Welcome to the AMTA paper archive. Select a category, publication date or search by author.
(Note: Papers will always be listed by categories. To see ALL of the papers meeting your search criteria select the "AMTA Paper Archive" category after performing your search.)
Compact ranges are well suited to perform accurate indoor RCS measurements. These facilities are limited at the lower end of their bandwidth by the size of the parabolic reflector. Therefore, when RCS characterizations are required in the UHF band, RCS measurement facilities usually operate large horns or phased array antennas in a near field measurement layout. However, these calibrated near field measurements cannot directly be compared to the plane wave RCS characteristics of the target. One way to compare simulation and measurement results is to take the near field radiation pattern of the antenna into account. This paper first presents the design of a phased array antenna developed for indoor UHF RCS measurements. Then a model of this antenna is derived and a simulation of the experimental layout is performed. In parallel, near field RCS measurements of a canonical target were performed with this phased array antenna in an anechoic chamber. As a conclusion, a comparison between simulation and experimental results on this particular canonical target is discussed.
The two most common test methods used to evaluate wireless test chambers are the Ripple Test Method standardized by CTIA - The Wireless Association and the Field Sensor Method standardized by the 3rd Generation Partnership Project (3GPP). Both methods sample the magnitude of the illuminating field at fixed spatial points in the Test Zone to determine the magnitude of the ripple in the test zone. This ripple data is then statistically processed to determine the expected measurement uncertainty attributable to chamber reflections at a given frequency. The strengths and weaknesses of each of these evaluation methods are discussed in detail. Test results using both methods to evaluate a single chamber are presented. A third wireless test chamber evaluation method is also described. In this method a series of Total Radiated Power (TRP) measurements are made on an antenna with the antenna positioned at various spatial locations in the test zone. If measured with a perfect plane wave, each TRP measurement should produce the same result regardless of the spatial location of the antenna. Variations in measured TRP relate directly to measurement uncertainty caused by deviations of the incident wave from a perfect plane wave.
Both mathematical simulations and experimental results have shown that it is possible to make accurate over-the-air measurements of wireless devices at much shorter range lengths than those indicated by the far-field criteria of 2D2/.. This paper describes a small shielded anechoic chamber designed to minimize the cost and floor space requirements of over-the-air measurements while at the same time providing measurement uncertainties that are comparable to larger chambers whose design is based on the far-field criteria. The design trade-offs are presented and the construction of the chamber described. The chamber was evaluated at different wireless frequency bands using the ripple test procedure from the CTIA Test Plan for Mobile Station Over The Air Performance. Total Radiated Power measurements were also made on gain standard dipoles to determine the uncertainty in integrated measurements. These measurement results are presented.
We report on the initial phase of our study to assess the electromagnetic interference and electromagnetic compatibility measurement and calibration procedures at the National Institute of Standards and Technology. We are developing a measurement-based uncertainty analysis of calibrations and measurements in the anechoic chamber. We intend to characterize all sources of uncertainty, which include power and probe-response measurements, noise, nonlinearity, polarization e.ects, multiple re.ections in the chamber, drift, and probe-position and probe-orientation errors. We present simple and repeatable measurement procedures that can be used to determine each individual source of uncertainty, which then are combined by means of root-sum-squares to state the overall measurement or calibration uncertainty in the anechoic chamber. We report on work in progress and future plans to characterize other EMI/EMC facilities at NIST.
Anechoic chambers simulate open air test conditions and are advantageous for testing avionics systems in a secure, quiet electromagnetic environment. The 412th Electronic Warfare Group’s Benefield Anechoic Facility (BAF), located at Edwards AFB, California was designed for testing systems in the radio frequency (RF) range from 500 MHz to 18 GHz. For frequencies below 500 MHz, the installed radar absorbent material (RAM) does not effectively absorb incident RF energy, thereby allowing undesired RF scattering off the chamber’s floor, ceiling, and walls. This leads directly to measurement inaccuracies and uncertainty in test data, which must be quantified for error analysis purposes. In order to meet the desired measurement accuracy goals of antenna pattern and isolation measurement tests below 500 MHz, RF scattering must be mitigated. BAF personnel developed a test methodology based on hardware gating, range tuning and improved RAM setup, to improve chamber measurement performance down to 100 MHz. Characterizing the chamber using this methodology is essential to understanding test zone performance and thus increases confidence in the data. This paper describes the test methodology used and how the test zone was characterized with resulting data.
We introduce a new type of planar near-field measurement technique for testing small antennas which, heretofore, have been traditionally tested via spherical or cylindrical scanning methods. Field acquisition in both these procedures is compromised to a certain extent by the fact that probe movement induces change in relative geometry with respect to, and thus interaction with, the anechoic chamber enclosure. Moreover, obstructing equipment, such as antenna pedestals, may significantly impede, or even reduce the available angular scope of any given scan. Our proposed procedure, by contrast, minimizes both the residual interaction contaminant and the threat of obstruction. We have in mind here a variant, a hybrid version of planar scanning wherein, on the one hand, we limit severely the size of the acquisition rectangle (and thus minimize the contaminating influence of a variable probe/chamber interaction), while, on the other, we really do collect near-field data throughout a complete range of solid angle around any candidate AUT, front, back, above, below, and on both sides. Such completeness is achieved through the mere stratagem of undertaking six independent planar scans with the AUT suitably rotated so as to expose to measurement, one by one, each of the faces of an enclosing virtual box. In the meanwhile, the inevitable AUT pedestal per se remains immobile and removed from any occupancy conflict with the scanned probe. We have accordingly named our new planar near-field data acquisition scheme the “Boxed Near-Field Measurement Procedure.” With subsequent use of our Field Mapping Algorithm (FMA), elsewhere reported, we obtain the entire field exactly, everywhere, both interior and exterior to the surrounding (virtual) box. In particular, we achieve enhanced accuracy in the far-field patterns of primary interest by virtue of the completeness of data acquisition and its relative freedom from spurious contamination. The angular completeness of data acquisition conferred by our procedure extends in principle to antennas of arbitrary size, provided, of course, that due provision is made for the necessary scope of measurement rectangles. The benefits are seen to be especially valuable in the case of narrow-beam antennas, whose back lobe pattern details, usually deemed as inaccessible and hence automatically forfeited during conventional (i.e., utilizing a “onefaced box,” in our new way of thinking) planar near-field testing, are thrust now into full view. Our new, full-enclosure planar acquisition technique as now described has been verified by analytic examples, as well as by hardware measurements, with excellent results evident throughout, as we are about to demonstrate.
ABSTRACT
The back wall is an important element in a high performance tapered or compact range anechoic chamber operating at VHF/UHF frequencies, as by design it is intended to absorb the non-intercepted portion of the incident plane wave containing the majority of the power transmitted by the chamber illuminator. Back wall reflections may interfere with the direct illumination signal and thus influence the test zone performance. Consequently, in order to ensure that the overall test zone reflectivity specification is met, the reflectivity produced by the back wall should be better than the reflectivity specified for the test zone. The conventional approach used to achieve good reflectivity is to apply high performance, high quality absorbing materials to the back wall. Further improvement of up to 10 dB can be achieved if a Chebyshev absorber layout is implemented [1, 2]. This layout consists of high performance absorbing pyramids of different heights, and assumes that the performance does not depend on a metallic backing plate. This approach is expensive, and presents technical challenges due to the complexity involved in the design and manufacturing of the absorbing material. In addition, installation and maintenance is an issue for such large absorbers. In this paper an alternative approach is presented which is based on an implementation of a shaped back wall as, for example, suggested in [3-5], and use of lighter, lower grade absorbing materials whose performance essentially depends on reflections from the metallic backing wall. This type of design can be optimized at the lowest operating frequency, if the back wall and absorber front face reflections cancel each other. Different back wall shapes are considered for a tapered chamber configuration, and the test zone reflectivity produced by a flat, inverted “open book” and a pyramidal back wall are evaluated and compared at VHF frequencies using a 3D EM transient solver [6].
The primary purpose of a chamber for Far–Field (FF) antenna measurements is to create a test zone surrounding the AUT, where the electric field is to be as uniform as possible, and multiple reflections are kept to a minimum. It is well known, that typical rectangular anechoic chambers for Far–Field (FF) antenna measurements are subject to increased reflectivity from specular regions on the side walls, floor and ceiling. The reflectivity further increases if a larger test zone and, consequently, longer source antenna/ AUT separation is required. The alternative to a rectangular chamber, which can be implemented to reduce the reflectivity, could be a chamber with a shaped interior, where the side walls are to be shaped based on GTD/GO principles so that the reflections are diverted out of the test zone. Even more reflectivity suppression is expected, if, in addition, wedge absorbers are used throughout the specular region or entire wall with a smoothly varied wedge orientation chosen according to GTD principles. The combination of two approaches constitutes a chamber design method termed a “Two – Level GTD”. The chamber shape and wedge orientation for delivering reduced reflectivity in the test zone are not unique. According to a “Two -Level GTD” a plurality of solutions exists and can be practically implemented. Freedom in choosing these parameters can be utilized to satisfy the additional requirements for the chamber design to reduce RCS clutter and/or secondary reflections in the chamber. In this paper the method validity is confirmed based on comparison of various chamber designs performed using 3D EM analysis tools.
This measurement paper presents the procedures used for the quick characterization of WiFi and WiMax commercially available antennas. Measurements were done in the 2.3 GHz to 2.6 GHz region using a short 101” spacing to measure the gain function of the antennas at boresight. Gain was measured against a Standard Gain Horn (made by IFI). The Agilent Vector Network Analyzer was used to perform the measurements and Microsoft Excel was used to plot the data and results. Results seem to indicate most commercially available antennas do not meet the advertised gain parameters and they normally fall short by as much as 50%. The paper shows all the antennas measured results, a comparison against the manufacturer’s advertised gain and their return loss as function of frequency. The measurements were conducted in a few hours per antenna and without the use of complex anechoic chamber.
RF guided missile developers require flight simulation of their target engagements to develop their RF seeker. This usually involves the seeker mounted on a Flight Motion Simulator (FMS) as well as an RF target simulator that simulates the signature and motion of the target. Missile intercept engagements are unique in that they involve highly dynamic relative motion in a short period of time. This puts demanding requirements on the RF target simulator to adequately present the desired phase slope, amplitude, and polarization to the seeker antenna and electronics under test. This paper describes a newly installed RF Target Simulator that addresses these requirements in a unique fashion. The design utilizes a compact range reflector, dynamically rotated in two axes as commanded by the flight simulation computer, to produce the desired changing phase slope and an RF feed network dynamically controlled to produce the desired changing polarization and amplitude. Physical optics analysis establishes an accurate correlation between reflector physical rotation and resulting angle-of-arrival of the wave front in the quiet zone. The RF Target Simulator is self contained in a two-man portable anechoic chamber that can be disengaged from the FMS and rolled to and from the FMS as needed. Measurements are presented showing the performance of the RF Target Simulator.
A large single reflector corner fed rolled edge compact range system, featuring an elliptical cylinder 12’ (H) x 16’ (W) x 16’ (L) quiet zone has been recently installed in a large anechoic chamber [1]. The Compact Range System parameters, such as reflector surface tolerance of better than 0.001” over the Quiet Zone section of the reflector and superior Quiet Zone field performance at frequencies down to 1.0 GHz were verified and validated. As a part of further studies of potential advantages delivered by the compact range system, the study of the compact range application to Antenna and RCS measurements at VHF/UHF frequencies was initiated. Though the reflector surface tolerance is not an issue at the VHF/UHF bands, successful compact range operation at these frequencies would be a significant expansion of the capabilities of the existing compact range system. In order to evaluate the system performance at VHF/UHF frequencies a number of challenging technical issues had to be resolved and performed. They include: Compact Range Quiet Zone Performance Analysis at the VHF/UHF bands Choice of a concept for a broadband feed suitable for the application and installation within the existing feed carousel Feed Design and Performance Validation Feed Installation in the existing feed carousel Quiet Zone Field Probing and Performance Verification All these issues were addressed in the development of a suitable low frequency feed, and are described in more detail below.
Hardware-In-The-Loop chambers provide the chamber designer with many difficult obstacles to overcome in order to establish a high performance environment for the measurement of missile seeker systems. One of the most difficult challenges is to overcome the low performance of absorbing materials at low grazing angles. To solve this problem Tapered R-Card Fences have been used in conjunction with Chebyshev absorbers. Last year we reported on the ATK chamber built in Woodland Hills which showed preliminary test results well within the system requirements. This paper will make a direct comparison of chamber performance with and without Tapered R-Card Fences. The establishment of a sister chamber built in the ATK Alliant Techsystems Inc. ABL facility has provided us with the unique opportunity to test the chamber prior to the installation of the R-Cards and then to test it again with the installation of the R-Cards. This unique opportunity has allowed us a direct comparison of an advanced chamber deign with Chebyshev absorbers as would be utilized in a conventional chamber and the performance increase directly attributable to the introduction of the Tapered R-Cards in the anechoic chamber. The chamber evaluation is carried out utilizing The Ohio State developed TDOA measurement method utilizing their proprietary measurement and analysis software.
Active phased array antennas are often considered for many applications in radar and communications, particularly in millimeter wavelengths. The ability of active phased array antennas to be reconfigured with different beam shapes and pointing directions makes them attractive to increase the flexibility of the next generation of communications satellites so that they can adapt to the needs of a fast changing communications market. The antenna noise temperature of an active array is an important performance parameter, which is difficult to measure compared to a classical passive antenna. Moreover, for a satellite antenna, which has to be evaluated in an anechoic chamber before integration on the spacecraft, the ability to characterise the noise contribution of the antenna itself independent of the environment noise would be very interesting as it would allow better prediction of the antenna performance when it is deployed in orbit on the spacecraft. The paper describes the results of a study undertaken for the French Space Agency (CNES) to devise a new method for the measurement of the noise temperature of a Ka band active phased array antenna when mounted in a Compact Antenna Test Chamber (CATR). An important objective of the study was to find a method which did not rely on the substitution of the antenna under test with a reference antenna, which is the method often used in practice. The method of measurement of noise was based on digital processing of signal to noise ratio rather than analogue detection of noise level, which improves the measurement precision.
Electromagnetic Anechoic Chamber has recently been built by Alenia Aeronautica at Caselle South Plant: The Anechoic Chamber is a full anechoic chamber, and it has been designed to carry out electromagnetic vulnerability tests mainly on fighter and unmanned aircraft. In addition measurement can be carried out on many different vehicles that can be brought into the chamber through the main access door. A system to extract exhaust gas was installed in order to carry out tests on a wide variety of vehicles. The Anechoic Chamber has been designed to carry out both HIRF/EMC test and High Sensitivity RF measurement: in particular HIRF/EMC tests in the frequency range 30MHz ÷ 18GHz with the capability of radiating a very high intensity electromagnetic field and High Sensitivity RF measurement, including antenna pattern measurements on antennas installed on aircraft in the frequency range 500MHz ÷ 18GHz. During the design phase a 1/12th scale model of the chamber had been fabricated to assess the desired electromagnetic performance. In this phase of design the model was tested at the scale frequencies for Filed Uniformity, Site Attenuation and Free Space VSWR results. This study was published at the AMTA 2004 meeting. In addition to the physical model, during the construction phase, various computer simulations were performed to further define the detailed internal absorber layout and to define test acceptance methods for procedures not covered by the standards. The computer model analysis was conducted to identify areas of scattering that could be treated with higher performance absorbers to improve the chambers quiet zone performance. The identified “Fresnel Zones." have been treated with high performance absorbers optimized to provide improved performance at microwave frequencies. The absorber optimization was reported at the AMTA 2006 meeting. This optimization has allowed validation of the chamber according to the requirements of CIRSP 16-1-4 2007-02 in the range of frequency 30 MHz - 18GHz. The size (shield to shield) of chamber is 30m wide, 30m long and 20m high, and the 18m wide by 8.5m high main door allows the SUT access. The shielded structure is a welded structure of 3mm-thick steel panels which guarantees shielding effectiveness of more than 100 dB in the frequency range 100 kHz to 20GHz. The chamber includes a 10 meter diameter turntable to rotate a 30 ton SUT with an angular accuracy of ± 0.02° and a pathway to allow SUT access. Both the pathway and the turntable are permanently covered by ferrite tiles. A hoist system permits lifting of the SUT (max 25 tons) up to 10 meters from the turntable centre enabling EMC testing on aircraft with the landing gear retracted.
Advantages of Far-Field (FF) anechoic chambers utilized for antenna measurements, as compared to conventional outdoor ranges, such as security, interference-free radiation, and immunity to weather conditions allowing broadband antenna measurements on a 24/7 basis, are well known. The dimensions of an anechoic chamber are primarily determined by the lowest operating frequency and are, therefore, significantly increased if operation is required down to VHF and UHF frequency bands. As a result, the advantages of indoor chambers are often disputed when considering low frequency applications. The main counter-argument is the real estate required for chamber construction. In addition, such chambers require the use of high performance absorbing materials, and consequently, chamber certification is always a challenging task. Therefore, rigorous and accurate 3D EM analysis of the chamber is an important procedure to increase confidence, reduce the risk associated with achieving the required test zone performance, and to make the design more efficient. Thus, an accurate simulation of the chamber is even more important these days due to a dramatically growing number of antenna manufacturers supplying products at VHF and UHF bands. Such analysis is a standard procedure at ORBIT/FR, and is described below for the example of a chamber with dimensions of 6m (W) x 6m (H) x 10m (L), operating down to 150 MHz.
This paper gives a detailed account of free space Voltage Standing Wave Ratio (VSWR) method. We first review the formulations and terms commonly used in this method. We then discuss errors involved in its direction determination of extraneous signals, contrasting them among plane wave, spherical wave and specular reflection. We highlight issues relating to its application in anechoic chamber electromagnetic performance. Also discussed is the practice of data processing through analyzing a measured VSWR pattern.
NSI’s MARS technique (Mathematical Absorber Reflection Suppression) has been used to improve performance in anechoic chambers and has been demonstrated over a wide range of frequencies on numerous antenna types. MARS is a post-processing technique that involves analysis of the measured data and a special filtering process to suppress the undesirable scattered signals. The technique is a general technique that can be applied to any spherical or far-field range or Compact Antenna Test Range (CATR). It has also been applied to extend the useful frequency range of microwave absorber to both lower and higher frequencies than its normal operating band. This paper will demonstrate the use of the MARS capability in evaluating the performance of anechoic chambers used for spherical near-field measurements, as well as in improving chamber performance.
This paper presented a theoretical and numerical investigation of the performance comparison of Compact Antenna Test Range (CATR) equipped with Hybrid and Super Hybrid modulated serrations. The performance of the quiet zone will be degraded for a traditional CATR without both an edge treatment of the reflector antenna and a high quality anechoic chamber. Usually, the ripples in both the phase and magnitude of the field intensity inside the quiet zone are caused by stray signals, which come from edge diffraction. In order to solve the edge diffraction problems, edge treatments such as a different shaped serrated edge are often applied along the edge of the reflector antenna. The quiet zone field analysis of such reflectors has traditionally taken the form of ray tracing using numerical integration of the reflector surface currents called physical optics (PO). PO technique is used to obtain Fresnel Zone field of a plane square aperture embedded with a hybrid and super hybrid serrated CATRs. It is observed that super hybrid serrated CATR gives lesser ripples and enhanced quiet zone width than hybrid serrated CATR.
A new method is here proposed to accurately evaluate the Specific Absorption Rate (SAR), e.g. of a mobile phone, through free-space measurements. The method takes advantage of the simple yet powerful plane-wave spectrum (PWS) representation of the electromagnetic (EM) field. The emitting device is tested in an anechoic chamber, where the two tangential components of the electric field are measured (amplitude and phase) and expanded into their PWS. These experimental data are subsequently fed to an equivalent transmission-line representation of the planar stratified structure composed by stacking the half-space made of free-space and the stratified flat phantom. Numerical simulations have shown that this method allows to accurately reconstruct the E field distribution inside a homogeneous phantom, with a worst-case error of 26 % in the estimation of the peak E field [1,2]. Furthermore, the proposed method is the first practical procedure for assessing the SAR in a stratified phantom, where the standard approach of moving a probe inside a liquid-filled phantom is no more feasible.