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RCS

Portable RCS diagnostic system
R. Harris,B. Freburger, D. Maffei, R. Redman, November 1993

This paper describes the most recent version of the Model 200 portable RCS diagnostic radar. The Model 200 was designed to provide high-resolution RCS measurements in unprepared rooms indoors as well as on outdoor ranges. The system can provide real aperture measurements, ISAR measurements, or SAR measurements without changing system configuration.

Ground-to-air RCS diagnostic system
R. Harris,A. Strasel, B. Freburger, C. Zappala, M. Lewis, R. Redman, November 1993

The initial phase of METRATEK's new Model 300 Radar System has been installed at the Navy's Chesapeake Tests Range (CTR) at Patuxent River, MD. This ground-to-air Multimode, Multifrequency Instrumentation Radar System (MMIRS) is a high-throughput frequency-and-polarization agile radar that is designed to drastically reduce the cost of measuring the radar cross section of airborne targets by allowing simultaneous measurements to be made at VHF through Ku Band.

High duty instrumentation radar transmitters
F.A. Miller, November 1993

Today's requirements for dynamic Radar Cross Section (RCS) test data set new demands upon instrumentation Radar systems. Transmitters must deliver high power and operate at high data rates. Additionally, noise floor reduction of coherent spurious signals improves raw data and minimizes the need for manipulation of data.

Demonstration of bistatic electromagnetic scattering measurements by spherical near-field scanning, A
M.G. Cote,R.M. Wing, November 1993

The far-field radar cross section (RCS) of a conducting sphere is obtained by transforming scattered near-fields measured on a spherical surface. A simple and convenient calibration procedure is described that involves measuring the incident field directly at the target location. Although a non probe-corrected transmission formula was used in this study the importance of prove correction in practice is demonstrated.

Vertical bistatic RCS measurements in the MDTI radar measurement center
J.,D. Weatherington, November 1993

This paper demonstrates vertical bistatic Radar-Cross-Section measurement capability in the McDonnell Douglas Technologies, Inc. (MDTI) Radar Measurement Center )RMC). Data will be presented showing the system configuration, system specifications, and predicted and measured vertical bistatic RCF data on a variety of generic targets.

What is RCS in an image?
G Fliss,D. Mensa, W. Nagy, November 1993

Extracting absolute RCS levels from radar images has become a prevalent practice, but is it valid? Scattering strengths associated with pixels in radar images are derived from responses of the target averaged over frequency and aspect angle. This paper presents theoretical and experimental data for simple and complex targets with frequency-and angle-dependent scattering to illustrate differences between results of narrowband and wideband RCS measurements.

Use of an infrared camera system in the analysis of phased array boresight errors, The
R.P. Gray,J.J. Kosch, November 1993

The use of electronically scanned phased array antennas in demanding rolls such as satellite communications and radar systems has led to an increasing desire to analyze the sources of error present in the boresight alignment of such systems. Not the least among these errors are those introduced by thermal effects on the various components which comprise the array structure. In an effort to understand this mechanism, this paper will discuss a technique which uses an infrared camera system to analyze the beam deflection errors caused by the effects of temperature gradients present in the antenna system.

Prediction of phased array antenna sidelobe performance based on element pattern statistics
H.M. Aumann,F.G. Willwerth, November 1993

Phased array antenna sidelobe levels are evaluated based on the statistics of the differences in element patterns. It is shown that the differences can be treated as random errors. The standard formula for predicting the average sidelobe level of an array due to random errors is valid if the interaction between the element patterns and the excitation function is taken into account. Sidelobes of a linear array with a variety of near-field perturbations are considered. The statistics indicate that for an N-element array, adaptive calibrations may lower the average sidelobe level by a factor of N.

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.

New antenna pattern recorder which reduces test time and provides advanced data management capabilities
A.R. Koster,D. Morehead, November 1993

As antennas have become more sophisticated, the testing requirements have grown tremendously. Testing often adds significantly to the cost of the system. A need has developed for test equipment more advanced than the completely manual systems of the past and less expensive than the completely automated systems of today. An antenna pattern recorder which helps to minimize test time is presented. The instrument utilizes a use friendly touch screen which facilitates user interaction with the unit. The pattern recorder is capable of measuring up to five channels of data simultaneously as a function of angle, linear position, or time. The data is stored on electronic media and may be saved, retrieved, zoomed, plotted, analyzed by internal programs or exported for analysis by external programs. The user may customize the plot format for test reports, proposal information, and other data requirements.

Low frequency RCS using the HP-8510
E. Ditata,C. Wegehenkel, November 1993

Northrop Corporation's Business and Advanced Systems Development Group has recently completed a very successful Radar Cross Section (RCS) measurements program on the USAF/Northrop B-2 bomber. One of the capabilities spawned from the program is a measurements radar system, comprised largely of off the shelf hardware, which provides high resolution whole body two-dimensional RCS images of large targets on the ground in the near field. Its high gain antennas allow operation in a space limited area and utilizes Synthetic Aperture Radar (SAR) data collection techniques. The system, though designed for use at VHF, has been expanded to operate from 100-2000 MHz in three bands. The hardware, associated signal processing, its applications and limitations are discussed.

X-band array for feeding a compact range reflector, An
J.P. McKay,L.U. Brown, T.J. DeVincente, Y. Rahmat-Samii, November 1993

The utility of array feeds for compact range reflector antenna applications is discussed. The goal is to feed a circular-aperture, offset parabolic reflector such that the central illumination is uniform and the rim illumination is zero. The illumination taper results in significant reduction of edge-diffracted fields without the use of reflector edge treatment. A methodology for designing an array feed requiring only two real excitation coefficients is outlined. A simple and cost effective array implementation is presented. The array beam forming network is realized as a radial transmission line which is excited at the center from a coaxial transmission line, and terminated at the perimeter with absorber and conductive tape. Energy is probe-coupled from the radial line to balun-fed dipole array elements. The required element amplitude excitation is obtained by adjusting the probe insertion depth, and the required element phase excitation is supplied by the traveling radial wave. Construction and test of an X-band array are summarized. The measured array patterns display a flat-topped beam with a deep null at angles corresponding to the reflector rim.

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.

Transverse pattern comparison method for characterizing antenna and RCS compact ranges, The
S. Brumley, November 1993

This paper briefly reviews existing compact range performance characterization methods showing the limitations of each technique and the need for an accepted and well understood technique which provides efficient and accurate characterization of compact range measurement accuracy. A technique known as the transverse pattern comparison method is then described which has been practiced by the author and some range users for the past several years. The method is related to the well known longitudinal pattern comparison method, however, comparisons are conducted in the transverse planes where the required span of aperture displacement is much smaller and does not exceed the dimensions of the quiet zone. This method provides several advantages for characterizing compact range performance as well as enables range users to improve achievable measurement accuracies by eliminating the impact of extraneous signal errors in the quiet zone.

Implementation of a 22' x 22' planar near-field system for satellite antenna measurements
G. Hindman,G. Masters, November 1993

Design and implementation of a large horizontal planar near-field system delivered to Space Systems/Loral for satellite antenna testing will be discussed. The 22' x 22' scan plane is 25' above the ground and employs real-time optical compensation for the X, Y, Z corrections to the probe position. The system provides high speed multiplexed near-field measurements using NSI's software and the HP-8530A microwave receiver. System throughput is enhanced through the use of a powerful and flexible test sequencer software module.

Application of Prony's method to software gating
C.W. Trueman,C.L. Larose, M. Flynn, S. Mishra, November 1993

Prony's method has been found useful in extracting the time domain response over extended time using data samples of limited time span. This paper describes results of studies underway to apply Prony's method to extracting the RCS time and frequency response from limited measured and computed data. The technique has been applied to characterize the RCS response of structures with inherent multiple resonances, e.g., a dielectric cube constructed using high-permittivity dielectric material. Implications of the technique to gated antenna and RCS measurements are discussed.

Effect of spherical measurement surface size on the accuracy of test zone field predictions, The
D.N. Black,E.B. Joy, J.W. Epple, M.G. Guler, R.E. Wilson, November 1993

The field present in the test zone of an antenna measurement range can be calculated from the range field measured on a spherical surface containing the test zone. Calculated test zone fields are accurate only within a spherical volume concentric to the measurement surface. This paper presents a technique for determining the probing radius necessary to create a volume of accuracy containing the test zone of the range. The volume of accuracy radium limit is caused by the spherical mode filtering property of the displaced probe. This property is demonstrated in the paper using measured field data for probes of differing displacement radii. This property is used to determine the volume of accuracy radium from the probing radius. This is demonstrated using measured far-field range data.

Generation of wideband information from a few samples of data
R. Adve,T.K. Sarkar, November 1993

The Method of Cauchy has been used to extrapolate a desired parameter over a broad range of frequencies using some information about the parameter as a few frequency points. The approach is to assume that the parameter, as a function of frequency, is a ratio of two polynomials. The problem is to determine the order of the polynomials and the coefficients that define them. For theoretical extrapolation/interpolation the sampled values of the function and, optically, a few of its derivatives with respect to frequency have been used to reconstruct the function. This technique also incorporates the method of Total Least Squares to solve the resulting matrix equation.

New extrapolation/spherical/cylindrical measurement facility at the National Institute of Standards and Technology, A
J. Guerrieri,D. Kremer, T. Rusyn, November 1993

A new multi-purpose antenna measurement facility was put into operation at the National Institute of Standards and Technology (NIST) in 1993. This facility is currently used to perform gain, pattern, and polarization measurements on probes and standard gain horns. The facility can also provide spherical and cylindrical near-field measurements. The frequency range is typically from 1 to 75 GHz. The paper discusses the capabilities of this new facility in detail. The facility has 10 m long horizontal rails for gain measurements using the NIST developed extrapolation technique. This length was chosen so that gain calibrations at 1 GHz could be performed on antennas with apertures as large as 1 meter. This facility also has a precision phi-over-theta rotator setup used to perform spherical near-field, probe pattern and polarization measurements. This setup uses a pair of 4 m long horizontal rails for positioning antennas over the center of rotation of the theta rotator. This allows antennas up to 2 m in length to be accommodated for probe pattern measurements. A set of 6 meter long vertical rails that are part of the source tower gives the facility that added capability of performing cylindrical near-field measurements. Spherical and cylindrical near-field measurements can be performed on antennas up to 3.5 m in diameter.







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