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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.
The feed support struts often cause noticeable strut lobes in the patterns of reflector antennas. For example, strut lobes are apparent in the measured and calculated patterns presented in Ref. [1] for the 8-foot diameter reflector with a prime focus feed. As pointed out in [1], the calculated strut lobes are higher than the measured ones. The reason for the difference is secondary scattering by the oppositely located strut, which was not modeled in the calculated pattern in [1]. Detailed examination showed a difference of about 2 1/2 dB caused by the secondary scattering for this reflector antenna design. The purpose of this paper is to present measured and calculated patterns which explicitly demonstrate the quantitative effect of the secondary strut scattering. This effort is shown by comparing the measured strut lobe levels with the oppositely located strut removed, i.e., by using 3 struts instead of 4 struts. Calculated patterns are also given in which the secondary scattering is modeled.
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.
The LX/LH organization of McClellan AFB has been using near-field (NF) technology for the past two years to test an Air Force phased array receiving antenna. McClellan uses both a close range surface RF scanner and a larger offset, fain and back-transform near-field scanner. NF testing is done for both trouble-shooting purposes to support repair efforts, and for acceptance-testing to certify the antenna. The purpose of this paper is to show how McClellan interprets its planar near-field data for diagnosing antenna faults. First the various near-field techniques used by the LX/LH organization will be discussed. Following, will be an examination of the antenna defects pointed out by the NF test date. Failures will be traced to the component level where possible. Techniques other than near-field, such as electronic test, will be used to verify these problems. Additionally, the repair methods will be discussed.
In this paper, a new error correction technique is introduced to improve the accuracy and efficiency of the traditional NRL Arch method. The use of this integrated technique allows one to correct the error terms in the traditional NRL arch setup so that a broadband evaluation of the performance of the absorber product can be performed with much better accuracy and efficiency. This technique also allows one to conduct large bistatic angle evaluation of absorbers without the cross talk and other error signal interferences. Design guidelines for a broadband NRL test arch are provided so as to successfully implement this improved NRL Arch method for a broadband evaluations of anechoic absorbers. Sample test results from Ray Proof's broadband test arch (0.5-6 GHz) are also presented.
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.
This paper describes the significant upgrades to METRATEK's Model 100 AIRSAR Dynamic Imaging System since the earlier version was discussed at last year's conference. This system consists of three wideband radars mounted on a A-3 aircraft. It can generate diagnostic images airborne targets up to 200 feet in length and width. We will present examples and discussions of the solutions found to the many difficulties involved in generating high quality, high resolution, fully-calibrated SAR images of aircraft in flight from aircraft in flight. Data collection and processing hardware and software, as well as lessons learned from over 6 months of flight tests will also be described.
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.
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.
Lockheed has recently completed the construction of a Large Compact Range (LCR) for antenna and RCS measurements. The dimensions of the facility are 60' (h) x 100' (w) x 120' (l) with a 20' x 20' cylindrical quiet zone and operational capabilities from 0.1 to 18.0 GHz. The requirement to measure low RCS levels in a room which is smaller that the desired has resulted in a unique system design. Elements of this design include a feed pit, a feed hood, and a rolled edge reflector; special absorber layouts to minimize background scattering, a high performance instrumentation radar, fast ring down feed antennas, and a unique string suspension and positioning system. This paper presents the various sub-systems that make up the LCR along with chamber validation methods and preliminary performance data. The subsystems listed in this paper are LCR's: Reflector, radar system, feed antennas, feed positioner, absorber, target handling equipment, and string positioning system. Initial design requirements are listed for some sub-systems along with range characterization data such as un-subtracted clutter levels, background subtraction performance, and theory vs. measured data for some simple conical shapes.
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.
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.
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.
In the past, various companies have installed large permanent Near-field antenna measurements systems. In many instances, a test range has been constructed for a particular project or purpose. After the conclusion of the project, the range may become dormant or under-utilized. In addition, a dormant range quickly becomes a potential source for spare parts. These factors combine quickly to render the once functioning range useless. With the current industry emphasis on cost reduction, minimizing new capital purchases, and utilization of existing resources, an upgrade of a dormant test facility is a preferable path. NSI has recently upgraded an existing Near-field antenna measurement system at Hughes Space and Communications Co. hereinafter referred to as Hughes S&C. This paper focuses upon the design considerations undertaken during the upgrade process.
The planar near-field measurement technique is a proved technology for measuring ordinary antennas operating in the microwave region. The development of very low-sidelobe antennas raised the question whether this technique could be used to accurately measure these antennas. We show that data taken with an open-ended waveguide probe and processed with the planar near-field methodology including the probe correction, can be used to accurately measure the sidelobes of very low-sidelobe antennas to levels of -55 to -60 dB relative to the main-beam peak. We discuss the major sources of error and show that the probe antenna interaction is one of the limiting factors in making accurate measurements. The test antenna for this study was a slotted-waveguide array whose low sidelobes were known. The near-field measurements were conducted on the NIST planar near-field facility
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.
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.
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.
Numerous monostatic radar cross-section (RCS) calibration routines exist in the literature. Many of these routines have been implemented at the RCS measurement facility built at the Houston Advanced Research Center in The Woodlands, TX. Key monostatic results are presented to give an indication of the measurement accuracy achievable with this chamber. Unfortunately, bistatic calibration routines are not nearly as common in the literature. As with the monostatic routines, a number of bistatic routines have been implemented and typical results are presented. Additionally, descriptions are given for some of the reference targets along with their support structures that are used during calibration.
Lockheed Sanders, Inc., has constructed a state-of-the-art electromagnetic measurement system. Cost considerations dictated the use of existing facilities and space, We took advantage of the lessons learned from the Lockheed Advanced Development Company's (LADC) Rye Canyon, California Facility [1]. Lockheed Sanders, Inc. now has a complete indoor measurement capability from VHF to MMW. Lockheed Sanders, Inc. needed a facility capable of making measurements over a broad range of frequencies. The system consists of a tapered chamber and a compact range. The system consists of a tapered chamber and a compact range. The tapered chamber has a measurement area of 28' x 28' x 34'. This range is capable of antenna and RCS measurements from .1 to 2 GHz. The compact range is designed for 2 to 40 GHz. Using a Scientific Atlanta, Inc. reflector scaled from the Rye Canyon reflector, a 6' x 6' quiet zone is possible. Feeds consist of a feed cluster aligned for phase and limiting parallax and horn cross-talk. Both chambers use the Flam and Russell 959 measurement system. This paper will discuss the chambers and their operation. The paper will close with a demonstration with measurements on standard, complex targets.