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The use of electromagnetic scale models for evaluating the expected performance of a large test facility prior to its construction has been found to be useful in providing insight on how various absorber layouts effect the ultimate performance of the full scale test chamber. This report details the expected and measured performance of a series of absorber sets used in a 1/12 scaled model of a very large anechoic chamber to be used for both EMC and microwave measurements. Electromagnetic scaling of dielectric absorbers involves not just the geometry of the absorbers but also the amount of conductive carbon loading required to achieve a given reflectivity at the scaled frequencies. The goal is to scale performance over a very broad frequency range. It was found that absolute physical scaling is not always possible. Expected and measured performance of the scaled absorbers is detailed over the scaled frequency range of 360 MHz to 12 GHz. Selected measured chamber performance is included for Free Space VSWR, site Attenuation, and Field Uniformity to demonstrate the effectiveness of the scaling.
This paper presents an analysis of the reflectivity performance of the anechoic chamber. Measurements indicating the performance of the chamber-installed foam absorbers (described in a companion paper) are used to complete this analysis. This is followed by a comparison of the analysis results to chamber measurements taken in accordance with the free-space VSWR procedure [1]. Agreement between the analysis results and worst-case VSWR test measurements is within 1dB for a majority of reflection angles. In addition to chamber performance predictions, this paper describes a method of identifying primary reflection paths through interferometer calculations that compare all single bounce reflection path lengths to the direct path length. The angular spacing between interferometer nulls is used to identify the primary reflection direction. This information can be used to improve the overall chamber reflectivity by identifying areas of significant reflections and enhancing absorber treatments in these areas.
Millimeter wave antennas are typically small in physical cross-section, and thus require only a small quiet or test zone illumination area when undergoing standard antenna tests. Lockheed Martin Missiles and Fire Control had a requirement for a test zone diameter of less than 1 foot in order to test millimeter wave antennas required as part of research and development programs. ORBIT/FR developed a unique portable test facility that is inclusive of a “minicompact range” reflector system featuring a rolled edge design with a nominal 12 inch diameter quiet zone. The compact range is integrally mounted into a portable anechoic chamber assembly that measures 60”H x 52”W x 84”L. The chamber features a “hatch” type opening that allows easy access inside the chamber interior, and the entire assembly is easily relocated using a built-in set of casters. An AL-060-1P miniature positioner allows for feed polarization adjustment, and an AL-160-1 provides azimuth rotation for the antenna under test. Corrugated feeds allow precise control of the reflector illumination within the small chamber assembly, allowing excellent quiet zone performance to be realized. Although the primary frequency band of operation is Ka band, the reflector exhibits excellent performance at Ku band, and is capable of operating down to X band as well. The integrated facility is utilized with the Agilent Performance Network Analyzer (PNA) and the 959Spectrum Antenna Measurement Workstation to provide a complete small antenna, high frequency measurement solution. A detailed description of the system, as well as performance results, are presented in this paper.
In order to reach the desired degree of confidence in verifying the required aircraft flight clearances, according to military and civil international standards, Alenia Aeronautica have developed test facilities and EMC test areas which are suitable to perform conducted and radiated tests on fighter and transport aircraft seen as a whole system. Up to now, all tests performed at Alenia’s facilities are intended to be performed in open space, due to several constraints and limitations such as weather conditions effects and future higher EMC certification field level Alenia Aeronautica are designing and implementing a shielded/anechoic chamber, suitable for both HIRF/EMC testing on fighters without engines running and measurement of patter of the antennas mounted on aircraft. This paper includes the modern techniques and the new facility that Alenia Aeronautica are studying and developing at our division of Caselle South (Turin, Italy) are described.
Accurate near-field calibration of a large 60 ft. diameter reflector can be accomplished with a minimal sampling technique. Near-field amplitude and phase is collected as the reflector scans across a receiving calibration tower. The near-field data is then transformed to a far-field pattern using a Fourier transform technique. Information on far-field EIRP, directivity, pointing, axial ratio and tilt, as well as encoder timing is obtained with accuracies comparable to anechoic chamber measurement techniques. The system was analyzed for sampling and multipath effects, as well as the effects of phase and amplitude stability. A spherical wave expansion technique was compared to a straight-forward summation technique for the Fourier transform.
More and more wireless services such as satellite radio (SDAR), navigation systems, OnStar, and mobile telephones are installed in GM vehicles. This has created a need to make quick and accurate vehicle antenna measurements. For the frequency range of 500 MHz to 6 GHz, one solution is to use a spherical near-field system. The Satimo rapid probe array technology was selected to develop a vehicle antenna test system (ATS) to reduce test time and maintain data accuracy. The ATS was designed to operate inside of an existing GM electromagnetic compatibility (EMC) anechoic chamber equipped with a nine-meter turntable. The ATS was completed and received XM certification in the first quarter of 2004. The ATS performs multi-frequency dual-polarized complex measurements for every one-degree in azimuth and elevation, over a full hemisphere, in approximately five minutes. The autonomous transport and deployment system, allows the ATS hardware to be removed and the chamber returned to its EMC configuration. This paper presents the ATS design and a summary of the verification test results. A detailed uncertainty budget, as defined by NIST, is also presented.
The optimum source antenna in an anechoic chamber provides adequate uniform amplitude illumination of the antenna under test, but it minimizes the level of energy reflected from the walls of the chamber. The selection is a function of the range length (R), test aperture (D), source antenna gain (G), and the chamber’s aspect ratio (AR) (range length/width). The latter sets the angle of incidence seen by the absorber on the chamber walls. Adequate phase uniformity is assumed.
The Institute for Integrated Circuits of the Fraunhofer Gesellschaft recently acquired a combined spherical nearfield / far-field (SNF/FF) antenna measurement range with a shielded anechoic chamber for verifying passive and active antenna design concepts. A single 9-pin digital control connector allows the range to remain sealed from external RF, while maintaining full motion and data acquisition control. This set-up uses two different illuminators, separated 180° as seen from the AUT. This combined SNF/FF configuration gives the opportunity to perform intra-range measurement comparisons (SNF vs. FF) with not only the distance between AUT and illuminator being varied, but also with the measurement zone being reversed. In this manner, a comparison between SNF and FF measurements also compares the quality of two sides of the measurement chamber.
An indoor far-field range consists of the appropriate instrumentation and an anechoic chamber. In most of cases, the construction of the anechoic chamber is a laboring task and costs at a great expense. To save the money and labor, efforts for the range design are needed before the chamber been constructed. In this paper, the finite-difference time-domain (FDTD) method is employed to establish the design criteria for the far-field ranges. The commercial package named “FIDELITYTM”, based on FDTD algorithm released by Zeland Software, Inc., is used for the numerical calculations. To emulate the test procedure of the free-space VSWR technique, the electric fields of the points on the scanning axis are recorded during the simulation. And then, by plotting the amplitude ripples calculated from the recorded data, the range performance can be evaluated. The criteria of chamber layout, absorber arrangement, and source antenna selection and placement will be presented and discussed.
Shielded anechoic chambers have been extensively used to measure antennas for various applications. Recent proliferation of mobile telecommunications presented high demands for measurements of antennas that are used in mobile wireless handsets. Since antennas in mobile handsets are low-directive for better mobile links to base stations, they are capable of transmitting or receiving nearly all unwanted reflected signals from imperfections through various reflection and scattering paths in the anechoic chamber in addition to desired signal from the direct path during the measurements. The Quiet Zone (QZ) characterization method has to be re-examined. This paper presents measurements and analyses comparing the difference in chamber designs and verifications of anechoic chamber QZ’s. Through this development, design guidelines are provided to improve the anechoic chamber QZ signal-to-noise ratio for measuring low-directive antennas. Techniques derived from this requirement can also benefit for measurements of high sensitivity Radar-Cross-Section.
This paper describes a family of new measurement systems, termed “test cells”, designed to satisfy the certification requirements of the Cellular Telephone & Internet Association’s (CTIA) “Method of Measurement for Radiated RF Power and Receiver Performance” test plan for wireless subscriber stations. These test cells employ simultaneous dual-axis mechanical scanning and operate in both far-field and near-field modes over the 750MHz to 6 GHz frequency range. Operation can be extended to higher frequencies through the use of suitable sampling antennas. Test cell facility configuration is detailed. Scanner layout and RF sampling antenna designs are discussed. Anechoic chamber characterization data is presented along with typical measured pattern and efficiency data for both broadbeam and directive AUT’s. Measurement test times for various test scenarios are discussed.
Bistatic radar cross sections of targets are computed from field measurements on a cylindrical scan surface placed in the near field of the target. The measurements are carried out in a radio anechoic chamber with an incident plane-wave field generated by a compact-range reflector. The accuracy of the computed target far field is significantly improved by applying asymptotic edge-correction techniques that compensate for the effect of truncation at the top and bottom edges of the scan cylinder. The measured field on the scan cylinder is a “total” near field that includes the incident field, the field of the support structure, and the scattered field of the target. The background subtraction method determines an approximation for the scattered near field on the scan cylinder from two measurements of total near fields. The far fields of metallic sphere and rod targets are computed from experimental near-field data and the results are verified with reference solutions.
The Radar Cross Section (RCS) measurement facility operated by the Stealth Materials Department of BAE SYSTEMS Advanced Technology Centre in the UK is an invaluable tool for the development of low observable (LO) materials and designs. Specifically, it permits the effect of signature control measures, when applied to a design, to be demonstrated empirically in terms of the impact on the RCS. The facility is operated within a 3m by 3m by 12m anechoic chamber where pseudo-monostatic, co-polar, stepped frequency data for a target can be collected in a single measurement run over a frequency range of 2- 18GHz, and for a range of azimuth and elevation angles using a Vector Network Analyser (VNA). The data recorded consists of the complex voltage reflection coefficients (VRC) for the chosen range of aspect angles. This includes data for the target, mount, calibration object, and the associated calibration object mounting where significant. All data processing is conducted offline using a bespoke post processing software routine which implements software time domain gating of the raw data transformed into the time domain prior to calibration. The significant sources of type A (random) and B (systematic) uncertainties for the range are identified, grouped, and an approach to the determination of an uncertainty budget for the complex S21 data is presented. The method is based upon the UKAS M3003 guidelines for the treatment of uncertainties that may be expressed by the use of real, rather than complex numbers. However, a method of assessment of the uncertainties in both real and imaginary parts of the complex data is presented. Finally, the uncertainties estimated for the raw VRC data collected are propagated through the calibration and the uncertainty associated with the complex RCS of a simple target is presented.
Undesirable antenna to antenna coupling has caused EMI problems between the WSC-6 SATCOM system and various systems in many shipboard installations. Long term solutions are currently being explored to resolve this EMI problem, which include adaptive interference cancellers and redesign of the WSC-6 feed and subreflector. However, these solutions are expensive and require several years to develop. An intermediate solution using RAM shrouds around the main reflector and subreflector edges of the WSC- 6 antenna has been proposed. The RAM shrouds were designed to reduce the spillover and diffraction of the antenna while having minimal impact on the antenna performances. A lightweight RAM was chosen to minimize the weight increase of the antenna. A prototype unit with the proposed modifications has been fabricated, assembled and tested in a tapered anechoic chamber, a near-field range, and a compact range. Significant reductions in the WSC-6 antenna sidelobes and backlobe have been verified via these measurements. Highlights of these modifications are described. Measured data (near field, compact range, tapered chamber, and shipboard) are presented.
One basic Direction Finding (DF) technique for Radar is Amplitude Based Comparison DF. Multiple directional antennas are placed around an aircraft to get a 360 deg view of the area. By placing these antennas on the aircraft, the antennas are subjected to reflections from the aircraft, which distorts the antenna characteristics. This antenna distortion causes errors in the measurement of the angle of arrival. The work presented here describes the measurement of the antenna characteristics of a cavity backed spiral antenna both by itself and attached to the nose of an MH- 47A helicopter nose measured in an anechoic chamber. The spiral antenna’s pattern was changed when it was measured on the helicopter. The effect this change in pattern has on the DF accuracy is discussed.
Software GPS receiver development has been undertaken. We are particularly interested in improving the GPS signal-to-noise/interference ratio using a beam forming techniques. The phase relationship among the antenna array elements requires careful calibration. In this study, we will report a phase calibration technique for a 2 by 2 GPS antenna array using both simulated and real GPS signals. This technique is based on the GPS signalprocessing algorithm developed for the software GPS receiver. A four-channel digital data collecting system was used in the experiment. For a simulated GPS signal, the experiment was conducted in an anechoic chamber in which a GPS simulation system was facilitated. For real GPS signals, we conducted the experiment on a rooftop to receive the signal from GPS satellites. The calibration verified the coherent nature of the signals among the elements. The results also allowed the source's direction to be determined.
The radiation properties of an antenna are defined in the far field, since this is the environment that they will operate. Creating far field conditions when testing a large aperture antenna is quite challenging. This is particularly true if testing occurs within the confines of an anechoic chamber, or if other complicating field characteristics (like angle-of-arrival simulation) are desired. Rather than attempt to generate a true planewave in the usual manner, we propose an instrument that creates a field distribution in the near field of a transmit array that is planewave-like in nature only over specified regions of interest (a region occupied by an antenna under test, for example); we do not require that the incident field be a true planewave at other locations. In these other locations the field is free to assume any value demanded by the governing equations of electromagnetics. By relaxing the requirement on the electromagnetic field in the test volume, we considerably reduce the complexity of the problem and define a tractable problem with a potential engineering solution.
This paper describes a novel system that overcomes the inaccuracies in antenna radiation pattern measurements caused by multipath propagation. The system operates by specifically compensating for the effects of unwanted signals rather than by attempting to remove, or minimize, their effects through the use of screens or baffles or an anechoic chamber. Compensation is achieved through the use of an equalization technique, the parameters of the equalizer(s) being determined from a special measurement of the antenna range under consideration. The method is generally applicable; it may be implemented ab initio in new indoor or outdoor ranges, or retrofitted to existing ranges to improve accuracy. Most importantly, however, the basic idea leads to the design of a completely new type of real-time 3-D range in which sensors are placed on the surface of an imaginary sphere surrounding the antenna under test (AUT), and an anechoic chamber is not required.
The true RF spectral response represents In- phase and Quadrature (I and Q) data in the frequency domain, and is identical to that mea- sured in many anechoic chambers including the one at Mission Research Corporation. Given a Linear FM (LFM) response, a method is derived that extracts the true RF spectral response. In the process some basic features of LFM systems are explained. The derivation depends on the assumption that the received signal is zero outside a de¯ned interval. Validation consists of applying the extraction process to both sim- ulated and measured LFM data from the ERIM DCS radar system.
The need for a well-defined accuracy estimate in antenna measurements requires identification of all possible sources of inaccuracy and determination of their influence on the measured parameters. For anechoic chambers, one important source of inaccuracy is the reflection from the absorbers on walls, ceiling, and floor, which gives rise to so-called stray signals that interfere with the desired signal. These stray signals are usually quantified in terms of the reflectivity level. For near-field measurements, the reflectivity level is not sufficient information for estimation of inaccuracy due to the stray signals since the near-to-far-field transformation of the measured near-field may essentially change their influence. Moreover, the inaccuracies are very different for antennas of different directivity and with different level of sidelobes, and for different parts of the radiation pattern. In this paper, the simulation results of a spherical near-field antenna measurement in an anechoic chamber are presented and discussed. The influence of the stray signals on the directivity at all levels of the radiation pattern is investigated for several levels of the chamber reflectivity and for different antennas. The antennas are modeled by two-dimensional arrays of Huygens' sources that allow calculation of both the exact near-field and the exact far-field. The near-field with added stray signals is then transformed to the far-field and compared to the exact far-field. The copolar and cross-polar directivity patterns are compared at different levels down from the peak directivity.