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Anechoic Chamber

Antenna measurements in the commercial world
J.F. Aubin,D.R. Frey, J.D. Berlekamp, November 1995

Due to rapid growth in the RF commercial market, new thinking is required in antenna measurement techniques. Certain customers, such as those designing cellular base station antennas, have unique requirements. One example of this is accurate front­ to-back ratio measurements. This is a difficult measurement to make inside an anechoic chamber, particularly at the currently used commercial frequencies. This paper focuses on a technique for measuring front-to-back ratio, which involves averaging patterns collected at different test antenna positions in order to resolve the chamber back wall reflection from the antenna back lobe measurement.

Method to transform measured Fresnel patterns to far-field based on a least-squares algorithm with probe correction, A
F. Las Heras,B. Galocha, J.L. Besada, November 1995

A method to transform Fresnel field data to far-field data with probe correction, based on a non linear least­ squares algorithm, is presented. The functional to be considered is the expression of the Fresnel field radiated by an array of isotropic sources located on the antenna aperture, and the complex excitations are the coefficients that minimize the rms error between the measured data and the functional values. The intermediate step of determining the complex excitations can be used as a diagnostic tool. Probe pattern correction has been included in the method, improving the performances of antenna measurement systems placed in small size anechoic chambers.

Performance of helicopter mounted HF antennas
C.R. Birtcher,C.A. Balanis, J. Peng, P.A. Tirkas, W.V. Andrew, November 1995

Finite-Difference Time-Domain (FDTD) is prov­ ing to be a practical and accurate technique for an­ alyzing and predicting the performance of anten­ nas mounted on complex structures. As part of an effort to develop and validate an FDTD code, the impedance and radiation patterns of helicopter mounted loop antennas are predicted and compared to full-scale and 1:10 scale measurements. The input impedance and coupling of HF loop an­ tennas on the scale model helicopter are measured in the ElectroMagnetic Anechoic Chamber facility at Arizona State University. Although made difficult by the large mismatch between the highly reactive HF antennas and the instrumentation, the scaled impedance measurements agree well with the full­ scale measurements and predictions. In addition, ro­ tor blade modulation effects on the input impedance are examined.

Anechoic chamber evaluation
K. Haner, November 1994

This paper details the evaluation of a major aerospace company's tapered anechoic chamber. Using an NSI 3' x 3' near-field scanner and software, the chamber was evaluated at 11 frequencies and two polarizations. SAR imaging techniques were used to map the chamber reflections. A new addition to the software provided the ability to map the difference between the measured phase front and the theoretical spherical phase front; the software also derives the x,y,z phase centers of the source. Error estimates for all aspects of the evaluation will be discussed.

Radar absorbing material thermal characteristics
R.M. Taylor,H.D. Reynolds, M. Matteson, November 1994

The Benefield Anechoic Facility, Edwards AFB, California contains a large anechoic chamber for avionic integration test and evaluation. Because of the large chamber size, operational tests can require high-power aircraft radar emissions. To define the range of energy safely accommodated by currently installed radar absorbing material (RAM), a detailed analysis was performed and the results presented. The incident radar energy generates a heat transfer to the RAM. The RAM boundaries dissipate heat through convection, conduction, and radiation. A finite-difference solution demonstrates the temperature distribution in the material varies with the angle and polarization of the incident electric field. Discussions include the use of the RAM thermal characteristic's pretest evaluation to improve operating capability determinations and to facilitate assessment of customer requirements.

Image processing techniques to remove absorber edge effects errors
W.D. Burnside,J-R, J. Gau, M. Beals, November 1994

Absorber is mounted in an anechoic chamber to attenuate stray signals. In this application the stray signals impinge on a whole continuous absorber wall. Consequently, to evaluate chamber performance, one must determine the reflection properties associated with an absorber wall instead of a finite absorber panel. Unfortunately, absorber is normally evaluated experimentally using a finite absorber sample. As a result, absorber measurements are corrupted by edge (end) effect errors. These errors have been observed in measured data using ISAR image techniques, especially for high performance absorbers. One can isolate these error terms by using image filtering. The corrected image is then transformed back to the frequency and angle domains, such that the resulting data will much better represent the true absorber performance. Measured and calculated results will be shown to validate this new method for high performance absorbers.

Influence of geometrical asymmetries on anechoic chamber performance
C. Bornkessel,W. Wiesbeck, November 1994

Due to the new EMC regulations of the European Union (EU) the number of anechoic chambers recommended as EMC test facilities is rapidly growing. Very often, the size and shape of the chamber are dictated by the surrounding building, which may cause asymmetries in the chamber geometry. Sometimes, however the walls and/or the ceiling of the chamber are intentionally sloped to prevent modes from becoming dominant in the cross section. In this contribution, an accurate numerical analysis method, presented and verified at the AMTA '92 and '93 meetings, us applied to evaluate systematically the influence of these geometrical asymmetries on a specific example. It is shown, that the influences of the asymmetries are strongly dependent on the frequency and the actual shape of the chamber. Both parameters wok together in such a complex manner, that ordinary trial and error methods, often used for this purpose are unsuitable for an accurate analysis.

Demonstration of test zone field compensation in an anechoic chamber far-field range
D.N. Black,D.A. Leatherwood, E.B. Joy, R.E. Wilson, November 1994

Test zone field (TZF) compensation increases antenna pattern measurement accuracy by compensating for non-plane wave TZFs. The TZF is measured over a spherical surface encompassing the test zone using a low gain probe. The measured TZF is used in the compensation of subsequent pattern measurements. TZF compensation is demonstrated using measurements taken in an anechoic chamber, far-field range. Extraneous fields produced by reflection and scattering of the range antenna field in the chamber causes the TZF to be non-planar. The effect of these extraneous fields on pattern measurements is shown. Measured TZFs are also shown. TZF compensation results for pattern measurements using a high-gain, X-band slotted waveguide array are presented.

Analysis of anechoic chamber performance
T-H. Lee,J-R, J. Gau, W.D. Burnside, November 1994

One critical issue in designing absorber for an anechoic chamber is the bistatic scattering performance of the absorber and its effect to the quiet zone field quality. The bistatic scattered fields from the absorber side walls, floor and ceiling of the range result in undesired stray signals which can cause significant measurement errors. Consequently, it is very important to analyze the performance of the absorber from the overall system point of view; i.e., the performance of the absorber in the range environment. This paper will address this issue and present calculated results of absorber wall performance for a compact range with a blended rolled edge reflector.

Simulation of actual antenna and chamber under test
T-H. Lee,W.D. Burnside, November 1994

For an anechoic chamber design, one normally spec­ ifies the field quality throughout the quiet zone in terms of the ripple level requirement. The ripple in the quiet zone field is caused by the interfer­ ence of various stray signals with the desired plane wave. The stray signals in an anechoic chamber can come from absorber or other parts of cham­ ber. However, from a range performance point of view, it is more important to know the ef­ fects of stray signals on the measurement accu­ racy of an antenna radiation or target scattering pattern. Consequently, it is very critical to eval­ uate how the chamber stray signals will affect a given measurement. This paper addresses this is­ sue by simulating pattern measurements of a phase scanned array in a compact range and discuss the effects of various stray signals associated with the scattering from absorber walls and feed spillover.

Automated production test facility for a MMW radar system
W.S. Arceneaux, November 1994

Martin Marietta has developed a new, automated facility for high-volume production testing of the Longbow millimeter wave missile. Two dedicated far field anechoic chambers were designed, both automated to support component test and analysis in the production environment. One standard far field chamber is used to perform the complete characterization of the antenna and rac1orne; it allows very accurate measurements of power sidelobes, monopulse errors, and cross­ polarization isolation. The completed radar missile sensor group is evaluated in the second far field chamber, which can reach higher-level parameters of the antenna, transceiver, and gimbal. This paper describes chamber and test station capabilities; time reduction benefits; and the novel, new assembly technique which allows for future portability of these chambers with limited downtime.

Evaluation of compact ranges for low sidelobe antenna measurements
I.J. Gupta,W.D. Burnside, November 1993

A method is presented to qualify a compact range measurement system for low sidelobe antenna measurements. The method uses the target zone fields (probe data) of the compact range. Using the method, one can identify the angular regions around which the measurement errors can be significant. The sidelobe levels which can be measured around these angular regions with less than a 3 dB error are also defined.

In flight VHF/UHF antenna pattern measurement technique for multiple antennas and multiple frequencies
J.S. DeRosa,D. Warren, November 1993

The Precision Airborne Measurement System (PAMS) is a flight test facility at Rome Laboratory which is designed to measure in-flight aircraft antenna patterns. A capability which provides antenna pattern measurements for multiple VHF and UHF antennas, at multiple frequencies, in a single flight, has recently been demonstrated. A unique half space VHF/UHF long periodic antenna is used as a ground receive antenna. Computerized airborne and ground instrumentation are used to provide the multiplexing capability. The new capability greatly reduces time and cost of flight testing. The design, construction, and calibration of the half-space log-periodic ground receiving antenna is discussed and the ground and airborne segments of the instrumentation are described.

HARC/STAR Microwave Measurement Facility: physical description and capabilities, The
B.D. Jersey,A.J. Blanchard, B.A. Williams, B.D. Krenek, W.N. Colquitt, November 1993

A complete description is given of the unique radar cross-section (RCS) measurement facility built at the Houston Advanced Research Center in The Woodlands, TX. The uniqueness of this chamber comes from its ability to independently move the transmit and receive antennas, which can each be moved to any position within their respective ranges of motion to a resolution of about 0.05 degrees. The transmit antenna is fixed in azimuth, but can be moved in elevation: the receive antenna is free to move in both azimuth and elevation. Additionally, the target can be rotated in azimuth by means of an azimuth positioner. Analysis has been performed to determine the impact of chamber effects on measurement accuracy. The most notable chamber effect comes from the two large aluminum truss structures, which are the mounting supports for the transmit and receive antennas. Fortunately, the scattering from these structures can be readily separated from the desired target return through the use of range (time) gating. Time domain results are presented showing the effects of these structures.

Antenna pattern measurement errors evaluation at the INTA compensated compact range
P.L. Garcia-Muller,J-L. Cano, November 1993

The plane wave quality of a compact range (CR) is usually specified in terms of the crosspolar level and the magnitude and phase ripple in the test zone. The way these deviations from the ideal plane wave affect the measurement of different antenna types can be treated by the application of the reciprocity principle between the transmitting and receiving antenna in a measurement set-up. By the application of the sampling theorem, it is found that the measured antenna pattern can be expressed as a summation of the plane wave spectrum components of the field at the test zone weighted by the true radiation pattern of the antenna under test (AUT) evaluated at the CR plane wave directions in the rotated coordinate system of the AUT. The inverse procedure can be used to extract the CR plane wave information (and therefore the CR field at the test zone by means of the Fourier series) from the measurement of a standard antenna with a known radiation pattern.

Design of triad steering antenna arrays for the testing of monopulse antenna seeker systems
J. Land, November 1993

This paper deals with the development of an approach to the design of triad steering antenna arrays which are used in anechoic chambers for hardware-in-the-loop testing of monopulse antenna seeker systems. In the design of a large array, such as those used for hardware-in-the-loop of guided weapons, it is important to optimize the array element spacing. An excessively narrow spacing results in an unreasonable number of required antennas and increased cost, while an excessively wide spacing will induce angle measurement errors in the seeker under test which can be significant. The specific objective of this effort is to quantitatively describe the monopulse discriminant efforts which result when a non-planar field, radiated by an antenna triad, illuminates a monopulse seeker under test. The approach to this problem is to calculate the triad field at the aperture of the monopulse seeker assuming various levels of triad element phase and amplitude error. Using this illumination field and the illumination function of the monopulse antenna, the resulting sum and difference patterns are calculated along with the monopulse discriminant. Software has been developed to perform these calculations. The resulting patterns are compared with the ideal far field pattern and the discriminant bias, or angle measurement error, is quantified.

High-polarization-purity feeds for anechoic chamber, compact, and near field test ranges
R. Gruner,J. Hazelwood, November 1993

With the recent use of dual-polarized transmission and reception on communications links, the capability to perform accurate polarization measurements is an important requirement of test-range systems. Satellite antennas are commonly measured in the clean, protected environment of compact and near-field ranges, and a circularly polarized feed/field probe is a primary factor in establishing their polarization properties. The feeds also provide excellent source-horn systems for tapered anechoic chambers, where their circular symmetry and decoupling of the fields from the absorber walls improve the often troublesome polarization characteristics of tapered chambers. Circularly polarized feeds are generally composed of four primary waveguide components: the orthomode transducer, quarter-wave polarizer, scalar ring horn, and circular waveguide step transformer. Linearly polarized feeds omit the quarter-wave polarizer. This paper discusses the design and performance of high-polarization-purity source feeds for evaluating the polarization properties of antennas under test. Circularly polarized feeds have been constructed which operate over 10- to 20-percent bandwidths from 1.5 to 70 GHz. Gain values are generally in the area of 12 to 18 dBi, with cross-polarization isolation in excess of 40 dB. Representative measured data are presented.

Transportable compact antenna range, A
J.H. Pape,C.L., Jr. Devor, D. Smith, J. Smiddie, November 1993

The Compact Range is becoming the method of choice for indoor testing of many types and sizes of antennas. Implementation of a compact range requires a suitable parent building structure in which to house the chamber. The chamber is located within the parent building and the compact range is then installed within the chamber. In some cases an existing building may not be available for the range and it may be difficult to acquire a new building due to local or proprietary requirements. Once a building has been located, many problems still exist with coordination installation of the chamber and compact range within this building. Overcoming these problems can be both time consuming and inefficient in terms of cost. This paper describes a Compact Antenna Range conceived and designed to be totally self contained and truck transportable. The compact range consists of a complete anechoic chamber facility with self contained electrical, lighting, HVAC and fire protection systems. The compact range provides a 3 foot test zone over the 5.8 to 94 GHz frequency range. Once completed and tested at the factory, the facility is transported and set in place at the user site. Details are presented which describe the structural requirements of the chamber, the RF performance of the completed facility, and the transport and installation process. The integrated test positioner and an automatic feed changing mechanism are also described.

Design and measurements of multi-purpose compact range antenna (CRA)
M. Winebrand,E. Katz, Y. Rosner, November 1993

Traditional Compact Range Antenna (CRA) applications are related to Antenna Pattern and RCS measurements. For these purposes, as a rule, CRA are installed within or outside of an anechoic chamber as stationary equipment. However, for some modern applications, such as Electronic Warfare development, radar tracking system testing, indoor RF environment simulation and others, where dynamic and pointing properties of an AUT are to be tested, the mobile and multi-beam CRA is of great importance, since it provides the designer with powerful simulation and testing capabilities. Such a CRA has been designed, built and tested at ORBIT ADVANCED TECHNOLOGIES, LTD. The design trade-offs, CRA analysis, test set-up and results are discussed in the presented paper.

Simulation and verification of an anechoic chamber
R.M. Taylor,E.S. Gillespie, S.R. Renegarajan, November 1993

This paper considers an electromagnetic field simulation of an anechoic chamber with experimental verification. The simulation is a Geometric Optics (Ray Tracing) mathematical model of the direct path between two antennas and interfering scattering. There are two separate models due to the frequency dependent nature of the pyramidal radar absorbing material (RAM). The model for the frequency range of 30 to 500 MHz was used to characterize the specular scattering. The specular scattering was modeled as a lossy, tapered, TEM transmission line in an inhomogeneous anisotropic tensor material. The frequency range from 500 MHz to 18 GHz was characterized by dominant tip diffraction of RAM patches and the model made use of a Uniform Theory of Diffraction code for a dielectric corner. The measurements and simulations were based on an azimuthal cylindrical scan. Diagnostic measurements were also performed by a cylindrical scan of a directional horn antenna to locate scattering sources in the chamber. A cylindrical wave, modal expansion of the diagnostic data which included a one dimensional Fast Fourier Transform with Hankel function expansions.







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