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Anechoic Chamber

Accurate Planar Near-Field Antenna Measurements Without Full Anechoic Chamber
Greg Hindman,Stuart Gregson, Allen Newell, November 2014

In recent times, planar near-field antenna measurements have largely been performed within fully absorber lined anechoic chambers.  However this is a comparatively recent development as, due to the nature of the electromagnetic radiation when measuring medium to high gain antennas, one can often obtain excellent results when testing within only a partially absorber lined chamber [1], or in some cases even when using absorber placed principally behind the acquisition plane. As absorber can be bulky and costly, optimizing its usage often becomes a significant factor when planning a new facility.  This situation becomes more pressing when the designated test environment is not exclusively devoted to antenna pattern testing with non-ideal absorber coverage being, in some cases, mandated, c.f. EMC testing.  Planar test systems lend themselves to deployment within multipurpose installations as they are routinely constructed so as to be portable [2] thereby allowing partial or perhaps complete removal of the test system between measurement campaigns. This paper will present measured data taken using a number of different planar antenna test systems with and without anechoic chambers to summarize what is achievable and to provide design guidelines for testing within non-ideal anechoic environments.  NSI’s Planar Mathematical Absorber Reflection Suppression (MARS) technique [3, 4] will be utilized to show additional improvements in performance that can be achieved through the use of modern sophisticated post processing. Keywords: Planar Near-Field, Reflection Suppression, Scattering, MARS. REFERENCES S.F. Gregson, A.C. Newell, G.E. Hindman, M.J. Carey, “Extension of The Mathematical Absorber Reflection Suppression Technique To The Planar Near-Field Geometry”, AMTA, Atlanta, October 2010. G.E. Hindman, “Applications of Portable Near-Field Antenna Measurement Systems”, AMTA, October, 1991. S.F. Gregson, A.C. Newell, G.E. Hindman, “Advances In Planar Mathematical Absorber Reflection Suppression”, AMTA, Denver, Colorado, October 2011. S.F. Gregson, A.C. Newell, G.E. Hindman, P. Pelland, “Range Multipath Reduction In Plane-Polar Near-Field Antenna Measurements”, AMTA, Seattle, October 2012.

Near-Field to Far-Field Transformation for ICs Using Dipole-Moment Models on EMI Measurement
Guochang Shi,Yuan Zhang, Yi Liao, November 2014

The electromagnetic compatibility (EMC) problems are becoming more challenging and noticeable due to the increasing complexity of integrated circuits (IC). Currently, most electromagnetic interference (EMI) standards specify that the measurements must be performed in the far field which is time consuming and expensive for the use of semi-anechoic chambers or open area test site. While near-field measurement is usually fast and much more flexible, especially for the complex structures, the near-field results could be obtained more efficiently for built-in ICs. The transformation between near-field and far-field data is of great significance as long as the near-field data is measured. Many methods including near-field scanning method and Huygens’ equivalence method are used to complete the transformation from near-field data to far-field radiation. However, the near-field scanning method is inherent complex and requires strict mathematical derivation, which is difficult to handle for some practical cases. Huygens’ equivalence method is restricted by the location of observation point and the results are hardly obtained under scanning plane. In contrast, near-field to far-field transformation based on inverse method appears to be more desirable by reconstructing a dipole-moment model instead of an IC. The dipole-moment model can be used to predict the far-field data, but also can be incorporated into a numerical full-wave tool as an equivalent source for complex systems. In this paper, the inverse method is firstly introduced. A noise source model from an IC is proposed based on an array of dipoles. These dipole moments can be extracted from the near-field measurement in a scanning plane above the IC. Each dipole is modeled as an equivalent combined source consists of wire antennas and loop antennas. Then the radiation of IC in far-field region can be easily obtained. Finally, an example of physical IC is given to validate the approach.

Uncertainty Analysis of Spherical Near Field Antenna Measurement System at VHF
Gwenn Le Fur,Francisco Cano-Facila, Daniel Belot, Lise Feat, Luc Duchesne, Anthony Bellion, November 2014

Recent enhancements in military telecommunication systems for monitoring and tracking in low VHF range (30-80MHz) imply the use of specific antenna measurement facilities to characterize either the antenna alone or the antenna mounted on a supporting structure which can be heavy and bulky. The indoor Near-Field approach shows benefits in terms of compactness. However this approach involves issues due to high levels of reflectivity of the anechoic chamber, the antenna under test positioner and the measurement probe structure at these larges wavelengths. Studies and simulations of each contribution have been performed in a previous paper. The proposed paper focuses on the improvement of measurement results using post-processing techniques and associated uncertainty analysis of the mono-probe near-field system at the CNES. First the new 50-400 MHz dual polarized probe and the measurement system are briefly presented. Then the estimation of each error term is detailed providing a global error budget in order to appreciate the benefit of post-processing technique. All the considered errors terms are all of those included in the well-known 18 NIST terms. Each of them is evaluated using the most appropriated approaches (specific measurement, simulation).

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

Simulating Antenna Measurements in an Anechoic Chamber
Derek Campbell, November 2013

Abstract— The measurement community has a substantial and increasing interest in utilizing computational electromagnetic (CEM) tools to minimize the financial resources and real estate required to design and construct a custom anechoic chamber without sacrificing performance. Although a full-wave numerical technique provides the most accurate solution, the computational resources can quickly become a hindrance as the electrical size increases with frequency. Fortunately, the assumptions underlying numerical solvers using asymptotic approximations become more valid and therefore more accurate as frequency increases. Simulations using these numerical solvers, available in commercial software such as FEKO [1], extend previous research [2] with a comparison between the power received by an antenna under test (AUT) and a reference antenna in an anechoic chamber across the UHF frequency band. The ability to simulate a measurement technique helps unite the measurement and computational communities by accounting for a variety of potential error sources. The respective antenna gain is then extracted with post-processing and ultimately provides further insight into the performance of anechoic chambers. Without loss of generality, a complete chamber measurement of half-wave dipole antennas at several frequencies has been simulated.

An Innovative Design of a Size-Reduced Anechoic Chamber for Antenna Measurements at Low Frequencies
Rong-Chung Liu,Teh-Hong Lee, Hsi-Tseng Chou, November 2013

The core technology to this innovative chamber design is the invention of a new feed structure which integrates the design of the chamber’s wall, and reduces the multipath effects from the walls. In this design, the absorbing materials are integrated as a part of its feeding wall thereof to produce a homogeneous property on the plane, i.e., the plane parallel to the feeding wall. The material attached to the other walls has a non-homogeneous property on the plane parallel to its corresponding attached wall, which allows the scattering of incident field in a widely spread fashion.

Positioner Effects in Measurements of Low-Medium Gain Antennas
Alford Chauraya,Terence West, Rob Seager, Will Whittow, Shiyu Zhang, Yiannis Vardaxoglou, November 2013

Abstract—In this paper, a bespoke, fully automated anechoic chamber is discussed and the positioner effects on measurements of antennas are investigated. Antenna measurements performed in this robust anechoic chamber are undertaken in two parts namely; acquisition and analysis, with the aid of low cost positioner hardware and low level software language. In order to get a measure of validation of our measuring system only the important parts of the chamber have been modelled and measurements carried out using a balanced sleeved dipole and a microstrip patch antenna, which have well-known characteristics. It was noticed from the results that the positioner, exaggerates the performance of some antennas particularly small antennas without a ground plane at certain distances and frequencies. The positioner has a tendency to reflect energy, and distort radiation patterns; hence, it was important to ensure that such antennas are placed at an appropriate distance away from the positioner. The comparison between the simulated and measured efficiency of a balanced sleeved dipole is good. The predicted and measured peak efficiency at 2.49 GHz was 95% and 94% respectively. It was also observed that the variability in efficiency measurements was less than 3% for measurements with different angular resolutions on different days.

Low-cost GNSS Antennas Phase Center Variations Characterization for UAV Attitude Determination Application
Serge Bories, Yann Mehut, Christophe Delaveaud, October 2013

In the present paper, a non-dedicated mass market GNSS antenna calibration method is discussed, with a special focus on the significant error component due to phase variations of receiving antennas in precise GNSS applications. Different calibration methods are compared from the literature; the indoor (anechoic chamber) calibration has been selected. The algorithm used to compute the mean Phase Center (PC) and its associated Phase Center Variation (PCV) for all angular directions is also described and has been validated on simulated canonical antennas. PC and PCV are then computed when four antennas are placed near the command unit of an unmanned aerial vehicle (UAV), which emulates the final application scenario. The impact of this structure is evaluated thanks to PCV cartographies. Two low-cost COTS antennas have been selected and their PCV maps are compared with regards to their geometry. Lastly, a reproducibility study based on the PCV characterization of ten copies of one of the selected COTS antennas concludes on the robustness of the PCV calibration.

Square Patch Antenna Design from Equivalent Circuit Models for MIMO Antenna Communications Application
Paul Oleski,US Air Force Research Laboratory, November 2012

Although the square patch antenna is a well known printed circuit antenna, there are gaps in the publications that prevented accurate design for practical dual polarization patch antennas. This paper describes (without gaps) the steps that allow rapid design of the dual polarized square patch antenna with typical commercial RF materials. Given a patch laminate material, the design process proceeds by using the Matlab program which is given in Appendix A. Typical values for a 5 GHz patch antenna are given. Dual polarization square patch antennas were constructed. Measurements show the two ports are well isolated, and they provide polarization diversity which is useful in our MIMO array development program. The scattering matrix of the two port antenna was measured with an Agilent PNA network analyzer. The antenna patterns were measured in our anechoic chamber and on our far field range. The pattern widths provide hemi­spherical coverage. The results which are given imply good efficiency for the antenna ports. When combined with the other patch elements in the MIMO array, robust communications are achieved for all look angles.

Modeling and Analysis of Anechoic Chamber using CEM Tools
C.J. Reddy,EM Software & Systems (USA) Inc., November 2012

Advances in computational resources facilitate anechoic chamber modeling and analysis at VHF frequencies using full-wave solvers available in commercial software such as FEKO. The measurement community has a substantial and increasing interest in utilizing computational electromagnetic (CEM) tools to minimize the financial and real estate resources required to design and construct a custom anechoic chamber without sacrificing performance. A full-wave simulation analysis provides a more accurate solution than the approximations inherent to asymptotic ray-tracing techniques, which have traditionally been exploited to overcome computational resource limitations. An anechoic chamber is simulated with a rectangular down-range cross-section to utilize the software’s capability to assess polarization performance. The absorber layout within the anechoic chamber can be optimized using FEKO for minimal reflections and an acceptable axial ratio in the quiet zone. Numerical results of quiet zone disturbances and axial ratios are included for both low- and medium-gain source antennas over a broad frequency range.

“Defects” of Specular Patches in Elongated Anechoic Chambers
John Aubin,ORBIT/FR Inc., November 2012

Specular patches comprising pyramidal absorber components are frequently used in anechoic chambers to suppress potential DUT coupling with the side walls, floor and ceiling of the chamber. However, these specular patches also interact with the incident field radiated by the source antenna, compact range reflector, or tapered chamber feed illuminating the chamber. If the specular patch reflects the incident field in GO fashion, then the reflected field is incident on the absorptive back wall and is sufficiently attenuated there, so that there is no significant degradation of the field uniformity in the Quiet Zone due to the reflected field. If, however, the chamber is long, and the grazing angle of the incident field on the specular patches is relatively low, “non-specular” reflections incident on the Quiet Zone will perturb the field, and accordingly will degrade the field uniformity. If the chamber is operating at high frequencies (e.g., above several GHz) and the distance between the Quiet Zone and side walls is significant in terms of wavelengths, then the “non-specular” reflections will not impact the field uniformity to a noticeable extent, as they are attenuated in free space while propagating from the specular patches to the Quiet Zone. If the chamber is intended for operation at VHF/UHF frequencies, as is prevalent in tapered chambers, then the “non-specular” reflections may be the dominant factor affecting the Quiet Zone uniformity. In this paper the measured reflectivity in a tapered chamber with pyramidal specular patches is presented, illustrating a significant rise of the reflectivity over a portion of the VHF/UHF bands. Thorough investigation has shown the source of the degraded reflectivity to be the specular patch. This effect has been confirmed by simulation, and is analyzed by modeling the specular area as a periodic structure. Replacement of the specular patches by wedges has materially improved the reflectivity in the chamber, as will be shown by comparative reflectivity measurement results. For the application under consideration, the coupling between the DUT and sidewalls was below the specified minimum and, thus, advanced coupling suppression techniques were not required. For more stringent coupling requirements, the use of the ORBIT/FR patented “Two Level GTD” technology (see, for example, [1-4]) is a good choice to minimize reflectivity and DUT/sidewall coupling simultaneously.

G/T Measurement in an Anechoic Chamber
Paul Kolesnikoff,Ball Aerospace, November 2012

Many modern antennas are incorporating LNAs into the aperture to maximize system receive performance. G/T (Gain over Temperature) quantifies the performance of these antenna systems. Historically, G/T measurements needed knowledge of absolute effective temperature of multiple noise sources, which is not practical in an anechoic chamber. A Y-factor method is presented which uses a reference antenna system with a known G/T to determine the G/T of the Antenna Under Test (AUT). This paper will review G/T, describe the measurement process, cover calibration of the reference antenna system and discuss error sources and their mitigation.

Absorber, Performance, and Advancements in Absorber Technology
Donald Gray,TDK RF Solutions, November 2012

All of us involved with antenna measurements or radar cross section measurements are familiar with the absorber seen on the walls, ceiling, and floor of anechoic chambers. It helps simulate free-space conditions. It comes in various shapes and lengths, and it reduces the reflections, or unwanted energy, from encroaching on the quiet zone. But what makes one absorber better than another? Further, what advances in composition have been made over the last 50 years to improve the simulation of free space? This paper will address differences in geometry and differences in materials and “ingredients” for optimizing performance. Also, it will discuss the advantages in using different materials to create stronger absorber to help maintain performance and for creating clean and safe environments, for such endeavors as measurements involving flight hardware.

Echo Suppression By Spatial Filtering Techniques In Advanced Planar And Spherical Nf Antenna Measurements
L. J. Foged, L. Scialacqua, F. Mioc, F. Saccardi, P. O. Iversen, L. Shmidov, R. Braun, J. L. Araque Quijano, G. Vecchi, November 2012

This paper presents a comparative investigation of two versatile error mitigation techniques applicable to general antenna near field measurement scenarios with echo signals of unknown origin. Both techniques are based on spatial filtering of the measured field taking advantage of the apriori knowledge of the antenna size. The first approach takes advantages of the spatial filtering properties of the spherical waves expansion of the measured field. The second approach is based on the reconstruction of equivalent currents and implements the spatial filtering as a direct consequence of the selected size and shape of the reconstruction surface. The investigation is performed using measured data on two different horns in both planar and spherical near field scanning geometries. The presence and levels of echo pollution in the measurements are controlled by introducing known scattering objects in the anechoic chambers and comparing to reference situations without disturbance.

Antenna Measurements: Test & Analysis Of The Radiated Emissions Of The Nasa/Orion Spacecraft ~ Parachute System Simulator
John Norgard, November 2012

For future NASA Manned Space Exploration of the Moon and Mars, a blunt body capsule, called the Orion Crew Exploration Vehicle (CEV), composed of a Crew Module (CM) and a Service Module (SM), with a parachute decent assembly is planned for reentry back to Earth. A Capsule Parachute Assembly System (CPAS) is being developed for preliminary parachute drop tests at the Yuma Proving Ground (YPG) to simulate high-speed reentry to Earth from beyond Low-Earth-Orbit (LEO) and to provide measurements of landing parameters and parachute loads. The avionics systems on CPAS also provide mission critical firing events to deploy, reef, and release the parachutes in three stages (extraction, drogues, mains) using mortars and pressure cartridge assemblies. In addition, a Mid-Air Delivery System (MDS) is used to separate the capsule from the sled that is used to eject the capsule from the back of the drop plane. Also, high-speed and high-definition cameras in a Video Camera System (VCS) are used to film the drop plane extraction and parachute landing events. To verify Electromagnetic Compatibility (EMC) of the CPAS system from unintentional radiation, Electromagnetic Interference (EMI) measurements are being made inside a semi-anechoic chamber at NASA/JSC at 1m from the electronic components of the CPAS system. In addition, EMI measurements of the integrated CPAS system are being made inside a hanger at YPG. These near-field B-Dot probe measurements on the surface of a parachute simulator (DART) are being extrapolated outward to the 1m standard distance for comparison to the MIL-STD radiated emissions limit.

Testing Large Wireless Devices In Small Anechoic Chambers
J. Huff,C. Sirles, November 2011

TESTING LARGE WIRELESS DEVICES IN SMALL ANECHOIC CHAMBERS 100_0967100_0973 James D. Huff -20.00-18.00-16.00-14.00-12.00-10.00-8.00-6.00-4.00-2.000.00050100150200Relative Power (dB) Theta Angle (deg) Uncorrected Dipole Patterns 0,0,00,0,120,0,18-20.00-18.00-16.00-14.00-12.00

Large Size, Light Weight, Broadband RF Lens for Far-Field Antenna Measurement
L. Matytsine,P. Lagoiski, S. Matitsine, November 2011

Large size, light weight, broadband convex RF lens was developed to meet far-field requirements for antenna measurements. The Lens was fabricated from low loss, low density meta-materials and has diameter of D=2 m, focusing distance 2.4m and weight of just 50 kg with operational frequency 0.8 to 6 GHz. The lens is able to produce a plane-wave zone with an approximate size of 0.7D, allowing a 2m diameter lens to test antennas up to 1.4m in relatively small anechoic chamber. Another possible application of large size, lightweight RF lens is RCS measurements that include bi-static measurements. Results of quiet zone measurements for different frequencies are presented.

Broadband Free Space Material Measurement System
R. Huang,L. Liu, L. Kong, S. Matitsine, R. Kumaran, R. Balakrishnan, November 2011

This paper introduces a broadband free space material measurement system in Temasek Laboratories at National University of Singapore (TL@NUS). The system is designed by TL@NUS and ST Aerospace for measuring permittivity, permeability, reflection and transmission properties of electromagnetic materials and structures from 1 to 40 GHz. The measurement system includes a pair of double convex spot-focusing lenses, horn antennas, a network analyzer and two arms that can be moved along a circular arc. The two arcs of the arms allow measurement to be done with different incident angles. Each of the double convex lenses is made from two plano-convex dielectric lenses of 77 cm in diameter. The plano-convex lenses can collimate the field from the source horn into uniform plane wave thus also allowing both mono-static and bi-static electromagnetic scattering measurement to be done in very limited space. The system is housed in an anechoic chamber of dimension 6.7 m (D) × 6.6 m (W) × 3.8 m (H) to reduce unwanted reflections and interference signals from the surroundings. Typical measurement results are presented in this paper for dielectric materials, magnetic materials, frequency selective surfaces, and metamaterials.

A "Two–Level GTD" Anechoic Chamber  for VHF/UHF Antenna Measurements:  Design and Experimental Validation
J. Aubin,M. Winebrand, V. Vinogradov, November 2011

Recently ORBIT/FR Inc. has introduced a far – field antenna measurement anechoic chamber design method called “ Two Level GTD “ , which combines shaped chamber walls with a specific absorber layout intended to achieve a better level of reflectivity in the test zone [1-3]. The sidewalls may have the shape of an “inverted open book “, while the back wall may be a pyramidal shape with a small subtended angle at the base. A wedge type foam absorber with a variable orientation of the wedge tips can treat the sidewalls in a “fishbone” layout, while the back wall may be treated by using conventional foam based pyramids. The’ fishbone’ like layout is intended to adverting the reflected waves by the sidewalls out of the test zone, while the back wall pyramid layout is applied to utilize both: the optimum pyramid reflectivity at almost normal incidence; the back wall shape diverting the reflected incident plane or quasi - plane wave out of the test zone. It well known that GO and GTD principles are widely applicable to electrically large structures, delivering a high quality simulation accuracy and good correlation with measurement results. Therefore, the application of the “Two –Level GTD “ is expected to deliver well predicted improvement in the reflectivity of anechoic chambers operating at relatively high frequencies , where the chamber sidewall characteristic dimensions may reach 30. where . is the wavelength at lowest operating frequency. The key question to be answered is - Can the method be successfully applied to cases where the chamber sidewall characteristic dimensions are only – 2-3.? This represents a typical situation in anechoic chambers designed for operation at VHF/UHF frequency bands. In order to answer the question, a full wave 3D simulation has been performed on two anechoic chambers having similar dimensions: 20’ x 20’ x 33’ (L). The two cases are a conventional anechoic chamber and a shaped wall chamber designed based on the “Two – Level GTD” principle. The simulation results were compared, and the “Two - Level GTD” has shown superior performance. Based on these encouraging results, the anechoic chamber was constructed and measurements were performed in the tests zone at a number of frequencies down to 100 MHz. The chamber construction, simulation and measurement results are discussed in the paper below.

Optimization of an Array to Create a Plane Wave in a Chamber with Partially Reflective Walls
E. Walton,J. Holderle, November 2011

Far field measurements of ground vehicle antennas in anechoic chambers often require the creation of a plane wave by near field hemispherical probing with associated mathematical transformations to the far field/plane wave result. Direct far field measurements can be done to save time when the frequency is low enough. This paper discusses a method of extending the frequency band where direct measurements can be done by synthesizing a plane wave using a small array of antennas. The use of an array to create a plane wave in an anechoic chamber usually results in errors due to the reflections from the walls of the chamber. The technique to be described in this paper is to model the wall reflections and the array antenna characteristics and to use optimization techniques to derive an antenna placement and power distribution scheme to optimize the plane wave. Several optimization techniques will be described and results from testing in a 1.2 meter long sub-scale chamber model will be shown. Improvements in the far field measurements will be discussed.







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