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A new Compact Antenna Test Range (CATR) has been built, as a turnkey facility, with a cubic quiet zone (QZ) of 4.8m x 4.8m x 4.8m in the frequency range 0.9-18 GHz. The CATR has been installed in a new building with an isolated and stable foundation. The dimensions of a traditional CATR for such QZ size becomes impractical and requires a very large chamber. A new, diagonally fed, short focal length reflector has been developed to minimize the chamber size to fit the dimensions of 22 m x 14.5 m x 14.5 m.
This paper discusses about the design, fabrication and testing of a compact P-band (370 MHz) dual circular polarization (CP) patch antenna. The antenna is intended for reflectometry applications by measuring both direct and ground reflected 370 MHz signals transmitted from a satellite or airborne source. This design adopts quadrature-phase hybrid feeding network for achieving excellent polarization purity and supporting simultaneously LHCP and RHCP measurements. Another novel design aspect is placing the feeding network on top of the patch so that the antenna can be mounted directly on a ground plane. Therefore, the resonant modes inside the patch is excited from the top instead of from ground plane as in conventional designs. High dielectric material (ECCOSTOCK®HiK) with a dielectric constant of 9 and loss tangent of 0.002 was used as the substrate to reduce the antenna size. The final antenna has a dimension of 5.9" x 5.9" x 1.3" (excluding ground plane) and weight of 1620 gram. The measured performance on a 1-foot diameter circular ground plane showed 4.5 dBic gain and 23 dB co-polarization to cross-polarization isolation at the center frequency for both LHCP and RHCP. The 1-dB gain bandwidth is approximately 3.7%.
Accurate in situ antenna manifolds are desired for performance evaluation of radio frequency systems, including communication, navigation, and radar among others. In situ antenna measurements are the most accurate way to obtain antenna manifolds on the platform of interest, but are often impractical or impossible to obtain. Instead, combinations of simulations and measurements are used to estimate antenna manifolds on platforms. First, the gain and phase of the target antenna are measured on a simple ground plane, over the frequencies and field of view of interest. The measurements are then imported into computational electromagnetics codes to simulate platform scattering from the platform of interest. However, the representation of the measured data is not unique, which leads to inaccuracies and/or a large run time in computational electromagnetics codes. This paper presents a new method to represent measured antenna data in electromagnetics codes through aperture current distributions of simulated cross-slots and monopoles. A weighted sum of far-fields from the simulated equivalent elements approximates the measured antenna far-fields, with weights determined by the minimization of the L2-norm difference between measured far-fields and equivalent element far-fields. However, the least square solution may place large, unrealizable currents on the aperture. A constraint is introduced to limit the amount of current on the aperture, by minimizing the length of the solution vector. In this paper, the details of the suggested method will be presented. We will also illustrate the accuracy of the method through example simulations, where good agreement is achieved between truth data and equivalent antennas on complex platforms.
The effect of different decomposition techniques on the imaging and detection accuracy for polarimet-ric surface penetrating data is studied. We derive the general expressions for coherent polarimetric decomposition using the Stokes parameters and model based polarimetric decomposition using the Yamaguchi technique. These techniques are applied to multi-frequency (0.4-4.8GHz) full polarimetric near-field radar measurements of scattering from surface laid calibration objects and shallow buried landmine types and show in detail how the decomposition results provide effective surface and sub-surface clutter reduction and guide the interpretation of scattering from subsurface objects. Data processing methods assume cross-polar symmetry and a novel bistatic calibration procedure was developed to enforce this condition. The Yamaguchi polarimetric decomposition provides significant clutter reduction and image contrast with some loss in signal power; while Stokes parameters also provide imagery localising targets, complementary information on the scattering mechanism is also obtained. Finally a third novel polarimetric filter was formulated based on differential interferometric polarimetric decomposition. The three combined techniques contribute to a significant improvement of subsurface radar performance and detection image contrast.
This paper presents a time domain antenna measurement technique by using a low cost software defined radio receiver. The technique aims to resolve measurement challenges derived from antennas where the reference signal is not accessible. The phase reconstruction implemented in this work is based on calculating the Fast Fourier Transform of the time domain signal to estimate the power spectrum and the relative phase between measurement points. In order to do that a reference antenna is used to retrieve the phase, providing a full characterization in amplitude and phase of the electric field and allowing source reconstruction. The results demonstrate the potential of this technique for new antenna measurement systems and reveal some of the limitations of the technique to be optimized, like the undesired reflections due to the interactions between the probe and the reference antenna.
The need for thermal stability in a test chamber is a well-established requirement to maintain the accuracy and repeatability sought for high frequency planar near-field (PNF) scanner measurements. When whole chamber thermal control is impractical or unreliable, there are few established methods for maintaining necessary precision over a wide temperature range. Often the antenna under test (AUT) itself will require a closed-loop thermal control system for maintaining stable performance due to combined effects from transmission heat dissipation and the environment. In this paper, we propose a new approach for near-field system design that leverages this AUT stability, while relaxing the requirement of strict whole chamber thermal control. Fixed reference monuments strategically placed around the AUT aperture perimeter, when measured periodically with a sensing probe on the scanner, allow for the modeling and correction of the scanner positioning errors. This process takes advantage of the assumed stability of the reference monuments and attributes all apparent monument position changes to distortions in the scanner structure. When this monument measurement process is coupled with a scanner structure that can tolerate wide thermal variations, using expansion joints and kinematic connections, a robust structural error correction model can be generated using a bilinear mapping function. Application of such a structure correction technique can achieve probe positioning performance similar to scanners that require tightly controlled environments. Preliminary results as well as a discussion on potential design variations are presented.
This communication provides an experimental assessment of an accurate near-field-far-field (NF-FF) transformation with spherical scan, properly developed to take into account a mounting in offset configuration of a quasi-planar antenna under test (AUT). Such a technique relies on the nonredundant sampling representation of electromagnetic fields and, unlike the classical NF-FF transformation, it allows the reconstruction of the far field radiated by an AUT from a minimum number of NF data, which remains practically the same both when the AUT is mounted in onset and offset configuration, since this number is related only to the surface modeling the AUT. Such a surface has been here chosen coincident with that formed by two circular bowls with the same aperture and eventually different bending radii. Experimental results assessing the validity of such a technique are reported.
An ambitious resurfacing campaign was launched in late 2017 to correct for large reflector surface distortions present at the NASA LaRC Experiment Test Range (ETR) in support of performing Europa Clipper flight High Gain Antenna (HGA) measurements at X-and Ka-band frequencies. The effort was successful as the worst case peak-to-peak amplitude ripple was reduced from 4.0-dB to 1.5-dB across the 4.1-meter quiet zone.
In this paper, a novel algorithm for designing 3D-printed shaped inhomogeneous dielectric lens antennas is provided. The synthesis approach is based on a novel combination of Geometrical Optics (GO) and the Particle Swarm Optimization (PSO) method. The GO method can trace rays through inhomogeneous media and calculate the amplitude, phase, and polarization of the electric field. The algorithm is used to design an inhomogeneous lens antenna to produce an electronically scanned revolving conical beam to replace a mechanically scanned parabolic reflector antenna for spaceborne weather radar satellite antenna applications. Two breadboard model on-axis fed lens designs are presented and measured results given to validate the approach. A representative optimum off-axis design is presented which produces the revolving conically scanned beam. Imposition of a Body-of-Revolution restriction allows the optimization to be performed at a single offset feed location. The complex inhomogeneous engineered materials that results from optimization are printed using new 3D printers.
This contribution details a procedure to collect and process necessary data to describe the antenna patterns of PAAs using a planar near-field (NF) range. It is proposed that a complete characterization methodology involves not only capturing beam-steered antenna patterns, but also measuring embedded element patterns, exhaustive testing of the excitation hardware of the antenna under test (AUT), and performing a phased array calibration technique. Moreover, to demonstrate the feasibility of the proposed approach, the methodology is applied onto a 2x8 microstrip patch PAA, proving its utility and effectiveness. Finally, by means of the collected data, any array pattern could be predicted by post-processing, as proven by the great agreement found between a measured pattern and its computed predicted version.
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.
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.
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.
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.
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.
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 [1]. 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 [2] 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 [2], if the position of transmitters and receivers is properly designed, the obtained results are equivalent to the ones provided by a conventional dense sampling. [1] 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. [2] 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.
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
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.
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.
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.