AMTA Paper Archive
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Changes In The DO-213 Standard For Commercial Nose-Radome Testing
“RTCA DO-213 Minimal Operation Performance Standards For Nose-Mounted Radomes” is a document frequently referenced in nose-radome testing requirements for commercial aircraft. This document was produced and is maintained by the Radio Technical Commission for Aeronautics (RTCA). The specifications of weather-radar systems have recently changed within RTCA’s DO-220A, and as a result DO-213 was updated to DO-213A in March, 2016, to ensure that radome requirements are consistent with those of the weather radar. In addition to the new requirements for radome evaluation, several existing requirements were clarified. These clarifications addressed such things as suitability of near-field measurements, proper procedures and processing, and appropriate measurement geometries. RTCA coordinated the document revision, with the bulk of the technical inputs coming from a broad-based working group. This working group had representatives from radar, aircraft, and radome manufacturers, government agencies, and providers and users of radome-testing systems. When requirements were added or when common practice conflicted with existing requirements, there was considerable effort and analysis employed to ensure that each change or clarification was truly required. Nevertheless, DO-213A has some significant impacts to many existing radome-testing facilities. This paper discusses the significant changes in DO-213A and their implications for radome test facilities, concentrating on after-repair radome electrical testing.
Multiple Target, Dynamic RF Scene Generator
The evaluation of RF Sensors often requires a test capability where various RF scenes are presented to the Unit Under Test (UUT). These scenes may need to be dynamic, represent multiple targets and/or decoys, emulate dynamic motion, and simulate real world RF environmental conditions. An RF Scene Generator can be employed to perform these functions and is the focus of this paper. The total test system is usually called Hardware in the Loop (HITL) involving the sensor mounted on a Flight Motion Simulator (FMS), the RF Scene Generator presenting the RF Scene, and a Simulation Computer that dynamically controls everything in real time. This paper describes the system concept for an RF Scene Generator that simultaneously represents 4 targets, in highly dynamic motion, with no occlusion, over a wide range of power, frequency, and Field of View (FOV). It presents the test results from a prototype that was built and tested over a limited FOV, while being scalable to the total FOV and full system capability. The RF Scene Generator employs a wall populated with an array of emitters that enables virtually unlimited velocity and acceleration of targets and employs beam steering to provide high angular resolution and accuracy of the presented target positions across the FOV. Key words: RF Target Simulator, RF Scene Generator, Multiple Targets, Beam Steering Wall of Emitters, Steering Array Calibration, Plane-Wave Generator, Radar Environment Simulator.
Automotive Antenna Evaluation
The automotive industry is changing rapidly through the evolution of on board and embedded components and systems. Many of these systems rely on the over the air performance of a communication link and the evaluation of these links is a key requirement in understanding both the real world performance, and associated operating limits of a particular system. The operating frequency range of the installed communication systems now extends from the traditional AM bands from 540KHz to almost 6GHz for WiFi. There are several different antenna design options available to cover this range and in many cases, the performance of an antenna when installed on a vehicle differs from a measurement of the same antenna in isolation. There is also a growing use of high frequency RADAR systems operating at frequencies approaching 80GHz that also need to be included in the performance analysis. The behavior of the individual components using conducted methods for example, is an important step but the direct measurement of antenna pattern and data throughput under ideal steady state and also varying spatial and operating conditions is likely to be the most robust method of channel evaluation. There is a steady march toward vehicle autonomy that is pushing the development of increasingly complex and sophisticated sensors, receivers, transmitters and firmware, all installed on an already well populated platform. The interoperability and EMC performance of these embedded systems is an extension to the need for a fundamental understanding of performance. This paper will present some of the available measurement and evaluation options that could be used as part of an integrated test environment which takes advantages of a number of established techniques.
Utilization Of An Octocopter As A Two-Way Field Probe For Electro-Magnetic Field Measurements At An Outdoor Radar Cross Section Range
RCS and Antenna measurement accuracy critically depends on the quality of the incident field. Both compact and far field ranges can suffer from a variety of contaminating factors including phenomena such as atmospheric perturbation, clutter, multi-path, as well as Radio Frequency Interference (RFI). Each of these can play a role in distorting the incident field from the ideal plane wave necessary for an accurate measurement. Methods exist to mitigate or at least estimate the measurement uncertainty caused by these effects. However, many of these methods rely on knowledge of the incident field amplitude and phase over the test region. Traditionally the incident field quality is measured directly using an electromagnetic probe antenna which is scanned through the test region. Alternately, a scattering object such as a sphere or corner reflector is used and the scattered field measured as the object is moved through the field. In both cases the probe/scatterer must be mounted on a structure to move and report the position in the field. This support structure itself acts as a moving clutter source that perturbs the incident field being measured. Researchers at the Air Force Institute of Technology (AFIT) have recently investigated a concept that aims to eliminate this clutter source entirely. The idea is to leverage the advances in drone technology to create a free flying field probe that doesn’t require any support structure. We explore this concept in our paper, detailing the design, hardware, and software developments required to perform a concept demonstration measurement in AFIT’s RCS measurement facility. Measured data from several characterization tests will be presented to validate the method. The analysis will include an estimate of the applicability of the technique to a large outdoor RCS measurement facility.
Highly accurate fully-polarimetric radar cross section facility for mono- and bistatic measurements at W-band frequencies
New requirements in the field of autonomous driving and large bandwidth telecommunication are currently driving the research in millimeter-wave technologies, which resulted in many novel applications such as automotive radar sensing, vital signs monitoring and security scanners. Experimental data on scattering phenomena is however only scarcely available in this frequency domain. In this work, a new mono- and bistatic radar cross section (RCS) measurement facility is detailed, addressing in particular angular dependent reflection and transmission characterization of special RF material, e.g. radome or absorbing material and complex functional material (frequency selective surfaces, metamaterials), RCS measurements for the system design of novel radar devices and functions or for the benchmark of novel computational electromagnetics methods. This versatile measurement system is fully polarimetric and operates at W-band frequencies (75 to 110 GHz) in an anechoic chamber. Moreover, the mechanical assembly is capable of 360° target rotation and a large variation of the bistatic angle (25° to 335°). The system uses two identical horn lens antennas with an opening angle of 3° placed at a distance of 1 m from the target. The static transceiver is fed through an orthomode transducer (OMT) combining horizontal and vertical polarized waves from standard VNA frequency extenders. A compact and lightweight receiving unit rotating around the target was built from an equal OMT and a pair of frequency down-converters connected to low noise amplifiers increasing the dynamic range. The cross-polarization isolation of the OMTs is better than 23 dB and the signal to noise ratio in the anechoic chamber is 60 dB. In this paper, the facility including the mm-wave system is deeply studied along with exemplary measurements such as the permittivity determination of a thin polyester film through Brewster angle determination. A polarimetric calibration is adapted, relying on canonical targets complemented by a novel highly cross-polarizing wire mesh fabricated in screen printing with highly conductive inks. Using a double slit experiment, the accuracy of the mechanical positioning system was determined to be better than 0.1°. The presented RCS measurements are in good agreement with analytical and numerical simulation.
Measurements of Incident Radio Frequency Power levels from the L3 Technologies ProVision Body Scanner for the National Academy of Science
The Transportation Security Administration is tasked with the job of performing safety screening of millions of air travel passengers annually in a safe and efficient manner. One of the most widely deployed detection system is the L3 Technologies “Provision” body scanner, which utilizes millimeter wave radio frequencies (RF). Have you ever wondered what type and levels of RF energy are used to execute this routine security screening test? Recently, the Department of Homeland Security, Transportation Security Administration, tasked the National Academies of Science (NAS) to execute an updated safety analysis of the L3-Comm manufactured TSA ProVision Body Scanner units deployed in airports world-wide. In the process of executing their charter, the NAS realized there was very little peer-reviewed published data on calibrated field incident power within the ProVision scanner itself. While L3-Comm has their own factory acceptance program, the NAS wanted independent measurements executed on L3-Comm machines at four randomly selected airports. The NAS therefore contracted with the team of BerrieHill Research and Applied Research Associates to design a specialized field probe that could measure the RF emanations of the ProVision Units. This very challenging measurement environment required design ingenuity to fulfill the contract needs, since our team was not allowed to physically connect to any part of the ProVision machine. We had to place a field measurement device inside the unit where the passenger stands, and record all data over the air only. This paper will completely describe the BRC/ARA ProVision Scanner field probe measurement system, and present calibrated RF field measurements along with an uncertainty analysis of typical results.
Common Microwave Absorbers Evaluations in W-band (75-100 GHz)
Understanding absorber performance in the W band (75-100 GHz) has become increasingly important, especially with the popular use of W band radars for automotive range detections. Commercial absorber performance data is typically available only to 40 GHz. Measurements performed in the W band in anechoic chambers are often under the assumptions that high frequency absorber data can be extrapolated from the data below 40 GHz. In this paper, we provide a survey of common microwave absorbers in the W band. It shows that the extrapolated data from the lower frequencies are not accurate. Absorber analysis models for low frequencies such using homogenization concept are no longer valid. This is because, for the millimeter wave, microstructures of the foam substrate become important, and the dimensions of the pyramids are much greater than the wavelengths. We examine performance variations due to parameters, such as carbon loading, shape, and thickness of the absorbers. We will also show how paint on the absorber surface might affect the absorber reflectivity, and if the common practice of black-tipping (leaving the tip of the absorbers unpainted) is an effective technique to alleviate paint effects.
Analysis of Near-Field RCS Behavior for mm-Wave Automotive Radar Testing Procedures
Millimeter wave vehicular radar operating in the 77 GHz band for automatic emergency breaking (AEB) applications in detecting vehicles, pedestrians, and bicyclists, test data has shown that the radar cross section (RCS) of a target decreases significantly with distance at short range distances typically measured by automotive radar systems, where the reliable detection is most critical. Some attribute this reduction to a reducing illumination spot size from the antenna beam pattern. Another theory points to the spherical phase front due to measurement in the Fresnel region of the target, when the distance for the far-field zone is not met. The illumination of the target depends on the antenna patterns of the radar, whereas the Fresnel region effects depend on the target geometry and size. Due to fluctuations in measured data for RCS as a function of range in the near-field, upper and lower bounds for the target RCS versus range have been determined empirically as a method for describing the expected RCS of target. So far, the range-dependent RCS bounds used in AEB test protocols have been determined empirically. The study discussed in this paper aims to study the underlying physics that produces range-dependent RCS in near field and provide analytical model of such behavior. The resultant analytical model can then be used to objectively determine the RCS upper and lower bounds according to the radar system parameters such as antenna patterns and height. A comparison of the analytically predicted model and empirical near-field RCS as a function of range data will be presented for pedestrian, bicyclist, and vehicle targets.
Advances in SAR-ISAR Blending
Radar signature measurements of targets with or without camouflage in different backgrounds using airborne SAR is complicated and expensive. Measurements at many orientations as well as illumination angles have to be performed for each target for completeness. A more efficient solution is to use ground based ISAR measurements of the desired targets and then blend these images into measured SAR scenes. We are developing a SAR-ISAR blending method where the target and background are modelled by point scatterer representations. This can be formulated as an inverse problem described by the equation Ax = y, (1) where A is a forward operator describing the model, x is the image and y is the measured RCS data. The point scatterer representations for the target and the SAR background are determined by solving (1). The main contribution of this paper is that we use a combination of L1 and L2 regularization methods to solve the inverse problem. The target measured by ISAR is sparse in the image domain and (1) is therefore solved efficiently using a L1 regularization method. However, the SAR background is not sparse in the image domain and (1) is therefore solved using a L2 regularization method. We use the following procedure: Define the operators A and At, where At is the conjugate transpose of A. The same operators are used for both the target and the background. Solve (1) using L1 regularization for the target measured using ISAR. Edit the target point scatterers so that only target related scatterers are included. Solve (1) using L2 regularization for the SAR background. Edit the background point scatterers by removing the shadowed region, alternatively attenuate if there is a camouflage net. Combine the edited point scatterers for the target and background and calculate the RCS for the combination. Add estimated system noise. Create a blended SAR-image. The method is demonstrated with ISAR measurements of a full-scale target, with and without camouflage, signature extraction and blending into a SAR background. We find that the method provides an efficient way of evaluating measured target signatures in measured backgrounds.
MIMO Radar Scheme for mm-wave Portable Scanners
Millimeter-wave scanners are a powerful tool in multiple fields such as security and non-destructive evaluation. Recent advances in design and manufacturing at this frequency band, also boosted by automotive and communication industries, are resulting in the first generation of portable scanners based on different imaging paradigms. In the recent work of the authors, the capabilities of this kind of portable scanners have been considered. In particular the evaluation of different methods to combine multiple (potentially overlapped) acquisitions from arbitrary points has been considered . The proposed imaging method for each local acquisition was based on Synthetic Aperture Radar (SAR) techniques. Nevertheless, this kind of imaging method usually requires a dense sampling and, consequently, it can result in a large number of transceivers increasing the cost and weight of the device. For this reason, the use of multistatic arquitectures, also known as multiple-input multiple-output (MIMO), similar to the ones proposed in  is considered in this communication. This approach enables to reduce the total number of elements by almost an order of magnitude by placing transmitters and receivers at different positions along the scanner aperture in contrast to conventional SAR that considers a dense aperture of equally spaced independent transceivers. As demonstrated in , if the position of transmitters and receivers is properly designed, the obtained results are equivalent to the ones provided by a conventional dense sampling.  Jaime Laviada, Yuri Álvarez, Ana Arboleya, Fernando Las-Heras, Borja González Valdés, “Multiview Techniques for mm-Wave Imaging” Antennas and Propagation Simposium AP-S 2017, San Diego, USA, 2017.  S. Ahmed, A. Schiessl, and L. Schmidt, “A novel fully electronic active real-time imager based on a planar multistatic sparse array,” IEEE Transactions on Microwave Theory and Techniques, vol. 59, no. 12, pp. 3567–3576, 2011.
On the Disadvantages of Tilting the Receive End-Wall of a Compact Range for RCS Measurements
Abstract— Tilting the receive end wall of a compact range anechoic chamber to improve Radar Cross-Section (RCS) measurements has been a tool of the trade used since the earliest days of anechoic chambers. A preliminary analysis using geometrical optics (GO) validates this technique. The GO approach however ignores the backscattering modes from the reflected waves from a field of absorber. In this paper, a series of numerical experiments are performed comparing a straight wall and a tilted wall to show the effects on both the quiet zone and the energy reflected back towards the source antenna. Two Absorber covered walls are simulated. Both walls are illuminated with a standard gain horn (SGH). The effects of a wall tilted back 20° are computed. The simulations are done for 72-inch long absorber for the frequency range covering from 500 MHz to 1 GHz. The ripple on a 10 ft (3.05 m) quiet zone (QZ) is measured for the vertical wall and the tilted wall. In addition to the QZ analysis a time-domain analysis is performed. The reflected pulse at the excitation antenna is compared for the two back wall configurations Results show that tilting the wall improves measurements at some frequencies but causes a higher return at other frequencies; indicating this method does not provide a broadband advantage. Keywords: Anechoic Chamber Design, Radar Cross Section Measurements, Geometrical Optics
Automating RCS Measurements for High Speed Production Line In-Process Verification
In June of this year, DSC completed the installation of a turnkey RCS measurement system that is used for in-process verification (IPV) and final component validation using standard near field QC techniques in an echoic chamber. The delivered system included a radar, antennas, shroud, ogive pylon, foam column, elevators for each – column and pylon, automated pit covers, test bodies, target transport carts, and calibration targets. The system automatically loads test objects on the correct target support system, requiring no action by the operator to connect a target onto the azimuth over elevation “tophat” positioner – it is all automatic. The user interface is designed to be operated by production line workers, greatly reducing the need for experienced RCS test engineers. Simple pass/fail indicators are shown to the test technicians, while a full detailed data set is stored for engineering review and analysis. A wall display guides users through a test sequence for target handling and starting the radar. Radar data collection of all azimuth and elevation angles and target motion are initiated from a single button push. This is followed by all data processing necessary to conduct the ATP on the parts providing a pass/fail report on dozens of parameters. The application of production line quality automation to RCS measurements improves the repeatability of the measurements, greatly reduces both measurement time as well as overhead time, and allows systems operators to become more interchangeable. This highly successful project, which was completed on-time and on-budget, will be discussed. This discussion will include radar performance, antenna and shroud design, target handling, data processing and analysis software, and the control system that automates all the functions that are required for RCS measurements.
A Novel and Innovative Near Field System for Testing Radomes of Commercial Aircrafts
The maintenance of aircraft radomes is of particular importance for the commercial aviation industry due to the necessity to ensure the correct functioning of the radar antenna, housed within such protective enclosures. Given that the radar component provides weather assessment, as well as guidance and navigation functions (turbulence avoidance, efficiency of route planning in case of storms, etc.), it is imperative that every repaired radome be tested with accuracy and reliability to ensure that the enclosed weather radar continues to operate in accordance with the after-repair test requirements of the RTCA/DO-213. Recently, this quality standard was updated and published under the name RTCA/DO-213A, establishing more stringent measurement requirements and incorporating the possibility of measuring radomes using Near-Field systems. Consequently, a compliant multi-probe Near- Field system concept – AeroLab – has been specifically designed to measure commercial aircraft nose-radomes, in order to meet the new standard requirements. AeroLab performs Near-Field measurements. Near-Field to Far-Field transformations are then applied to the results. Such a Near-Field system allows the test range to be more compact than traditional Far-field test ranges, and thus be independent from the updated Far-Field distance which has progressed from D²/2l to 2D²/l in the new standard RTCA/DO-213A. AeroLab enables the evaluation of the transmission efficiency and beamwidth. It also allows for accurate evaluations of the side-lobe levels by providing improved visualization of principal cut views selected from 3D patterns. Moreover, depending upon the weather radar system inside the radome under test, 2 distinct scan sequences must now be taken into account: “elevation over azimuth” and “azimuth over elevation”. AeroLab emulates both of these motion sequences through a monolithic gimbal. Furthermore, thanks to its multi-probe array, such measurements are performed in a fraction of the time spent in current mono-probe test facilities (less than 4 hours, i.e. 1/3 less time than single probe scanners). Keywords: RTCA/DO-213A, radome measurement system, after-repair tests, multi-probe measurement system, Near-Field system.
A Low-sidelobe Ka-Band Array Antenna Design
In high-gain array applications such as satellite communications and radars, low sidelobe level is a key performance requirement. Applying a magnitude tapering profile across the aperture is a common way to suppress the sidelobes. Other realizes sidelobe level control method via phase variation across the aperture. Common ways for implementing aperture magnitude taper include applying lumped resistors or introducing proper series feeding, etc. This paper discusses about 33-35 GHz fixed-beam, low-sidelobe array antenna design. In order to minimize the number of the feed network, the 11x32 Ka-band array is composed of 32 sequentially-fed 11-element subarrays. Also, to maximize the antenna efficiency, the Chebyshev tapering selected for achieving -35 dB sidelobe levels was accomplished using a combination of un-even power divider, mismatched feed lines, and different feedline lengths. This approach allows it to achieve magnitude taper control from -0.5 dB to -26 dB from 33 to 35 GHz without using resistors. The unequal power divider arrangement reduces mismatch loss my 1.3dB compared to conventional 2-way even dividers in the corporate feeding network. The paper also studies the impact of the amplitude and phase uncertainty among array elements on the degradation of the sidelobe performance. An 11x32 planar patch array prototype is designed, fabricated and measured. The array obtains a realized gain of 19 dBi with Azimuth 3dB beamwidth of 1.0 degree and -23 dB sidelobe.
The 7 Common Habits of Highly Effective RF Target Simulators
The evaluation of RF Sensors often requires a test capability where various RF targets are presented to the Unit Under Test (UUT). These targets may need to be dynamic in time, represent multiple targets and/or decoys, emulate dynamic motion, and simulate real world RF environmental conditions. An RF Target Simulator can be employed to perform these functions and is the focus of this paper. The total test system is usually called Hardware in the Loop (HITL) involving the sensor mounted on a Flight Motion Simulator (FMS), the RF Target Simulator presenting the RF Scene, and a Simulation Computer that dynamically controls everything in real time. The realization of a highly effective target simulator, one that truly meets the user’s needs at an affordable cost, is the result of understanding the complex interrelationship of requirements, architecture and constraints. In this presentation, those relationships are examined in seven areas of discussion, employing examples of realized systems; Determining the necessary test zone volume Determining the necessary quality of RF target signal Sizing the field of view, range and facilities Creating each target’s RF signal Creating RF target motion Integration and real-time operation within the range Locating and minimizing the effects of error sources
An Experimental and Computational Investigation of High-Accuracy Calibration Techniques for Gain Reference Antennas
Gain is a principal property of antennas; it is essential in establishing the link budget for communication and sensing systems through its presence in Friis’ transmission formula and the radar range equation. The experimental determination of antenna gain is most often based on a gain-transfer technique involving a reference antenna for which the gain has been calibrated to high accuracy; this is typically a pyramidal horn antenna . The required accuracy of antenna gain obviously depend on the application; in some cases it can very high, ±0.1 dB or less, and this implies an even higher accuracy, of the order of ±0.01dB, for the gain reference antenna. This work investigates the accuracy to which a gain reference antenna can be calibrated; the investigation is based on experimental spherical near-field antenna measurements  and computational integral equation / method of moments simulations . While calibration of gain reference antennas has been studied in many previous works, even works from early 1950s -, this work is novel in systematically supporting measurements with full-wave simulations. Such simulations facilitate the study of e.g. the effect of multiple reflections between antennas at short distances. We study two absolute calibration techniques for the gain of pyramidal horn antennas. The first technique determines gain as the product of directivity and radiation efficiency; this technique has been referred to as the pattern integration technique  (which is not an entirely adequate designation since gain cannot be determined from the radiation pattern). The second technique determines the gain from Friis’ transmission formula  for two identical antennas; this technique is generally referred to as the two-antenna technique . These two calibration techniques involve very different steps and contain very different sources of error; for both techniques our investigation involves measurements as well as simulations. For the pattern integration technique we compare experimental and computational results for the directivity and demonstrate agreement within one-hundredth of a dB. The radiation efficiency is calculated by different techniques based on the surface impedance boundary condition for the metallic walls of the pyramidal horn. This technique is not influenced by proximity effects or by impedance mismatch between the measurement system and the gain reference antenna. For the two-antenna techniques we compare experimental and computational results for the gain and we compare the calculated distance-dependence with that of the extrapolation technique . It is demonstrated how the use of the phase center distance in Friis’ transmission formula notably decreases the necessary separation between the antennas for a required accuracy, but that multiple reflections may then become a limiting factor. This technique is highly influenced by the impedance mismatch that must be accurately accounted for. We compare the gain values resulting from the pattern integration technique and the two-antenna technique, including their very different uncertainty estimates, for a C-band standard gain horn. The work is related to an on-going ESA project at the DTU-ESA Spherical Near-Field Antenna Test Facility for the on-ground calibration of the scatterometer antennas of the EUMETSAT MetOp Second Generation B-series satellites. IEEE Standard – Test Procedures for Antennas, Std 149-1979, IEEE & John Wiley & Sons, 1979. J.E. Hansen, “Spherical Near-Field Antenna Measurements”, Peter Perigrinus Ltd., London 1987. www.wipl-d.com W.C. Jakes, “Gain of Electromagnetic Horns”, Proceedings of the IRE, pp. 160-162, February 1951. E.H. Braun, “Gain of Electromagnetic Horns”, Proceedings of the IRE, pp. 109-115, January 1953. W.T. Slayton, “Design and Calibration of Microwave Antenna Gain Standards”, Naval Research Laboratory, Washington D.C., November 1954. A. Ludwig, J. Hardy, and R. Norman, “Gain Calibration of a Horn Antenna Using Pattern Integration”, Technical Report 32-1572, Jet Propulsion Laboratory, California Institute of Technology, October 1972. H.T. Friis, “A Note on a Simple Transmission Formula”, Proceedings of the I.R.E. and Waves and Electrons, pp. 254-256, May 1946. A.C. Newell, R.C. Baird, P.F. Wacker, “Accurate Measurement of Antenna Gain and Polarization at Reduced Distances by an Extrapolation Technique”, IEEE Transactions on Antenna and Propagation, vol. 21, no. 4, pp. 418-431, July 1973.
Using Standard Wideband Antennas as Probe in Spherical Near Field Measurements with Full Probe Correction: Experimental Validation
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.
A Compressed Sampling for Spherical Near-Field Measurements
Spherical near-field measurements are regarded as the most accurate technique for the characterization of an Antenna Under Tests (AUT) radiation. The AUT's far-field radiation characteristics can be calculated from the Spherical Mode Coefficients (SMC), or spherical wave coefficients, determined from near-field data. The disadvantage of this technique is that, for the calculation of the SMC, a whole sphere containing the AUT must be Nyquist-sampled, thus directly implying a longer measurement time when only a few cuts are of interest. Due to antennas being spatially band-limited, they can be described with a finite number of SMC. Besides, the vector containing the SMC can be proved sparse under certain circumstances, e.g., if the AUT's radiation pattern presents information redundancy, such as an electrical symmetry with respect to coordinate system of the measurement. In this paper, a novel sampling strategy is proposed and is combined with compressed-sensing techniques, such as basis pursuit solvers, to retrieve the sparse SMC. The retrieved sparse SMC are then used to obtain the AUT's far-field radiation. The resulting far-field pattern is compared for both simulated and measured data. The reduced number of points needed for the presented sampling scheme is compared with classical equiangular sampling, together with the estimated acquisition time. The proposed sampling scheme improves the acquisition time with a reasonable error.
Aircraft Radome Characterization via Multiphysics Simulation
Altair Engineering Inc. Troy, MI USA-https://www.altairhyperworks.com Figure 1. The electromagnetic, aerodynamic, and structural performance of a nose cone radome can be characterized by computational simulation, allowing for early design concept validation and reducing the dependence on physical testing. Abstract-Radomes protect antennas from structural damage due to wind, precipitation, and bird strikes. In aerospace applications, radomes often double as a nose cone and thus have a significant impact on the aerodynamics of the aircraft. While radomes should be designed not to affect the performance of the underlying antennas, they also must satisfy structural and aerodynamic requirements. In this paper, we demonstrate a multiphysics approach to analysis of airborne radomes not only for electromagnetic (EM) performance, but also for structural, aerodynamic, and bird strike performances, as depicted in figure 1. We consider a radome constructed using composite fiberglass plies and a foam core, and coated with an anti-static coating, paint, and primer. A slotted waveguide array is designed at X-band to represent a weather radar antenna. The transmission loss of the radome walls is analyzed using a planar Green's function approach. An asymptotic technique, Ray-Launching Geometric Optics (RL-GO), is used to accurately simulate the nose cone radome and compute transmission loss, boresight error, and sidelobe performance. In addition to EM analysis, Computational Fluid Dynamics (CFD) analysis is used to predict pressures resulting from high air speeds, which are then mapped to an implicit structural solution to assess structural integrity using the Finite Element Method (FEM). We also demonstrate damage prediction due to a "bird strike" impact using an explicit structural FEM solver. The multiphysics simulation techniques demonstrated in this paper will allow for early design validation and reduce the number of measurement iterations required before a radome is certified for installation.
Geometric Effects on Radar Echoes from a Corner Reflector
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.
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