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AMTA Paper Archive

The Commissioning of a fast planar near-field facility
K.S. Farhat (ERA Technology Ltd.),N Williams (DRA (Maritime Division)) E H England (DRA (Maritime Division)), November 1992

Some of the novel mechanical and electronic subsystems features on a recently installed high specification planar near-field scanner are described together with a discussion of the problems encountered during the commissioning period. The test facility incorporates a number of novel design concepts both in terms of its instrumentation, control and processing subsystems. Features of the facility are the speed of data acquisition and the accuracy of the acquired near-field data. Scan speeds of up to 0.8 m/s and positional accuracies of 30 microns in the Z-axis have been achieved, and the near-field data is acquired, displayed and measured on the fly, hence allowing a typical 3m x 3m scan to be executed and the measured near-field results to be displayed and processed within a period of thirty minutes.

Anechoic Chamber Specifications: A Guide
James A. Graham, Jr. (Rantec Microwave & Electronics, Inc.), November 1992

As many institutions and companies have constructed anechoic chambers in the past few years, there has been little work done to codify the specification requirements. Often chambers have been constructed from woefully inadequate specifications resulting in chambers that may be too costly, unable to meet the performance criteria, and in some cases, be unsafe. This paper shall present various model specifications and guidelines to properly specify a chamber complex. Compact ranges, tapered chambers, as well as traditional rectangular chambers will all be examined. How to specify absorbing materials and quiet zone sizes, as well as tradeoffs associated with them, will be discussed. Finally, a guide for coping with facility concerns such as civil, structural, RF shielding, HVAC, electrical, and fire protection will be presented. Examples of good specifications and inadequate specifications will be demonstrated and reviewed.

A High Speed Fiber Optic Remote Receiver Link for Improved Antenna Measurements
Gerard J. Matyas (ORBIT Advanced Technologies, Inc.), November 1992

The remote capability of the ORBIT AL-8000-5 Microwave Receiver is described. The use of a high speed fiber optic link between the remote receiver and the control room unit allows range distances of up to 19,000 feet. With repeaters, the range distance limitation is removed. This eliminates many of the distance cable and EMI problems associated with receivers which use a remote LO. The small size and weight of the remote unit, allow the system to be mounted on the probe carriage of near-field scanner systems. This eliminates the high frequency phase errors as well as the phase error due to cable bending and temperature variation during the measurement. The result is a lower cost and more accurate measurement system. The advantages of this type of remote system are discussed for both near-field and far-field applications. Measurement data which show the performance of the fiber-optic system are presented. A description and pictures of recent installations are to be provided.

A Bistatic RCS Measurement Facility With Imaging Capability
Roland Erickson (National Defence Research Establishment),Olof Lunden (National Defence Research Establishment), November 1992

This paper describes the electrical and mechanical design of an outdoor bistatic RCS test range at the National Defence (sic) Research Establishment (FOA) in Linkoping, Sweden. Some experimental bistatic ISAR imaging results will also be discussed. The 100 m RCS test-range uses a curved rail system. The transmitter rail cart can be moved on a constant distance from the target. This can be illuminated in bistatic angles from 0 to 105 degrees. The measurement system uses fiber optic links for transferring reference signals for coherency. The system has an excellent phase stability that enables ISAR imaging and background subtraction techniques to be used.

Conversion of a Sonar Tank Facility to a Near-Field Scanner
J.E. Friedel (McClellan Air Force Base),H. Luong (McClellan Air Force Base), R.E. Johnson (McClellan Air Force Base), November 1992

Implementation of planar near-field (PNF) technology has become more practical in recent years due to the availability of turn-key measurement systems. McClellan AFT (SM-ALC) has developed an automated PNF measurement system by re-configuring a sonar immersion tank positioner. Modifications to the hardware and software have produced an integrated PNF scanner capable of accurate gain and diagnostic measurements. This paper describes the evolution of the SM-ALC near-field measurement system from proof-of-concept diagnostic scanner to a production tool capable of repeatable gain measurements. Analysis of accuracies, limitations, and processing capabilities is provided. Comparative analysis of data for a transfer standard antenna measured on the SM-ALC measurement system and the PNF measurement system at the National Institute of Standards and Technology is also included.

Measurement of the TDRSS Satellite Deployable Reflector Antennas
Gregory M. Turner (Harris Corporation), November 1992

This paper describes the range design and measurement techniques used for acceptance of the TDRSS Flight 7 Single Access Antennas (SAAs) performed at Harris Corporation Government Aerospace Systems Divison. The TDRSS SAA is a dual-band, shaped, cassegrain antenna that utilizes a 4.9 meter deployable reflector made of a wire mesh fabric. Acceptance testing of the SAAs required a high quality test range and a measurement methodology that would account for the gravity effects in a terrestrial environment. The SAA flight feeds were integrated with a solid reflector that precisely emulates the on-orbit surface of the mesh reflector. Antenna measurements were carried out on a far0field range and the mesh loss and mesh reflector surface effects were subsequently factored into the measured data. Multi-path interference, the primary inhibitor to accurate measurements (gating) as well as conventional methods. The range design included automated data acquisition, data processing and control. The test range and measurement techniques described herein resulted in a successful and efficient evaluation of the TDRSS Flight 7 SAA performance.

Planar and Spherical Near field Range of C.A.S.A.
J.L. Besada (University of Madrid),J. Molina (University of Madrid), A. Valero (University of Madrid), L. de la Fuente (University of Madrid), C.E. Montesano (CASA), A. Montesano (CASA), November 1992

The new antenna measurement facility in C.A.S.A. Space Division is described. The system, designed and installed by Grupo de Radiación of the Polytechnic University of Madrid , provides antenna measurement set-up for Far Field and both Planar and Spherical Near Field.

String support system for RCS measurements
L.A. Snover (Boeing Defense & Space Group), November 1992

The target support system at Boeing Defense & Space Group's 9-77 Compact Range includes a new string support system. The string support system consists of twelve string reels, six each of the High Capacity String Reels (HCSR). The string reel system is used to suspend and manipulate a target for radar cross section (RCS) measurements, primarily at frequencies below 1.5 GHz. The string reel system is capable of supporting targets up to 10,000 pounds and 40' in length and width. The manipulation and handling of targets, is a major consideration in a RCS measurement test plan. The following paper discusses the newly installed string reel system, enhancements to the 9-77 Range equipment which directly affect the use of the string support system, and future developments planned for the system.

Dual polaized constant beamwidth RCS reflector antenna
S. Hendler (Israel Aircraft Industries),G. Lazar (ECI Telecom Ltd.) S. Shammas (Israel Aircraft Industries), November 1992

A reflector antenna has been designed for the RCS measurements. The antenna is dual linearly polarized and exhibits constant beamwidth over an octave bandwidth. The design principle has been to keep the effective antenna aperture constant in terms of the wavelength over an octave bandwidth. The theoretical design lead into the choice of the antenna and the feed. The reflector was an offset parabolic reflector. The feed was a ring-loaded conical corrugated horn. The measurement results of the designed reflector antenna showed very good agreement with the computer results. The V- and H- polarization characteristics of the antenna are almost identical.

The HP 85330A multi-channel, multi-event system controller for high speed antenna and RCS measurement systems
J.M. Moorehead (Hewlett-Packard Company), November 1992

The HP-85330A multi-channel, multi-even system controller was designed to take advantage of the speed, performance and flexibility of automated HP 8530A microwave receiver antenna and RCS measurement systems. In its simplest form, the HP85330A is a VXI mainframe and card that controls high-speed, high-isolation solid-state microwave switch modules. Using the FAST data modes and internal data buffers of the HP 8530A, the measurement system is allowed to run at the maximum speed and performance specification. It accomplishes this through hardware control of the triggering and timing of the HP 8530A microwave receiver, HP8360 series sources, positioner controllers, and external switch modules. For outdoor ranges, the HP 85330A is capable of hardware handshaking with other HP 85330As through balanced twisted-pair wires. Accompanying the HP85330A controller are the HP 85331A and HP 85332A SP2T and SP4T external switch modules. To facilitate remoting the switches, communication between the HP 85330A system interface and 85331/2A is by parallel, twisted pair balanced lines. The lines are capable of distances of 40 meters. The switch modules are individually addressable or can be cascaded to form complex switch trees. Actual measurement throughout data is presented.

Application of the HP 85301A antenna measurement system to receiver pattern measurements
B. Newton (Thorn EMI Electronics),N. Mott (Hewlett-Packard Company), November 1992

As part of the Phalanx weapons system refurbishment program, Thorn EMI Electronics was required to perform a sequence of monopulse feed-pattern measurements at microwave frequencies and receiver-pattern measurements at IF frequencies. The feed-pattern measurements were accomplished using a HP 85301A antenna measurement system. Measurement of the receiver IF patterns, however, required a more novel approach. Because of dual downconversion within the Phalanx receiver, adaptation of the HP 85301A system was required to allow for this frequency translation characteristic. Reconfiguration time between the feed RF and the receiver IF test systems was to be kept to a minimum. The antenna measurement system is described in this paper and IF patterns are presented. The results show that the performance of the test system is not compromised in any way.

Broadband RAM plasma-absorber system theory and experiment
R.J. Vidmar (SRI International),D.G. Watters (SRI International), November 1992

A plasma-absorber system consists of a membrane that confines a collusional gas at atmospheric pressure and an ionization source. The ionization source generates a dense plasma that tenuous near the confinement membrane. An electromagnetic wave propagating through this plasma is attenuated. The mechanism for absorption is momentum transfer among electrons, driven by an incident wave, and a gas. Because the momentum-transfer collision rate, v, at atmospheric pressure can be as high as 870 x 10^9 s-1, the 3-dB bandwidth for absorption (~v/20) is approximately 40 GHz. The plasma thickness between the source and confinement membrane is approximately one wavelength at the lowest frequency. The magnitude of absorption depends on this thickness, the maximum electron number density, and the electron density gradient. A smooth gradient reduces reflections. This paper discusses a theoretical model that predicts general absorption and reflectivity phenomena. Experiments have quantified 40-dB performance at VHF using a 4-mil polyethylene vessel, and at X-band using a 2-mil Mylar inflatable support system. Applications to precision RCS measurements include reduction of backwall reflections and target interaction with the ground plane, and a shutter for reference targets.

New wedge and pyramidal absorber designs
W.D. Burnside (Ohio State University),C.F. Yang (Ohio State University), R.C. Rudduck (Ohio State University), November 1992

For the last few years, the Periodic Moment Method (PMM) has been used to analyze the scattered fields from an infinite absorber wall. Using this approach the absorbe4r can have different periodicities in the x and y directions, as well as arbitrary shapes and any dielectic (sic) distribution. This makes this analysis method very general such that it can treat any conventional wedge or pyramidal designs. Plus, it has been used to develop new ones, which is the subject of this paper. Traditionally there have been chamber uses for both wedge and pyramidal absorber (sic). In a normal RCS range, one uses pyramidal material in forward sector around the feed, wedge absorber through the target zone and pyramids on the backwall. Using this approach, one takes advantage of the basic features of the two types of absorber. To improve wedge material, one is interested in reducing its normal incidence reflection coefficient because the long straight edge is a rather large scatterer. Through the use of the PMM analysis, curved and serrated wedge absorber designs have been developed and tested. Both show significant improvement relative to conventional material. As for the pyramidal model, one would hope to improve its size requirements especially for lower frequencies. Recall that two wavelengths at 100 MHz is twenty feet. By placing twenty foot material throughout a chamber, one greatly restricts the size of the room. Again, the PMM analysis has been used to develop a new curved pyramid design which can perform as well as a conventional pyramid twice its size. Thus, one could use curved pyramids that are ten feet at 100 MHz and achieve the same performance as the commercially available twenty-foot material.

Equivalent layer modeling of pyramid absorbers for field computation in anechoic chambers
C. Bornkessel (University of Karlsruhe),H. Uhlmann (University of Ilmenau), November 1992

Most of the present available pyramid absorber lined anechoic chambers do not meet the new stringent requirements (plus or minus 4 dB criterion of CISPR) for EMC measurements at lower frequencies, say below 100 MHz, due to poor absorber efficiency. In this paper the actual field configuration in those chambers at these critical frequencies is numerically computed for extracting frequency dependent correcting relations for EMC measurements. To this end a finite difference formulation in frequency domain is used. The absorbers are modeled as planar dielectric layers. Examples of computed field configurations are presented and compared with measurement values. The results show the frequency response of the electrical field configuration with respect to the position of device under test, the test antenna, as well as the effect of chamber asymmetries.

Measurement of Constitutive Parameters at Microwave Frequencies
T.P. Fontana (Westinghouse Electronic Systems Group),E.A. Urbanik (Lockheed Sanders, Inc.), November 1992

To characterize a material's electrical performance or to understand a material's affects (sic) on electromagnetic systems, the constitutive parameters (e, u) of the material must be determined accurately. Materials with high dielectric constants, high loss tangents, or which are layered or complex (e.g., frequency selective surfaces, radomes, radar absorbing material, etc.) are difficult to measure and analyze. For example, germanium is an infrared window substrate in high performance aircraft. The germanium is doped to 1 - 4 ()-cm to raise the maximum operating temperature and to provide electromagnetic shielding. The material is very brittle. The standard methods (coaxial, waveguide, and cavity) are difficult to use. The brittle germanium pieces cannot be made thin enough or have a center conductor hole inserted making coaxial donuts are nearly impossible to fabricate and use. Usable waveguide samples absorb the transmitted energy needed in standard waveguide tests. The brittle sample cannot be made thin enough for X-band measurements or above. The sample, having a high dielectric constant, and having a high conductivity, reduces the Q of resonance techniques difficult and not repeatable. This paper discusses our methodology and shows comparisons with calculations. The technique is based on reflection measurements against a ground plane standard. This technique requires more measurements than other techniques, but the results are numerically more stable.

Surface resistivity measurements using a VHF ohms per square test system
D. Jones (Denmar, Inc.),J.E. Lutz (Denmar, Inc.), R.H. Campbell (Denmar, Inc.), November 1992

Many aerospace applications specify materials with precise surface resistivities to satisfy electromagnetic requirements. Frequently, composite materials are physically protected by a layer of dielectric (such as Mylar) or by virtue of being imbedded in a dielectric (such as fiberglass). Conventional resistivity measurement techniques, such as four point probes and busbars, fail in measurement of surface resistivity due to the inability to establish direct electrical contact with the composite structure. This paper describes a new methodology for measuring resistivity of aerospace materials. Capacitive coupling is used in a VHF surface resistivity test system. This system measures ohms per square in a non-destructive, non-intrusive fashion using a capacitive coupled probe technique. Several probe designs have been fabricated and tested. Baseline measurements are presented that illustrate performance against surface resistance which vary from ten to two thousand ohms per square. The effects of different types and layers of resistive materials are also discussed.

Resistivity measurement techniques using a portable reflectometer
J.E. Lutz (Denmar, Inc.),D. Jones (Denmar, Inc.), R.H. Campbell (Denmar, Inc.), November 1992

A quality assurance approach to qualifying target components before and after assembly requires special tools to check specific electromagnetic areas of concern. Of particular concern are the reflection characteristics of multilayered assemblies that may be damaged or have altered performance due to construction practices. This paper describes a methodology for measuring construction for measuring construction or materials problems across a broad frequency range. Bistatic and transmissive test configurations are used to determine specific materials characteristics, including conductivity/resistivity properties. The test methods discussed here and non-destructive, so they may be used on any configuration of interest. The test results that will be presented include Ku-band transmissive measurement of resistive materials for uniformity and general performance. Bistatic measurement results at Ku-band are presented as a technique for isolating problems under or near the surface.

Time domain characterization of UWB antennas
J.S. Gwynne (Ohio State University),J.D. Young (Ohio State University), November 1992

Calibrated measured results are presented that characterize the performance of a rhombic shaped TEM parallel plate horn antenna to transmit and receive ultra-wide bandwidth (UWB) waveforms, the standard narrow band antenna parameters, such as gain, are inadequate in characterizing the antenna. In this paper, the antenna is viewed as a transducer in which the transmitting and receiving antenna can be fully described by complex transfer functions. These functions provide a more natural means of characterizing an antenna for UWB applications. The time domain transmit and receive transfer functions of our test antenna are presented in a contour map as a function of angle for the two principal planes, and the responses are correlated to physical attributes. In addition, the waveform dispersion and the total received energy for a bandwidth limited impulse excitation are used to characterize its use for UWB synthetic aperture radar (SAR) applications.

Measurement distance effects on sum and difference patterns
R.C. Hansen, November 1992

The effects of measurement distance on the sidelobe sum and difference patterns are examined. Highly efficient and robust aperture distributions, the Taylor ñ and the Bayliss ñ, are used to generate date representative of all such distributions. Patterns are obtained through numerical integration of the near-field inegral with exact phase term. Taylor ñ patterns are computed for sidelobe levels to -60 db (published in 1984), and Bayliss patterns for sidelobe levels down to -50 db (new results). For both sum and difference patterns, the change in first sidelobe height, in db, is linear with the log of the measurement distance normalized by 2D(squared)/(lambda). In both cases the lines for different sidelobe levels have the same slope. These results, and typical patterns showing sidelobe changes, will be presented.

Measurements with a superconducting Array Antenna
J.P. Kenney (Hanscom A.F.B. Massachusetts ),J.S. Herd (Hanscom A.F.B. Massachusetts ), L.D. Poles (Hanscom A.F.B. Massachusetts ), November 1992

Using superconducting Yttrium Barium Copper Oxide (YBCO) film for the RF power distribution network of large microwave array antennas can reduce Ohmic losses considerably and thereby increase antenna efficiency. To achieve these benefits the array must be cooled down to liquid Nitrogen temperatures. Typically, in the laboratory, the cooling of the array is achieved by means of a cryostat. Performing transmission measurements with the array antenna encapsulated inside the cryostat housing however, becomes a very challenging measurement procedure. In this paper we will describe this measurement technique and demonstrate the measured performance of a sixteen-element superconducting proximity coupled array antenna.







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