AMTA Paper Archive
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Development of Standardized Procedures for Antenna Measurement Ranges
The DREO-DFL Antenna Research Lab (DDARLing), contains far-field and planar near-field antenna measurement ranges. Measurements can be made on both ranges from 1.0 to 62.5 GHz. In the early implementation stages of our antenna measurement ranges, most of our energy was absorbed in mastering the mechanics of the positioners and the intracies of the operation of the software, and addressing component failures. To make useful measurements, it is necessary to minimize system errors. Early experience and frustration has led us to the development of an ordered series of standardized procedures that are aimed at careful set-up, calibration, and operation of the ranges. Within these procedures, attention is paid to the identification and minimization of errors due to alignment, equipment calibration, linearity, leakage, multipath, and drift. Following a brief description of the two ranges in the DDARLing facility, the paper provides details of one of these procedures.
Squat cylinder and modified bicone primary static RCS range calibration standards, The
This paper describes the current status of the present cylinder family, and introduces theoretical and experimental RCS data for a modified "bicone" calibration standard. These standards, when used appropriately, greatly improve the quality and efficiency of primary RCS calibration measured within indoor or outdoor ranges. These techniques should offer range owners fairly simple methods to monitor the quality of their primary calibration standards at all times.
Polarimetric calibration of anisotropic materials measurements
Because the incident wave on an anisotropic material is likely to be depolarized, a complete characterization of such a media requires to measure its whole scattering matrix, which afterwards complicates the calibration process. A suitable technic is the Wiesbeck calibration method . In this paper, we apply this method to two configurations, the reflection configuration and the transmission configuration, and obtain very good agreements between theoretical and experimental results.
Wide band feed for a virtual vertex reflector, A
Sanders, A Lockheed Martin Company, measures radar cross section (RCS) and antenna performance from 2 to 18 GHz at the Com pany's Compact Range. Twelve feed horns are used to maintain a constant beam width and stationary phase centers, with proper gain. However, calibration with each movement of the feed tower is required and the feed tower is a source of range clutter. To Improve data quality and quantity, Sanders and The Ohio State University ElectroScience Laboratory designed, fabricated, and tested a new wide band feed. The design requirement for the feed was to maintain a constant beam width and phase taper across the 2 - 18 GHz band. The approach taken was to modify the design of the Ohio State University's wide band feed . This feed provides a much cleaner range which reduces the dependence on subtraction and other data manipulation techniques. The new feed allows for wide band images with increased resolution and a six fold increase in range productivity (or reduction in range costs). This paper discusses this new feed and design details with the unique fabrication techniques developed by Ohio State and its suppliers. Analysis and patterns measured from the feed characterization are presented as well. This paper closes with a discussion of options for further improvements in the feed.
Calibration of probes for near-field scanning at NPL, The
The adoption of planar near-field scanning techniques by many industrial organisations to meet their measurement requirements for large, directive antennas has led to a significant demand for calibrated probes. To compensate for the effects of the probe used in near-field scanning measurements one requires an accurate knowledge of the gain, axial ratio, tilt and pattern. While NPL has been measuring the gain of microwave antenna standards for over seventeen years, it is only in the last two years that facilities and techniques have been developed to measure the polarisation parameters and pattern of probes. For the gain and polarisation, three antenna techniques are employed and both linearly and circularly polarised probes can be calibrated. Since calibration data is required at each frequency at which the planar scanner is to be operated, the measurement techniques and software have been developed to allow measurements to be performed at a large number of frequencies simultaneously. This reduces the turn round time, cost and the need for interpolation between measurement points.
Turnkey near-field measurement system for pulse mode applications, A
NSI recently delivered a Turnkey Near-field Antenna Measurement System (TNAMS) to the Naval Surface Warfare Center - Crane Division (NSWC-CD) in Crane Indiana. The system supports characterization and calibration of the Navy's active array antennas. TNAMS includes a precision 12' x 9' vertical planar near-field robotic scanner with laser optical position measurement system, dual source microwave instrumentation for multiple frequency acquisition, and a wide PRF range pulse mode capability. TNAMS is part of the Active Array Measurement Test Bed (AAMTB) which supports testing of high power active arrays including synchronization with the Navy's Active Array Measurement Test Vehicle (AAMTV), now under development. The paper summarizes the hardware configuration and unique features of the pulse mode capability for high power phased array testing and the TNAMS interface to the AAMTV and AAMTB computers. In addition, range test data comparing antenna patterns with various pulse characteristics is presented.
W-band free space permittivity measurement system for candidate radome materials
This paper presents a measurement system used for W-band complex permittivity measurements performed in NASA Langley Research Center's Electromagnetics Research Branch. The system was used to characterize candidate radome materials for the passive millimeter wave (PMMW) camera experiment. The PMMW camera is a new technology sensor, with goals of all-weather landings of civilian and military aircraft. The sensor was developed by TRW as part of a cooperative agreement for the Defense Advanced Research Projects Agency (DARPA) and the dual-use technology program. NASA Langley manages the program on behalf of DARPA and also supports the technology development and flight test operations. Other members of the consortium include McDonnell Douglas, Honeywell, and Composite Optics, Inc. The experiment is scheduled to be flight tested on the Air Force's "Speckled Trout" aircraft in late 1997. This paper details the design, set-up, calibration and operation of a free space measurement system developed and used to characterize the candidate radome materials for this program.
Bistatic cross-polarization calibration
Calibration of monostatic radar cross section (RCS) has been studied extensively over many years, leading to many approaches, with varying degrees of success. To this day, there is still significant debate over how it should be done. In the case of bistatic RCS measurements, the lack of information concerning calibration techniques is even greater. This paper will present the results of a preliminary investigation into calibration techniques and their suitability for use in the correction of cross-polarization errors when data is collected in a bistatic configuration. Such issues as calibration targets and techniques, system stability requirements, etc. will be discussed. Results will be presented for data collected in the C and X bands on potential calibration targets. Recommendations for future efforts will also be presented.
Interlaboratory comparisons in polarimetric radar cross section calibrations
The National Institute of Standards and Technology (NIST) is coordinating a radar cross section (RCS) interlaboratory comparison study using a rotating dihedral. As an important component of measurement assurance and of the proposed RCS certification program, interlaboratory comparisons can be used to establish repeatability (within specified uncertainty limits) of RCS measurements within and among measurement ranges. The global importance of intercomparison studies in standards metrology, recently conducted comparison studies at NIST, and the status of the first national RCS intercomparison study using a set of cylinders are discussed in . In a companion program, we examine full polarimetric calibration data obtained using dihedrals and rods. Polarimetric data is essential for the complete description of scattering phenomena and for the understanding of RCS measurement uncertainty. Our intent is to refine and develop polarimetric calibration techniques and to estimate and minimize the correstponding measurement uncertainties. We apply theoretical results  to check on (1) data and (2) scattering model integrity. To reduce noise and clutter, we Fourier transform the scattering data as a function of rotation angle , and obtain the radar characteristics using the Fourier coefficients. Calibration integrity is checked by applying a variant of the dual cylinder calibration technique . Future directions of this measurement program are explored.
Improved validation of IER results
Image Editing and Reconstruction (IER) is used to estimate the RCS of component parts of a complex target. We discuss the general areas of controversy that surround the technique, and present a set of practical data processing procedures for assisting in validation of the process. First, we illustrate a simple technique for validating the end-to-end signal processing chain. Second, we present a procedure that compares the original unedited, but fully calibrated, RCS data with the summation of all IER components. For example, if we segregate the image into two components - component of interest, remainder of the target mounting structure plus other clutter - we require that the two patterns coherently sum to the original. This indirectly references the results to the calibration device. In addition, it provides a quantitative means of assessing the relative contribution of the component parts to overall RCS. We demonstrate the procedures using simulated and actual data.
Alignment errors and standard gain horn calibrations
The DTU-ESA Spherical Near Field Antenna Test Facility in Lyngby, Denmark, which is operated in a cooperation between the Danish Technical University (DTU) and the European Space Agency (ESA), has for an ex tensive period of time been used for calibration of Standard Gain Horns (SGHs). A calibration of a SGH is performed as a spherical scanning of its near field with a subsequent near-field to far-field (NF-FF) transformation. Next, the peak directivity is determined and the gain is found by subtracting the loss from the directivity. The loss of the SGH is determined theoretically. During a recent investigation of errors in the measurement setup, we discovered that the alignment of the antenna positioner can have an extreme influence on the measurement accuracy. Using a numerical model for a SGH we will in this paper investigate the influence of some mechanical and electrical errors. Some of the results are verified using measurements. An alternative mounting of the SGH on the positioner which makes the measurements less sensitive to alignment errors is discussed.
Study of a corner reflector of finite thickness
New measurements on the complete polarimetric responses from a 4" dihedral corner reflector from 4 to 18 GHz are presented. As a function of the azimuth, the vertically suspended object may present itself to the radar as a dihedral, a flat plate, an edge, a wedge, and combinations of these. A two dimensional method-of-moment (2-D MOM) code is used to model the perfectly electrical conducting (PEC) body, which allows us to closely simulate the radar responses and to provide insight for the data interpretation. Of particular interest are the frequency and angular dependences of the responses which yield information about the downrange separation of the dominant scattering centers, as well as their respective odd- or even-bounce nature. Use of the corner as a calibration target is discussed.
Mutual coupling measurements of a synthetic aperture Ka-band waveguide array
NASA Langley Research Center (LaRC) is participating in a technology program element in Synthetic Aperture Microwave Radiometry. This includes deployable antenna technology for "sparse" arrays to provide improved spatial resolution with lower mass and less packaging volume. One instrument under consideration includes a deployable L-Band antenna made up of 16 slotted waveguide array elements. Mutual coupling between elements is known to be critical to the calibration and performance of these systems. Currently, waveguide element portions of a 37 GHz "minimum redundancy" array, on loan from the US Naval Research Laboratory, were characterized in an effort to develop a computer model of such a system. Coupling measurements were performed on the WR-28, slotted array elements at spacings out to 50 wavelengths. Measurement results will be used for radiometric modeling and validation of a new coupling prediction code developed at NASA LaRC.
RCS characterization on a portable pit with a foam column at VHF/UHF
The RATSCAT radar cross section (RCS) measurement facility at Holloman AFB, NM is working to satisfy DoD and customer desires for certified RCS data. This paper discusses the low frequency characterization of the RATSCAT VHF/UHF Measurement System (RVUMS). The characterization was conducted on a portable pit with a 30' foam column at the RAMS site. System noise, clutter, backgrounds and generic target measurements are presented and discussed. Potential error sources are examined. The use of background subtraction and full polarimetric calibration are presented. Potential errors, which can occur from using certain cross-pol calibration techniques, are discussed. The phase relationship between each polarization components of the scattering matrix and cross-pol validation techniques are considered.
UWB noise radar using a variable delay line
The Ohio State University ElectroScience Laboratory (OSU/ESL) has built a series of radars that transmit UWB random noise. On receive, the signal is cross correlated with a delayed version of the transmitted signal. When the response of the system is taken as a function of the delay time, the result is proportional to the impulse response of the system. After background subtraction and calibration, the impulse response of the target results. We will present a description of the variable delay line system and show an example ISAR image made from measurements taken in the OSU compact range.
Study of interference between simple objects
New results on the complete scattering matrix measurements during the interference between a sphere and a second object are presented. The objects involved are strings of two sized, a rod, and a dihedral. In cases for the strings or the rod, in-phase oscillations in HH and VV are observed. For the dihedral, the HH and VV responses are exactly out-of-phase. We find that the results are in excellent agreement with the characteristics of the scatterer types. Use of targets other than the sphere for cross-polarized calibrations is discussed.
Accurate gain calibration procedure for large antennas
Gain calibration of circular horns and radiation pattern integration applying patterns in two principle planes only is accurate and does not require large computational or measurement effort. This technique is thus more practical than the integration over the entire angular domain, required in case of rectangular horns. However, for many types of AUT’s, additional errors may occur due to the differences in aperture size of the AUT and standard gain horn. The AUT will in many cases have physically larger aperture dimensions. Consequently, unknown test-zone field variations across this aperture can result in additional errors in gain determination. The new method uses a flat plate as a reference target. An RCS measurement of the flat plate is used to derive test-zone field characteristics over the same physical area as the AUT. Combined with the accurate gain calibration described above, field information is available over the entire area of interest and the accuracy in gain determination is increased. In this paper, experimental results and practical considerations of the method will be presented.
ISAR imaging using UWB noise radar
It is possible to build a very inexpensive radar which transmits wide band radio noise. On receive, the signal is cross correlated with a delayed version of the transmitted signal. In this paper we will discuss the design and operation of a UWB noise radar which was installed in the OSU compact RCS measurement range. Scattering measurements were made for a number of targets over 360 degrees of aspect angle. Calibration was performed, and then the data converted to ISAR images. Example ISAR images will be shown.
Indoor low frequency radar cross section measurements at VHF/UHF bands
In recent years there has been much interest in developing low frequency radar cross section (RCS) measurement capability indoors. Some of the principal reasons for an indoor environment are high security, all-weather 24-hour operation, and low cost. This paper describes recent efforts to implement VHF/UHF RCS measurement capability down to 100 MHz using the large compact-range collimator in the Bistatic Anechoic Chamber (BAC) at Point Mugu. The process leading to this capability has given rise to a number of technical insights that govern successful test results. An emphasis is placed on calibration and processing methodology and on measurement validation using long cylindrical targets and comparing the results with method-of-moment computer predictions and with measurements made at other facilities.
A 160 GHz polarimetric compact range for scale model RCS measurements
A fully-polarimetric compact range operating at 160 GHz has been developed for obtaining X-band RCS measurements on 1:16th scale model targets. The transceiver consists of a fast switching, stepped, CW, X-band synthesizer driving dual X16 transmit multiplier chains and dual X16 local oscillator multiplier chains. The system alternately transmits horizontal (H) and vertical (V) radiation while simultaneously receiving H and V. Software range-gating is used to reject unwanted spurious responses in the compact range. A flat disk and a rotating circular dihedral are used for polarimetric as well as RCS calibration. Cross-pol rejection ratios of better than 40 dB are routinely achieved. The compact range reflector consists of a 60” diameter, CNC machined aluminum mirror fed from the side to produce a clean 20” quiet zone. A description of this 160 GHz compact range along with measurement examples are presented in this paper.
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