Due to the non-ideal nature of absorbers, multiple reflections in the chamber affect the quality of the QZ. The constructive and destructive interferences from the reflections form standing waves in the QZ. The maximum value of the standing wave is used as the figure of merit for validation of testing facilities. Site Voltage Standing Wave Ratio (sVSWR) as specified in CISPR 16-1-4 is broadly used for the validation of test sites above 1 GHz. This method requires the measurement of six positions along a linear 40 cm transmission path at various locations in the QZ, with a frequency step of no greater than 50 MHz using an omnidirectional-like antenna (e.g. a dipole). Concerns have been raised that this method delivers an overly optimistic result due to both spatial and frequency domain undersampling. In this work, an alternative method to sVSWR for the validation of EMC chambers based on Spherical Mode Coefficients (SMC) is proposed. Two 90 •-rotated measurement cuts of an omnidirectional-like antenna are acquired around the periphery of the circular QZ. The measured situation and cut is replicated by applying translation and rotation of spherical waves to the known SMCs of the used omnidirectional-like antenna and transforming using the spherical wave expansion. The generated and measured cut are compared and the characteristics of the chamber are extracted. The major advantage of this method is the relatively high measurement speed and reliability.
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