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Imaging

Channel De-embedding and Measurement System Characterization for MIMO at 75 GHz
Alexandra Curtin, David Novotny, Alex Yuffa, Selena Leitner, October 2017

As modern antenna array systems for MIMO and 5G applications are deployed, there is increased demand for measurement techniques for timely calibration, at both research and commercial sites.[1] The desired measurement method must allow for the de-embedding of information about the closed digital signal chain and element alignment, and must be performed in the near-field. Current means of measuring large arrays cover a variety of methods. Single-element gain and pattern calibration must cover the parameter space of element weightings and is extremely time-consuming, to the point where the measurement may take longer than the duration over which the array response is stable.[4] Two other popular methods are the transmission of orthogonal codes and the use of holography to reconstruct a full-array pattern. The first of these methods again requires extremely long measurement time. For an array of N elements and weightings per element W_n, the matrix of orthogonal codes must be of an order greater than NW_n.[4][3]. This number varies with the form of W_n depending on whether the array is analog or digital, but in both cases for every desired beam configuration, an order-N encoding matrix must be used. The second method relies on illuminating subsets of elements within an array and reconstructing the full pattern.[2] Each illuminated subset, however, neglects some amount of coupling information inherent to the complete system, making this an imperfect method. In this work we explore the development of a sparse set of measurements for array calibration, relying on coherent multi-channel data acquisition of wideband signals at 75 GHz, and the hardware characterization and post-processing necessary to perform channel de-embedding at an elemental level for a 4x1 system. By characterizing the complete RF chain of our array and the differential skew and phase response of our measurement hardware, we identify crucial quantities for measuring closed commercial systems. Additionally, by combining these responses with precise elemental location information, we consider means of de-embedding elemental response and coupling effects that may be compared to conventional single-element calibration information and full-pattern array measurements. [1] C. Fulton, M. Yeary, D. Thompson, L. Lake, and A. Mitchell. Digital phased arrays Challenges and opportunities. Proceedings of the IEEE, 104(3):487–503, 2016. [2] E. N. Grossman, A. Luukanen, and A. J. Miller. Holographic microantenna array metrology. Proceedings of SPIE, Passive Millimeter-Wave Imaging Technology VIII, 5789(44), 2005. [3] E. Lier and M. Zemlyansky. Phased array calibration and characterization based on orthogonal coding Theory and experimental validation. 2010 IEEE International Symposium on Phased Array Systems and Technology (ARRAY), pages 271–278, 2010. [4] S. D. Silverstein. Application of orthogonal codes to the calibration of active phased array antennas for communication satellites. IEEE Transactions on Signal Processing, 45(1):206–218, 1997.

Meteosat Third Generation (MTG) DCS & GEOSAR Antenna testing at ESA/ESTEC
Luis Rolo, Luca Salghetti Drioli, Damiano Trenta, Eric van der Houwen, Paolo Noschese, Enrico D'Agostino, Roberto Flamini, Marcello Zolesi, November 2016

The Meteosat Third Generation series will comprise four imaging and two sounding satellites.  The MTG-I imaging satellites will carry the Flexible Combined Imager (FCI) and the Lightning Imager.  The MTG-S sounding satellites – a first for Meteosat – will carry an Infrared Sounder (IRS) and an Ultraviolet Visible Near-Infrared spectrometer, which will be provided by ESA as the GMES Sentinel-4 mission. On the MTG-I satellites, FCI will scan the full Earth disc every 10 minutes using 16 spectral channels at very high spatial resolutions, from 2 km to 0.5 km.  In fast imagery mode it will be capable of a repeat cycle of 2.5 minutes over a quarter of the disc.  The MTG-I satellites include a Data Collection System (DCS) & Geostationary Search and Rescue (GEOSAR) payload.  The DCS supports meteorology and weather prediction.  The GEOSAR transponder will be operated within the COSPAS-SARSAT system.  Distress alert signals are received by MTG-I in UHF band and transmitted to ground in L-band for distribution to rescue mission control centers. Developed by Thales Alenia Space Italy, the DCS and GEOSAR UHF and L-band patch array antennas have been designed to operate aboard MTG-I satellites. The Engineering Model of the MTG antenna assembly with mockup has been tested inside ESA’s Hybrid European RF and Antenna Test Zone (HERTZ) chamber. The spherical near field tests performed on the antenna stand-alone and on the antenna mounted on the mockup were aimed at identifying impact of the large satellite structure on radiation pattern of the two medium gain antennas at UHF- and L-band.  Taking into account the frequency of operation and the type of antenna under test, the major contributors to the measurement error are the room scattering and the probe-AUT mutual coupling.  For this reason, dedicated measurements and analysis have been performed, in order to estimate the uncertainty in the most realistic way.  The other parameters have been estimated based on past experience and knowledge on the measurement system.  Several additional measurements were performed in order to produce dedicated uncertainty budgets for the stand-alone and with mockup tests and for the two frequency bands UHF and L-Band.

Source reconstruction by far-field data for imaging of defects in frequency selective radomes
Bjorn Widenberg, Kristin Persson, Mats Gustafsson, Gerhard Kristensson, November 2016

An inverse source reconstruction method with great potential in radome diagnostics is presented. Radomes are designed to enclose antennas to protect them, from e.g. weather conditions. Frequency selective surface (FSS) radomes are designed to conceal the antennas and provide stealth properties, by transmitting specific frequencies and be reflective for other frequencies. Ideally, the radome is expected to be electrically transparent. However, tradeoffs are necessary to fulfill properties such as aerodynamics, robustness, lightweight, weather persistency, stealth properties, etc. One tradeoff is the existence of inevitable defects. Specifically, for examples, seams in large radomes, lightning strike protection, Pitot tubes, rain caps, or lattice dislocations in frequency selective radomes. In all these examples of defects, it is essential to diagnose their influences, since they degrade the electromagnetic performance of the radomes if not carefully attended and analyzed. In this contribution, we investigate if source reconstruction can be employed to localize and image the disturbances from the defects on the surface of the radome. Employing far-field measurements remove the need for probe compensation. An artificial puck plate (APP) radome with dislocations in the lattice is investigated. An APP radome is a frequency selective surface (FSS) and it consists of a thick perforated conducting frame, where the apertures in the periodic lattice are filled with dielectric pucks. Due to the double curvature of an FSS surface, gaps and disturbances in the lattice may cause deterioration of the radome performance. Source reconstruction methods determine the equivalent surface currents close to the object of interest. The reconstructions are established by employing an integral representation in combination with an integral equation. The geometry of the object on which the fields are reconstructed is arbitrary. However, the problem is ill-posed and needs regularization. The equivalent surface currents are reconstructed on a body of revolution with the method of moment (MoM), and the problem is regularized with a singular value decomposition (SVD). The aim is to back-propagate a measured far field to determine the field components on the radome surface. The purpose is to investigate if defects on a frequency selective surface (FSS) lattice can be localized.

Indoor 3D Spherical Near Field RCS Measurement Facility: 3D RADAR Images From Simulated And Measured Data
Pierre Massaloux, Pierre Minvielle, November 2016

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. This paper details a RCS measurement technique and the associated-post processing of raw data dedicated to the localization of the scatterers of a target under test. A specific 3D radar imaging method was developed and applied to the fast 3D spherical near field scans. Compared to classical radar images, the main issue is linked with the variation of polarization induced by the near-field 3D RCS facility. This method is based on a fast and efficient regularized inversion that reconstructs simultaneously HH, VV and HV 3-D scatterer maps. The approach stands on a simple but original extension of the standard multiple scatterer point model, closely related to HR polarimetric characterization. This algorithm is tested on simulated and measured data from a metallic target. Results are analyzed and compared in order to study the 3D radar imaging technique performances.

Inverse Scattering and Imaging of Compensated Compact Ranges by Plane Wave Analysis
Engin Gülten, Josef Migl, Thomas Eibert, November 2016

The Compensated Compact Range (CCR) 75/60 of Airbus DS GmbH is the state-of-the art indoor test facility for real-time RF measurements of satellite antennas within a frequency range from 1 to 200 GHz. The CCR is composed of a two reflector system, a main reflector and a sub-reflector, to create a cross-polar-compensated plane wave in the test zone. However, even such a sophisticated design has residual cross-polar components due to the contribution of the range feed, edge diffraction from the reflector system, as well as from the serrations and imperfect absorbers. To improve and optimize the RF performance of the CCR, detailed EM simulation models are developed in order to solve the related forward scattering problem [1, 2, 3]. In spite of this it is also of great importance to analyze the CCR in a different perspective to gain insight into the CCR. To this aim, an approach based on plane wave spectrum analysis combined with inverse scattering and imaging techniques is proposed. The proposed approach firstly computes the plane wave spectrum of the measured or simulated data taken in the quite zone by using 2D Fast Fourier Transform (FFT).  Then, the measured or simulated field is back-propagated by using an inverse scattering approach. By considering the geometrical shape information of the main reflector, the current distribution on the reflector is imaged. The reconstructed images help to clearly identify the effects of. Appropriate windowing is applied to the computed plane wave (angular) spectrum in order to locate and image the echoes. Based on the investigation carried out with the proposed approach, it turns out that the area of the main reflector should be increased to reduce the disturbing impact of the serrations. This investigation also shows that increasing the size of the sub-reflector does not help to improve the plane wave uniformity of the fields in the test zone.  In order to test the proposed method against the experimental data, which is not in a suitable format for FFT, the measured data is interpolated to equally spaced data in a Cartesian coordinate system. The experimental results, which are obtained by processing both co and cross polar measurements, show very good agreement with the results obtained by using synthetic data.      References [1] A. Geise, J. Migl, J. Hartmann, H-J. Steiner, “Full Wave Simulation of Compensated Compact Ranges at Lower Frequencies”, AMTA 33th Annual Symposium, 16 – 21 October 2011 in Englewood Colorado, USA. [2] C. H. Schmidt, A. Geise, J. Migl, H-J. Steiner, H.-H. Viskum, “A Detailed PO/ PTD GRASP Simulation Model for Compensated Compact Range Analysis with Arbitrarily Shaped Serrations”, AMTA 35th Annual Symphosium, 6 – 11 October 2013 in Colombus Ohio, USA. [3] O. Borries, P. Meincke, E. Jorgensen, C. H. Schmidt, “Design and Validation of Compact Antenna Test Ranges using Computational EM”, AMTA 37th Annual Symphosium , 11 – 16 October 2015 in Long Beach, CA, USA.

Near to Far Field Transformation of RCS Using a Compressive Sensing Method
Christer Larsson, November 2016

Near field Inverse Synthetic Aperture Radar (ISAR) Radar Cross Section (RCS) measurements are used in this study to obtain geometrically correct images of full scale objects placed on a turntable. The images of the targets are processed using a method common in the compressive sensing field, Basis Pursuit Denoise (BPDN). A near field model based on isotropic point scatterers is set up. This target model is naturally sparse and the L1-minimization method BPDN works well to solve the inverse problem.  The point scatterer solution is then used to obtain far field RCS data. The methods and the developed algorithms required for the imaging and the RCS extraction are described and evaluated in terms of performance in this paper.  A comparison to image based near to far field methods utilizing conventional back projection is also made. The main advantage of the method presented in this paper is the absence of noise and side lobes in the solution of the inverse problem. Most of the RCS measurements on full scale objects that are performed at our measurement ranges are set up at distances shorter than those given by the far field criterion. The reasons for this are, to mention some examples, constraints in terms of available equipment and considerations such as maximizing the signal to noise in the measurements. The calibrated near-field data can often be used as recorded for diagnostic measurements but in many cases the far field RCS is also required. Data processing is then needed to transform the near field data to far field RCS in those cases.   Separate features in the images containing the point scatterers can be selected using the method presented here and a processing step can be performed to obtain the far field RCS of the full target or selected parts of the target, as a function of angle and frequency. Examples of images and far field RCS extracted from measurements on full scale targets using the method described in this paper will be given.

Optimization of the Reflectarray Quiet Zone for use in Compact Antenna Test Range
Daniel Rodríguez Prado,Álvaro Fernández Vaquero, Manuel Arrebola, Marcos Rodríguez Pino, Fernando Las-Heras, November 2015

Reflectarrays have been widely studied in the past 3 decades and several techniques have been developed for the synthesis of shaped-beam far-field radiation patterns [1]. Also, some near-field applications have been studied, such as imaging [2] or RFID [3]. In this contribution, a near-field synthesis technique is proposed for the reflectarray quiet zone optimization, which can be of interest in the design of probes for compact antenna test ranges (CATR) at high frequencies. The near-field of the reflectarray is characterized by a simple radiation model which computes the near field of the whole antenna as far-field contributions of each element. The reflectarray unit cell is considered the unit radiation element and its far field is computed employing the second principle of equivalence. Then, at each point in space, all contributions from the elements of the reflectarray are added in order to obtain the near field [4]. This simple model has been validated through simulations with GRASP [5] and also through near-field measurements. Then it has been used to optimize the near field of the reflectarray. The Intersection Approach algorithm is used to optimize both amplitude and phase of the near field radiated by the antenna, and uses the Levenberg-Marquardt algorithm [6] as backward projector. This optimization increases the size of the quiet zone generated by the reflectarray. References [1] J. Huang and J. A. Encinar, Reflectarray Antennas Wiley-IEEE Press, 2008. [2] H. Kamoda et al., "60-GHz electronically reconfigurable large reflectarray using single-bit phase shifters," IEEE Trans. Antennas Propag., vol. 59, no. 7, pp. 2524–2531, July 2011. [3] Hsi-Tseng Chou et al., "Design of a near-field focused reflectarray antenna for 2.4 GHz RFID reader applications," IEEE Trans. Antennas and Propag., vol. 59, no. 3, pp. 1013–1018, March 2011. [4] D. R. Prado, M. Arrebola, M. R. Pino, F. Las-Heras, "Evaluation of the quiet zone generated by a reflectarray antenna," International Conference on Electromagnetics in Advanced Applications (ICEAA), pp. 702–705, 2-7 Sept. 2012. [5] "GRASP Software", TICRA, Denmark, http://www.ticra.com. [6] J. Álvarez et al., “Near field multifocusing on antenna arrays via non-convex optimisation,” IET Microw. Antennas Propag., vol. 8, no. 10, pp. 754–764, Jul. 2014.

Scattering Effects of Traveling Wave Currents on Linear Features
Dean Mensa,Donald Hilliard, Tai Kim, November 2015

Backscattering responses of range-extended objects include those attributed to traveling-wave effects, typically caused by the termination of the object.  Diagnosing the nature of scattering mechanisms contributing to the composite response is essential to modifying the object's radar signature. ISAR images reveal essential information on the location and nature of scattering features by decomposing the frequency/angle data into basis functions corresponding to independent point scatterers.  When applied to responses of objects exhibiting other basis functions, such as those for traveling-wave scattering, ISAR images reveal unexpected results that can obscure proper interpretation of the scattering mechanisms.  Because traveling-wave lobes are restricted to limited ranges of grazing angles and are frequency dependent, however, localizing their effects from high-resolution images can be elusive.  Specifically, the traveling-wave responses are not readily distinguished from direct or diffracted responses. The paper deals with backscattering data collected on a slender cylindrical rod of 183cm length and 0.635cm diameter for aspect angles 0-180 degrees over a frequency range of 2-10GHz with polarization parallel to the plane of incidence, intended to emphasize effects of traveling waves at the rod's grazing angles.  In spite of its relatively simiple geometry, the linear rod object presents complicated responses owing to the combined effects of traveling waves and multiple diffractions.  Although ISAR images properly locate point scatterers, an understanding of the imaging process provides clues on the expected location of image elements corresponding to more complicated scattering features.  The angle and frequency dependence of each scattering mechanism is illustrated in the paper by frequency responses, range responses, and ISAR images.  The total scattering resonse of the rod for grazing incidence is characterized by at least 5 distinct scattering mechanisms, each interacting with the others as a function of viewing angle.  Because images of traveling- and diffracted-wave components overlap for some grazing angles, their relative responses preclude separation.  The results provide an example of the complex nature of scattering from a simple shape subject to traveling-wave effects.

Advances in Instrumentation and Positioners for Millimeter-Wave Antenna Measurements
Bert Schluper,Patrick Pelland, November 2014

Applications using millimeter-wave antennas have taken off in recent years. Examples include wireless HDTV, automotive radar, imaging and space communications. NSI has delivered dozens of antenna measurement systems operating at mm-wave frequencies. These systems are capable of measuring a wide variety of antenna types, including antennas with waveguide inputs, coaxial inputs and wafer antennas that require a probing station. The NSI systems are all based on standard mm-wave modules from vendors such as OML, Rohde & Schwarz and Virginia Diodes. This paper will present considerations for implementation of these systems, including providing the correct RF and LO power levels, the impact of harmonics, and interoperability with coaxial solutions. It will also investigate mechanical aspects such as application of waveguide rotary joints, size and weight reduction, and scanner geometries for spherical near-field and far-field measurements. The paper will also compare the performance of the various mm-wave solutions. Radiation patterns acquired using some of these near-field test systems will be shared, along with some of the challenges encountered when performing mm-wave measurements in the near-field.

Source Reconstruction for Radome Diagnostics
Bjorn Widenberg,Kristin Persson, Mats Gustavsson, Gerhard Kristensson, November 2014

Radome enclose antennas to protect them from environmental influences. Radomes are ideally electrically transparent, but in reality, radomes introduce transmission loss, pattern distortion, beam deflection, etc. Radome diagnostics are acquired in the design process, the delivery control, and in performance verification of repaired and newly developed radome. A measured near or far-field may indicate deviations, e.g., increased side-lobe levels or boresight errors, but the origin of the flaws are not revealed. In this presentation, source reconstruction from measured data is used for radome diagnostics. Source reconstruction is a useful tool in applications such as non-destructive diagnostics of antennas and radomes. The radome diagnostics is performed by visualizing the equivalent currents on the surface of the radome. Defects caused by metallic and dielectric patches are imaged from far-field data. The measured far-field is related to the equivalent surface current on the radome surface by using a surface integral representation together with the extinction theorem. The problem is solved by a body of revolution method of moment (MoM) code utilizing a singular value decomposition (SVD) for regularization. Phase shifts, an effective insertion phase delay (IPD), caused by patches of dielectric tape attached to the radome surface, are localized. Imaging results from three different far-field measurement series at 10 GHz are presented. Specifically, patches of various edge sizes (0.5?2.0 wavelengths), and with the smallest thickness corresponding to a phase shift of a couple of degrees are imaged. The IPD of one layer dielectric tape, 0.15 mm, is detected. The dielectric patches model deviations in the electrical thickness of the radome wall. The results from the measurements can be utilized to produce a trimming mask, which is a map of the surface with instructions how the surface should be altered to obtain the desired properties for the radome. Diagnosis of the IPD on the radome surface is also significant in the delivery control to guarantee manufacturing tolerances of radomes.

Nearfield RCS Measurements of Full ScaleTargets Using ISAR
Christer Larsson, November 2014

Near field Radar Cross Section (RCS) measurements and Inverse Synthetic Aperture Radar (ISAR) are used in this study to obtain geometrically correct images and far field RCS. The methods and the developed algorithms required for the imaging and the RCS extraction are described and evaluated in terms of performance in this paper. Most of the RCS measurements on full scale objects that are performed at our measurement ranges are set up at distances shorter than those given by the far field criterion. The reasons for this are e.g., constraints in terms of budget, available equipment and ranges but also technical considerations such as maximizing the signal to noise in the measurements. The calibrated near-field data can often be used as recorded for diagnostic measurements. However, in many cases the far field RCS is also required. Data processing is then needed to transform the near field data to far field RCS in those cases. A straightforward way to image the RCS data recorded in the near field is to use the backprojection algorithm. The amplitudes and locations for the scatterers are then determined in a pixel by pixel imaging process. The most complicated part of the processing is due to the near field geometry of the measurement. This is the correction that is required to give the correct incidence angles in all parts of the imaged area. This correction has to be applied on a pixel by pixel basis taking care to weigh the samples correctly. The images obtained show the geometrically correct locations of the target scatterers with exceptions for some target features e.g., when there is multiple or resonance scattering. Separate features in the images can be gated and an inverse processing step can be performed to obtain the far field RCS of the full target or selected parts of the target, as a function of angle and frequency. Examples of images and far field RCS extracted from measurements on full scale targets using the ISAR processing techniques described in this paper will be given.

An Novel Near-Field Imaging Method Using FrFT Technology
Xin-Yi He,Li Li, Ru-Jiang Zhou, November 2014

We present a microwave near-field imaging method using Fractional Fourier Transform (FrFT). Since the FrFT of finite chirp signal including a Fresnel integral, the scattering field of target in Fresnel zone can be described in the FrFT form. It is a valuable expression of scattering, because it unified the expression of scattering field in three different range along with distance, Rayleigh zone, Fresnel zone, and Fraunhofer zone. Then, one can get the target image from the Fresnel scattering field using the inverse FrFT (IFrFT) technology. The fast algorithm of FrFT or IFrFT has the same algorithm complexity as Fast Fourier Transform (FFT), which made a fast near-field imaging method compare to conventional method like Back Projection (BP), Time Domain Correlation (TDC), etc. Two near-field imaging results, one simply target and one complex target, will be presented to prove the method.

Enhanced Spherical Near-Field Imaging of the Quiet Zone by Combining Mode Rotation and the CLEAN Deconvolution Algorithm
Marc Dirix,Dirk Heberling, November 2013

Abstract—It has been shown that it is possible to get a good estimation of the location of the largest centers of reflection causing ripple in the quiet zone using spherical near-field scanning of the quiet zone in combination with back projection to far-field. This method however, suffers from poor resolution at lower frequencies making it hard to distinguish small contributions from the main beam if they are closely spaced. For this purpose the CLEAN algorithm has been adapted and is presented here.

Surface and Internal Temperature versus incident field measurements of Polyurethane based absorbers in the Ku band
Zhong Chen,Vince Rodriguez, November 2013

I. INTRUDUCTION In the heating process of microwave absorbers under incident electromagnetic waves, two disciplines of physics are intertwined, i.e., electromagnetic waves behavior governed by Maxwell’s equations and heat transfer process dictated by laws of thermodynamics. The power density in the absorbers due to the electromagnetic .eld is given by p= s|E|2 =2po0 o ' f|E|2 (1) where, E is the total electric .eld (V/m) in the material, s is electrical conductivity of the material (S/m), o0 is the free space permittivity (8.854 × 10-12 F/m), o' is the imaginary part of the relative dielectric constant, and f is the frequency in Hz. This is point form of the Joule’s law, and is well understood by RF engineers. The EM behavior of the polyurethane absorbers can be numerically computed. The EM .eld acts as the heating source, and its distribution in the absorber can provide a good indication on the locations of hot spots. Polyurethane foam is an excellent insulator, so the conductive heat loss may be minimal. The heat exchanges can be reasonably described by radiation and convection transfers. Radiation takes place in the form of EM wave, mainly in the infrared region. The net power transferred from a body to the surroundings is described by Stefan-Boltzmann’s law [1], prad = osA(T4 -T04 ) (2) where A is the surface area, T is the surface temperature of the radiation body in K, and T0 is the ambient temperature in K. Unfortunately, the conventional symbols used in heat transfer s and o are not the same as those in Eq. (1). s here is the emissivity or emission coef.cient, and is de.ned as the ratio of the actual radiation emitted and the radiation that would be from a black body. o in Eq. (2) is the Stefan-Boltzmann constant (5.67 × 10-8 W/m2 K4 ). The context in the paper should make it clear which symbols the authors are referring to. Otherwise, we will make explicit references. The convective heat transfer is due to the motion of air surrounding the absorbers. Two forms can take place, naturally or by forced air. The relationship is described by Newton’s law of cooling [1]: pconv = hA(T -T0 ) (3) where h is the convection heat transfer coef.cient in (W/m-2 K-1 ). h is often treated as a constant, although it can be a function of the temperature. Eq. (3) assumes that the ambient air is abundant, and is taken to be constant. This is a reasonable assumption, because the heating is typically con.ned to a small localized area in a relatively large anechoic chamber. Combining the two mechanisms of heat transfer, the total heat loss is given by p= osA(T4 -T4 )+ hA(T -T0 ) (4) 0 It is possible to solve for the temperatures from coupled Maxwell’s and heat transfer equations. Realistic results require accurate electrical and thermal properties of the materials. It is often a non-trivial process to obtain the material properties in and of itself. Careful validation is warranted before we can have full con.dence in the results. In this paper, we adopt a measurement approach instead. We conduct a series of experiments to measure the temperature both on the surface of the absorbers using an infrared imaging camera, and internally using thermocouple probes inserted into the absorbers. Temperature pro.les versus applied E .eld are experimentally established. From the measured data, we curve .t to Eq. (4) or other mathematical functions. These functions are useful to calculate results at other .eld levels, e.g., extrapolating to a higher .eld where measurement results cannot be readily obtained. II. FIELD DISTRIBUTION INSIDE THE ABSORBERS Numerical analysis was performed using Ansys HFSS, a commercially available Finite Elements software package. As it was described in [2], symmetry is taken advantage of, so only one quarter of the pyramidal absorber is solved. The quarter pyramid is located inside a square cross section prism that bounds the computational domain. The structure is fed using a port located on the top of the geometry and the side boundaries of the domain are set as perfect electric conductor (PEC) or perfect magnetic conductor (PMC). The base is modeled as PEC. This is exactly the same approach taken in [2]. The structure of a CRV-23PCL-4 is analyzed at 12.4 GHz, the same frequency as used in the measurements. The resulting .eld is extracted at one plane. The plane is one of the two orthogonal planes that cut the pyramid in 4 sections. Fig. 1 shows the .eld distribution at 12.4 GHz. The curvature of the absorber pro.le has been added for clarity. The results are an approximation. The permittivity of the material is assumed to be fairly constant from 6 GHz to 12 GHz. The purpose of the numerical analysis is to check the expected .eld distribution in the pyramid, which we can use to compare with the infrared (IR) images of the absorbers taken during the measurements. Fig. 1. Electric Field distribution at 12.4 GHz The .eld distribution data shows that most of the .eld exists on the upper third of the pyramid. It also shows that there is a region of high .eld existing in the valleys between the pyramids. The surface temperature pro.le from the IR pictures shows that this is an real phenomena. On the other hand, the .eld is higher at the very tip of the absorber. Measurements from the IR images seem to contradict this result. This can be explained. Since the tip is smaller, it cools faster to the surrounding ambient temperature. III. EXPERIMENTAL SETUP AND DATA Experiments were performed on ETS-Lindgren CRV-23PCL-8, and CRV-23PCL-4 absorbers at 12.4 GHz. Both types are 23” long from tips to bases. A piece has a base size of 2’ × 2’. A CRV-23PCL-8 piece consists of 8×8=64 pyramids, whereas a CRV-23PCL-4 piece consists of 4×4=16 pyramids. The two types are designed to have similar RF performances, but the CRV-23PCL-8 is made of slender pyramids to facilitate better heat transfers to the surroundings [2]. The absorbers are mounted on a particle board with metallic backings, and are placed in front a Ku band horn antenna with a circular aperture (the gain is approximately 20 dBi). A 300W ampli.er is used, and the power to the antenna is monitored through a 40 dB directional coupler connected to a power meter. The test setup is shown in Fig. 2. The ambient temperature is at 23.C. Fig. 2. Test setup using a conical horn antenna to illuminate the absorbers As a .rst step, a 200 V/m .eld is generated by leveling to a calibrated electric .eld probe. The distance from the probe to the antenna is 30”. At this distance, near .eld coupling is assumed neglegible, and the incident wave uniform (numerical simulation also validated these assumptions). The power needed to generate 200 V/m .eld is recorded. Next, the .eld probe is replaced with the absorbers under test. The tips of the absorbers are placed at the same distance (30”) from the antenna. Other .eld strengths can be leveled by scaling from the power for 200 V/m. A. Surface Temperature Figs. 3 and 4 show two examples of the infrared images taken after the temperature reached equilibrium under a constant 700 V/m CW at f=12.4 GHz for the two types of absorbers described earlier. There is no forced air.ow during the measurement. Table 1 summarizes the resulting temperatures on the absorber surfaces at different .eld levels. Tests were performed on two .nishes of otherwise identical CRV-23PCL-8 absorbers, i.e., fully covered with rubberized paint, or with latex paint. The data indicates that the paint has minimal effects on absorber temperatures. Table 1 also lists data for the wider CRV-23PCL-4 absorbers (with latex paint). B. Internal Temperature of the Absorber recorded by Thermocouples Three thermocouples are inserted in the CRV-23PCL-8 which are painted with rubberized coating. They are inserted at distances of 4”, 6”, and 8” from the tip of the pyramid, as illustrated in Fig. 5. Fig. 6 shows the temperatures measured by the three sensors. The temperatures at 8” from the tip are consistently higher than at other locations. There is a gap in the data at 700 V/m because RF power was turned off brie.y. Internal temperature reached 115 .C under 1.7 kW/m2 Fig. 3. Infrared camera image for incident electric .eld of 700 V/m. The absorber is the slender CRV-23PCL-8. Fig. 4. Infrared camera image for incident electric .eld of 700 V/m. The absorber is the wider CRV-23PCL-4. (800 V/m). Since the maximum allowed temperature for the polyurethane foam material is 125 .C, the incident power density is recommended to stay less than 1.7 kW/m2 for CRV-23PCL-8 absorbers mounted vertically and with natural convection in a 23.C room. After the temperature reached equilibrium under 800 V/m, additional air.ow was introduced by turning on a 6” diameter fan at 45” in front of the absorbers. The air.ow rate was measured to be approximately 80 ft/min at this distance. Note that this is a rather moderate air.ow, which can arise naturally from air-conditioning vents in a chamber. As shown in Fig. 6, the internal temperature quickly dropped to 102.C from 115.C. TABLE I MAXIMUM SURFACE TEMPERATURE RECORDED BY THE IR CAMERA (AT EQUILIBRIUM). T0 =23. C. E Power CRV-23PCL-8 CRV-23PCL-8 CRV-23PCL-4 (V/m) Density rubberized latex (. C) rubberized (kW/m2 ) (. C) (. C) 200 0.11 24 300 0.24 28 360 0.34 30 400 0.42 35 36 43 500 0.66 41 50 600 0.95 54 67 700 1.30 63 82

Broadband Optically Modulating Scatterer probe for near field measurements
Ghattas Lama,Serge BORIES, Mervi HIRVONEN, Dominique PICARD, November 2013

In the literature, one can find a low scattering photodiode modulated-probe for microwave near field imaging. The frequency response of the probe is computed at 2.45 GHz. In this paper, however a new formulation for computing the scattered field for low frequencies (from 150 MHz) by a broadband near field probe based on the impedance matrix is developed. In addition, a method to increase the scattered power by controlling matching will be shown.

A 100 GHz Polarimetric Compact Radar Range for Scale-Model Radar Cross Section Measurements
Guy DeMartinis,Michael Coulombe, Thomas Horgan, Brian Soper, Jason Dickinson, Robert Giles, William Nixon, November 2013

Abstract— A fully polarimetric compact radar range operating at a center frequency of 100 GHz has been developed for obtaining radar cross section, inverse synthetic aperture radar imagery and high range resolution profiles on targets and structures of interest. The 100 GHz radar range provides scale-model RCS measurements for a variety of convenient scale factors including W-Band (1:1 scale), C-band (1:16 scale), and S-band (1:26 scale). An overview of the radar range is provided in this paper along with measurement examples of ISAR scale-model imaging, scale-model through-wall imaging, and preliminary kHz sweep-rate Doppler that demonstrate a few of the diverse and unique applications for this system. The 100 GHz transceiver consists of a fast-switching, stepped, CW microwave synthesizer driving dual-transmit and dual-receive frequency multiplier chains. The stepped resolution of the system’s frequency sweep is sufficient for unambiguous resolution of the entire chamber. The compact range reflector is a CNC machined aluminum reflector edge-treated with FIRAM™-160 absorber serrations and fed from the side to produce a clean quiet zone. This range is the latest addition to a suite of compact radar ranges developed by the Submillimeter-Wave Technology Laboratory providing scale-model radar measurements at nearly all of the common radar bands.

Focusing 3D Measured Field-Probe Data To Image A Compact Range Reflector
Scott McBride, October 2013

A diagnostic technique was published over 20 years ago on imaging compact-range reflectors by focusing plane-polar field-probe data. At that time, only synthesized data had been evaluated. Since then, a few reflectors have exhibited performance lower than expected, and this technique has been successfully employed to improve that performance based on their measured data. This paper reviews the technique and discusses the results of processing those measured data sets. The technique produces an image of the estimated field amplitudes at the reflector surface that do not contribute to the desired quiet-zone plane wave. Point sources, line sources, and deformations over an area have all been successfully identified, often outside the projected circular boundary of the field-probe data. All measurements to date have used very coarse angular spacing with acceptable degradation in image quality.

A 200-500 GHz Bi-Static Scattering System for Material Characterization
David Novotny,National Institute of Standards and Technology, November 2012

We present performance results of a bi-directional scattering measurement system in the 200-500 GHz range. The goal is to provide dense-spectrum, bi­directional reflectance distribution function (BRDF) of sample materials and small objects that can be propagated into detection models and used as standard materials to compare performance of various detection and imaging systems. Our system is built upon a commercial frequency-domain, vector network analyzer system. The system is designed to minimize drift due to movement and temperature changes. The initial data, presented here, of reflectance from a variety of standard targets and sample materials show operation from 200-500 GHz and highlight stability, repeatability, and dynamic range of the system.

Robotically Controlled mm-Wave Near-Field Pattern Range
Joshua Gordon,NIST, November 2012

The Antenna Metrology Lab at the National Institute of Standards and Technology in Boulder Colorado has developed a robotically controlled near-field pattern range for measuring antennas and quasi-optical components from 50 GHz to 500 GHz. This range is intended to address the need for highly accurate antenna pattern measurements above 100 GHz for a variety of applications including remote sensing, communications and imaging. A new concept in near-field range systems, this system incorporates the positioning repeatability of a precision industrial six-axes robot, six-axes parallel kinematic hexapod, and high precision rotation stage, integrated with a highly accurate laser tracking system. Programmable robot positioning allows the system geometry to be configured for spherical, planar, and cylindrical scans, as well as gain extrapolation measurements. Variable scan volume accommodates different test antenna sizes. Positioning accuracy better than 10 µm is predicted. Specifics of the system design, operating specifications and configurability will be presented.

GROUND TO AIR IMAGING OF AIRCRAFT IN FLIGHT
Louis Sheffield,STAR Dynanics Corporation, November 2012

Ground to air imaging poses numerous technical challenges, a number of which relate to target motion throughout its inverse synthetic aperture. A well-tracked target benefits not only its illumination, but provides an accurate description of the target’s position as a function of time. Tracking may be accomplished using a monopulse tracking radar or precision GPS/telemetry techniques; either of which are sufficiently accurate for coarse translational motion compensation. However, without target attitude telemetry, the inverse synthetic aperture may still be inferred from the target’s spherical coordinates and their first derivatives, and coarse rotational motion compensation may be performed. Further refinement is available from the in-phase/quadrature data. Residual translational motion may be characterized and corrected by considering both intra-chirp (i.e., within stepped chirps) and inter-chirp phase migration, accommodating fine translational motion compensation. The data become reasonably bandlimited, allowing rotational motion compensation to be performed via bandlimited resampling, yielding a focused ISAR image.







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