AMTA Paper Archive
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Wideband Double-Ridged TEM Horn for Nondestructive Evaluation and Imaging Applications
Antenna performance plays a significant role in synthetic aperture radar (SAR) image quality, particularly for nondestructive evaluation (NDE) applications. To obtain high image quality and target detectability, SAR imaging systems should possess good resolution (cross- and along-range), and a relatively large penetration depth. Consequently, the antenna used must be wideband with a relatively wide beamwidth for high resolution and operate at low starting frequency for sufficient penetration depth. Meanwhile, antenna aperture size should be small rendering it sufficiently portable for scanning purposes or when employed within imaging arrays. However, increasing frequency bandwidth, reducing minimum frequency of operation while maintaining small aperture size (resulting in wide beamwidth), all at the same time is difficult. To this end, double-ridged horn (DRH) antenna, with flared aperture for improved radiation efficiency and performance is found to provide a good compromise among these parameters. Therefore, an improved modified design of DRH is proposed. The dimensions of its geometry are optimized to provide low unwanted reflections. Curved surfaces are attached at the end of the two ridged walls for better aperture matching. The final aperture size of the antenna is 230 ? 140 mm2, operating in the 0.5-4.0 GHz frequency range, and with a relatively wide beamwidth in its near-field region where most NDE imaging measurements are conducted. Measured reflection coefficient by using the fabricated antenna is used to verify the simulation results. Comparisons are also made with similar designs of DRH found in the literature showing that the proposed antenna has smaller electrical length with respect to the lowest operating frequency for designs without using absorbing material. Moreover, to conduct wideband SAR imaging, a new phase calibration method, using a small electric field monopole probe, to measure the phase change between the antenna aperture center and the input feed port for each frequency component is developed. Imaging results over a large concrete slab with delamination and voids simulated by foam and plastic sheets show that the proposed calibration approach works well, and the proposed antenna can effectively detect all of these defects with different scattering properties.
2D RCS Prediction from Multistatic Near-Field Measurements on a Plane by Single-Cut Near-Field Far-Field Transformation and Plane-Wave Synthesis
A near-field far-field transformation (NFFFT) technique with a plane-wave synthesis is presented for predicting two-dimensional (2D) radar cross sections (RCS) from multistatic near-field (NF) measurements. The NFFFT predicts the FF of the OUT illuminated by each single source, then the plane-wave synthesis predicts the FF of the OUT each illuminated by each plane-wave by synthesizing the FFs given in the NFFFT step. The both steps are performed in the similar computational procedure based on a single-cut NFFFT technique that has been proposed previously. The method is performed at low cost computation because the NF and source positions are required only on a single cut plane. The formulation and validation of the method is presented.
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.
Indoor 3D Spherical Near Field RCS Measurement Facility: A new high resolution method for 3D RCS Imaging
Indoor RCS measurement facilities are usually dedicated to the characterization of only one azimuth cut and one elevation cut of the full spherical RCS target pattern. In order to perform more complete characterizations, a spherical experimental layout has been developed at CEA for indoor Near Field monostatic RCS assessment . This experimental layout is composed of a 4 meters radius motorized rotating arch (horizontal axis) holding the measurement antennas while the target is located on a polystyrene mast mounted on a rotating positioning system (vertical axis). The combination of the two rotation capabilities allows full 3D near field monostatic RCS characterization. 3D imaging is a suitable tool to accurately locate and characterize in 3D the main contributors to the RCS. However, this is a non-invertible Fourier synthesis problem because the number of unknowns is larger than the number of data. Conventional methods such as the Polar Format Algorithm (PFA), which consists of data reformatting including zero-padding followed by an inverse fast Fourier transform, provide results of limited quality. We propose a new high resolution method, named SPRITE (for SParse Radar Imaging TEchnique), which considerably increases the quality of the estimated RCS maps. This specific 3D radar imaging method was developed and applied to the fast 3D spherical near field scans. In this paper, this algorithm is tested on measured data from a metallic target, called Mx-14. It is a fully metallic shape of a 2m long missile-like target. This object, composed of several elements is completely versatile, allowing any change in its size, the presence or not of the front and / or rear fins, and the presence or not of mechanical defects, … Results are analyzed and compared in order to study the 3D radar imaging technique performances.
Feasibility of Coaxial Resonators for Permittivity Measurements of Pressurized Gases
This paper investigates numerically the feasibility of using quarter wavelength coaxial resonators for permittivity measurements of pressurized gases, as found in the subsurface. The non-short-circuited end of the resonator is facing the inside of a pressure cell and is filled with pressure resistant, low-loss dielectric material. Results show that as pressure increases, the corresponding increase in dielectric constant can be detected through a shift in the resonant frequency of |S11| and confirmed by a change in the phase of S11.
Measurements on extended objects for radar field probes
An extended long object usually gives rise to a bright reflection (a glint) when viewed near its surface normal. To take advantage of this phenomenon and as a new concept, a discrete Fourier transform (DFT) on the RCS measurements, taken within a small angular range of broadside, would yield a spectrum of incident wave distribution along that object; provided that the scattering is uniform per unit length, such as from a long cylinder [1, 2]. In this report, we examine the DFT spectra obtained from three horizontal long objects of different lengths (each of 60, 20, and 8 feet). Aside from the end effects, the DFT spectra looked similar and promising as an alternative to the conventional field probes by translating a sphere across a horizontal path. Keywords: RCS measurements, compact range, field probes, extended long objects 1. The Boeing 9-77 compact range The Boeing 9-77 indoor compact range was constructed in 1988 based on the largest Harris model 1640. Figure 1 is a schematic view of the chamber, which is of the Cassigranian configuration with dual-reflectors. The relative position of the main reflector and the upper turntable (UTT) are as shown. The inside dimensions of the chamber are 216-ft long, by 80-ft high, and 110-ft wide. For convenience, we define a set of Cartesian coordinates (x: pointing out of the paper, y: pointing up, z: pointing down-range), with the origin at the center of the quiet zone (QZ). The QZ was designed as an ellipsoidal volume of length 50-ft along z, height 28-ft along y, and width 40-ft along x. The back wall is located at z = 75 ft, whereas the center of the roll-edged main reflector (tilted at 25 o from vertical) is at z =-110 ft. It is estimated that the design approach controls the energy by focusing 98% of it inside the QZ for target measurements. The residual field spreading out from the main reflector was attenuated by various absorbers arranged in arrays and covering the chamber walls.-, Tel. (425) 392-0175 2. Anechoic chamber In order to provide a quiet environment for RCS measurements, the inside surfaces of an anechoic chamber are typically shielded by various pyramidal and wedged-shaped absorbers, which afford good attenuation at near-normal incidence for frequencies higher than ~2 GHz. At low frequencies and oblique angles , however, Figure 1. A schematic view of the Boeing 9-77 compact range with dimensions as noted. insufficient attenuation of the radar waves by the absorbers may give rise to appreciable backgrounds. Figure 2 shows a panorama view inside the compact range, as viewed from the lower rear toward the main reflector and the UTT. With the exception of the UTT, all other absorbers are non-moving or stationary. A ring of lights on the floor shows the rim around the lower turntable (LTT), prior to the installation of absorbers. In order to minimize the target-wall interactions, the surfaces facing the QZ from the ceiling, floor, and two sidewalls are covered with the Rantec EHP-26 type of special pyramidal absorbers.
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.
Near-field multi-focusing transmitarray optimization for multi-position feed
A transmitarray antenna is proposed as a multi-focusing antenna in the near-field region with capability for focus scanning and/or simultaneous and independent focus spots generation at 28 GHz. The transmitarray optics is defined for a centred configuration and the elements are designed to focus the radiated near-field at a given point. Then, a number of feeds is placed along arcs in the principal planes and the near-field generated by the transmitarray when its illuminated by each one is obtained, demonstrating the capability to generate multiple independent near-field spots. The focusing performance is improved for the centered feed through a Phase-Only synthesis technique based on the generalized Intersection Approach in near-field. Finally, the spots produced by the whole cluster are calculated, demonstrating the overall improvement and validating the designing process. This configuration can be applied in near-field systems as radar for surface inspection, measurement systems or wireless power transfer among others.
Portable Laser Guided Robotic Metrology System
This paper introduces the new Portable Laser Guided Robotic Metrology (PLGRM) system at the National Aeronautics and Space Administration's (NASA) Glenn Research Center. Previous work used industrial robots in fixed facilities to characterize antennas and required fixtures that do not lend themselves to portable applications. NASA's PLGRM system is designed for in-situ antenna measurements at a remote site. The system consists of a collaborative robot arm mounted on a vertical lift and a laser tracker, each on a mobile base. Together, they enable scanning a surface larger than the robot's reach. To accomplish this, the robot first collects all points within its reach, then the system is moved and the laser tracker is used to relocate the robot before additional points are captured. The PLGRM implementation will be discussed including how safety and planning are combined to effectively characterize antennas. Software defined triggering is a feature, for flexible integration of vector network analyzers and antenna controllers. Lastly, data will be shown to demonstrate system functionality and accuracy.
Recent Changes to the IEEE std 1502 Recommended Practice for Radar Cross-Section Test Procedures
Radar scattering is typically represented as the RCS of the test object. The term RCS evolved from the basic metric for radar scattering: the ratio of the power scattered from an object in units of power per solid angle (steradians) normalized to the plane-wave illumination in units of power per unit area. The RCS is thus given in units of area (or effective cross-sectional area of the target, hence the name). Note that the RCS of the test object is a property of the test object alone; it is neither a function of the radar system nor the distance between the radar and the test object, if the object is in the far field. Because the RCS of a target can have large amplitude variation in frequency and angle, it is expressed in units of decibels with respect to a square meter and is abbreviated as dBsm (sometimes DBSM or dBm2). In terms of this definition, the RCS of a radar target is a scalar ratio of powers. If the effects of polarization and phase are included, the scattering can be expressed as a complex polarimetric scattering (CPS) matrix. The measurement of the RCS of a test object requires the test object to be illuminated by an electromagnetic plane wave and the resultant scattered signal to be observed in the far field. After calibration, this process yields the RCS of the test object in units of area, or the full scattering matrix as a set of complex scattering coefficients. This paper describes the planned upgrades to the old IEEE Std 1502™-2007 IEEE Recommended Practice for Radar Cross-Section Test Procedures . The new standard will reflect the recent improvements in numerical tools, measurement technology and uncertainty estimates in the past decade.
Rydberg Atom-based RF Power Measurements
The power transmitted through a waveguide was determined using in-situ atom-based electric field measurements. The field distribution in the waveguide was measured using Rydberg atoms to find the maximum field, which was used to determine the power. For a proof-of-concept, the power of radio frequency fields at 17.86, 19.63, 26.53, and 33.03 GHz were measured in a WR42 waveguide. A section of waveguide was sealed and filled with cesium atoms. These atom-based measurements are self-calibrated and independent of typical power measurement methods.
Reflection-Based Inverse Scattering Image Reconstruction for Non-Destructive Testing
Non-destructive testing (NDT) is a fundamental step in the production chain of aircraft structural components since it can save both money and time in product evaluation and troubleshooting. This paper presents a reflection-based imaging technique for electromagnetic (EM) testing of composite panels, with the device under test (DUT) being metal backed and both the transmitting and receiving components of the NDT system situated on the same side of the DUT. One of the key properties of the presented technique is the complete redundancy of a reference measurement, thereby making it feasible to retrieve a high quality image of the DUT with only a single measurement. Data for both a proof-of-concept DUT and an industrially manufactured composite panel is provided, and the retrieved images show the applicability of both the measurement technique and the imaging algorithms.
Evaluation of the Monostatic-Bistatic theorem applied to the radar signature of aerial platforms in low frequency
In this paper, we explore the capabilities of the Monostatic-Bistatic Theorem (MBT) applied to Radar Cross Section (RCS) in low frequency. Originally, the validity of this theorem has been shown in high frequency for targets whose RCS is produced by elementary interactions (specular reflection in particular). We are interested in aerial platforms and in particular some Low Observable targets that have relatively "pure" geometries limiting the presence of complex interactions. Several variants of the MBT from the field of electromagnetism  and acoustics  are used. Their performances are compared from data obtained from a MoM method that is recognized to produce accurate scattering data. To highlight the discrepancies produced by the different variants, we use both a metric to compare the quality of the bistatic holograms obtained and also radar imaging which allows locating the areas of the target where the echoes are not correctly restored.
Precision Optical Antenna Alignment System for Tracking Antennas in 6-DOF
We present on an all-optical spatial metrology system , the PiCMM, that aids in the alignment and tracking of antennas with accuracies on the order of 25 microns and 0.01 deg. This system speeds up millimeter-wave antenna alignment, does not require contact, and links spatial measurements to a laser tracker world coordinate frame. An automated Pixel Probe and dark-field imaging are used to directly measure the aperture geometry and its pose. These measurements are absolute in the world-frame of a laser tracker and associated coordinate metrol-ogy space of the antenna scanner. Thus, aperture geometries can be linked directly to any laser tracker target (i.e. 6DOF, 3DOF) and data such as that used to calibrate positioner kinematics. For example, the links and joints defining the Denavit-Hartenberg kinematic model of a robotic arm scanner. The new automated aspect of the system reduces alignment time to under an hour. The synergy with laser tracker targets allows for a high level of repeatability. Furthermore, antennas can be exchanged or realigned in the antenna scanner autonomously because antenna geometry and kinematic models reside in the same laser tracker coordinate metrology space.
Imaging a Range's Stray Signals with a Planar Scanner
The fundamental purpose of absorber treatment in an anechoic chamber is to ensure that only the direct-path signal is coupled between the range antenna(s) and the device under test. For many simple and standard geometries, this is readily accomplished with conventional processes and procedures. When the geometry and/or stray-signal requirements deviate from the norm, however, it can be very beneficial to have an easy and reliable way to locate and quantify sources of stray signals. This paper discusses a straightforward algorithm for creating images of those stray signals in a range when a planar scanner and broad-beamed probe are available in the test zone. Measured data from multiple facilities are evaluated, along with absorber-treatment improvements made based on some of the images produced.
Effective Polarization Filtering Techniques for Ground Penetrating Radar Applications
The effect of different decomposition techniques on the imaging and detection accuracy for polarimet-ric surface penetrating data is studied. We derive the general expressions for coherent polarimetric decomposition using the Stokes parameters and model based polarimetric decomposition using the Yamaguchi technique. These techniques are applied to multi-frequency (0.4-4.8GHz) full polarimetric near-field radar measurements of scattering from surface laid calibration objects and shallow buried landmine types and show in detail how the decomposition results provide effective surface and sub-surface clutter reduction and guide the interpretation of scattering from subsurface objects. Data processing methods assume cross-polar symmetry and a novel bistatic calibration procedure was developed to enforce this condition. The Yamaguchi polarimetric decomposition provides significant clutter reduction and image contrast with some loss in signal power; while Stokes parameters also provide imagery localising targets, complementary information on the scattering mechanism is also obtained. Finally a third novel polarimetric filter was formulated based on differential interferometric polarimetric decomposition. The three combined techniques contribute to a significant improvement of subsurface radar performance and detection image contrast.
Evaluation of Software Defined Radio Receiver for Phaseless Near-Field Measurements
This paper presents a time domain antenna measurement technique by using a low cost software defined radio receiver. The technique aims to resolve measurement challenges derived from antennas where the reference signal is not accessible. The phase reconstruction implemented in this work is based on calculating the Fast Fourier Transform of the time domain signal to estimate the power spectrum and the relative phase between measurement points. In order to do that a reference antenna is used to retrieve the phase, providing a full characterization in amplitude and phase of the electric field and allowing source reconstruction. The results demonstrate the potential of this technique for new antenna measurement systems and reveal some of the limitations of the technique to be optimized, like the undesired reflections due to the interactions between the probe and the reference antenna.
RCS Measurements and Imaging on Arrays of Retro-reflectors
For features of very weak scattering while masked by background and clutter, care must be taken in the measurement design as well as data processing, in order to extract the true RCS values. A good example of flush-mounted fasteners for a low-observable (LO) aircraft, arranged in an array, was reported by Lutz, Mensa, and Vaccaro . In the Boeing 9-77 range, retro-reflectors (called retros) were routinely taped on a test-body for monitoring its 3-D locations and angles during measurements. Though the retro’s RCS may be several orders below that of a test-body, a challenge was to discover their exact values. In the millimeter wave range (MMWR), we measured 2-D arrays of retros arranged in both square and hexagonal lattices taped onto a flat metal surface pitched 20o down. RCS measurements were made as a function of frequency and aspect angle. From the 2-D FFT images, the nominal RCS for a retro in VV polarization was found to be -75 dBsm, independent of the geometry and number of retros even down to one unit-cell . But for the HH polarization, there is no backscattering from such a flat metal surface. References . J. Lutz, D. Mensa, and K. Vaccaro, "RCS measurements of LO features on a test body," Proc. 21st AMTA, pp. 320-325 (1999). . P. S. P. Wei and J. P. Rupp, " RCS measurements on arrays of weak scatterers enhanced by diffraction," Proc. 26th AMTA, pp. 263-268 (2004). ---------------------------------------------- ** Sam Wei is at: 4123 - 205th Ave. SE, Sammamish, WA 98075-9600. Email: firstname.lastname@example.org, Tel. (425) 392-0175
Development of A New Atom-Based SI Traceable Electric-Field Metrology Technique
One of the keys to developing new science and technologies is to have sound metrology tools and techniques. Whenever possible, we would like these metrology techniques to make absolute measurements of the physical quantity. Furthermore, we would like to make measurements directly traceable to the International System of Units (SI). Measurements based on atoms provide such a direct SI traceability path and enable absolute measurements of physical quantities. Atom-based measurements have been used for several years; most notable are time (s), frequency (Hz), and length (m). There is a need to extend these atom-based techniques to other physical quantities, such as electric (E) fields. We are developing a fundamentally new atom-based approach for that will lead to a self-calibrated, SI traceable E-field measurement and has the capability to perform measurements on a fine spatial resolution in both the far-field and near-field. This new approach is significantly different from currently used field measurement techniques in that it is based on the interaction of radio-frequency (RF) E-fields with Rydberg atoms (alkali atoms placed in a glass vapor-cell that are excited optically to Rydberg states). The Rydberg atoms act like an RF-to-optical transducer, converting an RF E-field strength to an optical-frequency response. In this new approach, we employ the phenomena of electromagnetically induced transparency (EIT) and Autler-Townes splitting. This splitting is easily measured and is directly proportional to the applied RF E-field amplitude and results in an absolute SI traceable measurement. The technique is very broadband allowing self-calibrated measurements over a large frequency band including 500 MHz to 500 GHz (and possibly up to 1 THz and down to 10's of megahertz). We will report on the development of this new metrology approach, including the first fiber-coupled vapor-cell for E-field measurements. We also discuss key applications, including self-calibrated measurements, millimeter-wave and sub-THz measurements, field mapping, and sub-wavelength and near-field imaging. We show results for mapping the fields inside vapor cells, for measuring the E-field distribution along the surface of a circuit board, and for measuring the near-field at the aperture in a cavity.
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.
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