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An automated instrumentation system has been configured for the purpose of evaluating advanced composites, radar absorbing materials, and frequency selective surfaces (FSS) in free space. Electrical test frequencies are divided into three bands that range from 18 to 60 GHz for any linear polarization. Software has been incorporated to calculate dielectric properties from the measured transmission and reflection characteristics. Using the HP9836 computer, software was written to automate and integrate the Anorad 3253 positioner with the HP8510 network analyzer. This system allows for the input of up to five incident angles at vertical, horizontal, and cross polarization. The measured transmission loss (amplitude and phase) at multiple incident angles is then plotted for comparison. This paper gives a complete description of the system configuration, calibration techniques, and samples of output data. Material properties are computed and compared to specified and theoretical values. Measured results of an FSS structure are compared to its predicted response.
Present day manufacturing techniques often employ composite materials in the fabrication of many structures. Graphite is one common material used to form structurally strong fibers for use in a resin binder. The material characteristics of graphite composites naturally differ from those of metallic materials. An interesting characteristic is the smoothness or roughness of composite materials as examined from an electromagnetic viewpoint. Radar backscatter measurements of several different planar panels were performed near grazing incidence to compare their scattering characteristics against a smooth metallic surface. These results show the "electrical" smoothness of the surfaces in terms of fabrication and material dependencies.
Low RCS signatures require verification of test body conductivity and material performance. A miniaturized radar system with a unique horn antenna was designed for the detection of conductivity gaps and material imperfections in radar absorbing material. The antenna system has a small aperture and low VSWR permitting direct placement against a surface for localization of electromagnetic phenomena. Test results indicate that test body construction gaps and material imperfections are readily detectable using the test system in either a handheld or robotic-type configuration. Preliminary results also indicate delaminations, conductive panel penetration, and structural component steps will be detectable.
Measurements of the microwave reflectivity of materials is often performed with complex test setups using probes attached to a vector network analyzer. The lack of portability of these systems prevents the user from measuring reflective properties of surfaces that are not easily moved to an appropriate test facility. This paper describe a small, hand held microwave reflectometer which is designed to perform rapid reflectivity measurements in the field. The reflectometer consists of a tuneable Ku band source, a dual polarization sampling horn, a pair of crystal detectors, and a battery powered microcomputer.
In practical ISAR applications the quality of the image obtained depends upon the distortions in the wavefront illuminating the target, effects introduced by the radar-target path, the accuracy of the angle and frequency steps used in obtaining the data, vibration, and multiple reflections from neighboring objects. Results of analysis, simulation and data obtained in an RCS compact range are presented to quantify the relationships of the image degradation introduced by these effects.
The objectives of RCS imaging are to spatially isolate and quantitatively measure the strength of scattering mechanisms on complex objects. Although some isolation can be provided directly by using radars with high spatial resolution, most current RCS systems achieve the required resolution by synthesizing the image from measurements of the object response to variations in frequency and rotation angle.
A study of RF testing methods was conducted for the Radarsat SAR antenna. The implementation tolerances of a planar and a cylindrical near-field facility were computed, by simulation of the effects of different types of measurement errors on the reconstructed far field. The results are presented and the two types of near-field facility are compared.
We describe a portable system for performing microwave holography of reflector antennas. This technique derives the complex (amplitude and phase) aperture current distribution from the measured complex far field of an antenna. The amplitude of the current distribution displays directly the effects of feed and support leg shadowing, and illumination taper. The phase of the current distribution is used to optimize feed and/or sub-reflector location, and to generate a table of recommended panel adjustments.
In this presentation we consider the features and performance of a large serrated-edge compact range reflector. This is a straightforward innovation from earlier compact range reflectors. The virtual vertex reflector is a paraboloidal surface truncated to exclude the vertex. This layout provides the advantages of better use of reflector surface area, reduced feed blockage, and reduced feed backscatter. The design is made economical by the use of serrations.
This paper presents a conceptual overview of the instrumentation system and signal processing involved in dynamic RCS and Imaging measurement systems.
This paper will address low RCS target mounting systems. Structural and electromagnetic aspects will be considered. The 4:1 vs the 7:1 ratio ogival shell pylons will be evaluated with consideration given to structural integrity, electromagnetic scattering, and positioner size. Measured and analytic data will be used in these evaluations.
A simple computer program which performs a direct calculation of the Discrete Fourier Transform (DFT) was written to generate the plane wave spectrum of a compact range from sampled field probe data. This program was used to analyze idealized quiet zone fields, computer predicted quiet zone fields and measured field probe data. Data from this analysis is presented along with suggestions for the correct interpretation of the results.
The use of serrated edge treatment in the design of a compact range collimating reflector is one method of mitigating the effects of edge diffraction on quiet zone performance. In this note a physical optics analysis is applied to the serrated reflector. The computational procedure is described and several results are presented. In particular, computed results are presented for the S-A Model 5755 compact range reflector and compared with experiment.
The design, fabrication, and testing of a high directivity, constant beamwidth feed horn is presented in this paper. The subject feed horn is designed to illuminate a shaped reflector compact range operating from 140 to 170 GHz. Design considerations related to pattern control and VSWR are discussed. Fabrication challenges are also considered. Primary pattern test results are presented and compared to predictions. Integration (into the reflector system) considerations are reviewed and quiet zone performance is discussed.
The capabilities of micro computer systems have jumped dramatically in the last few years; their performance now rivals that of mini computers, which have traditionally been used for acquiring and processing antenna measurement data. This paper discusses the significant advantages of using new-generation micro computers for antenna data processing, and provides specific, practical examples of how to use advanced features to process antenna measurement data efficiently.
This paper reports on a project carried out at Georgia Tech to reduce forward scattering from the top edge of far-field range diffraction fences over a wide frequency band. It is shown that the addition of serrations with length greater than ten wavelengths and a flower petal shape reduce the stray radiation in the quiet zone by as much as 10 dB. Several variations on the basic shape are investigated and computed results are shown.
This paper first presents an overview of the types of errors present in phase length measurements. Next, a minimum-error test method is formulated. The first test results using the test method are included, demonstrating accuracies on the order of plus-or-minus 3 electrical degrees at 18 GHz. Measurement time using currently available ANAs is accomplished in less than 1/2 minute when run in the reflective, time-domain mode.
The National Bureau of Standards (NBS) has been measuring antennas and dealing with the problems of leakage for the past twenty years. This paper will discuss the various methods of detecting leakage, typical sources of leakage, how to correct leakage problems, and the effects that leakage can have on calibration.
A new spherical near field test facility is under development by Canadian Astronautics Limited at the David Florida Laboratory in Ottawa. It provides for a wide range of antenna measurements, including far-field, far-field from near field, and near-in and very near-in field reconstruction. Many user-friendly, user-interactive, and graphics features are incorporated. This paper outlines some of the underlying concepts for the facility.
Conventional receivers for pulsed radar systems employ a wideband final filter that is matched to the pulse width and risetime. However for pulsed RCS measurements on small test ranges, instrumentation receivers with narrow IF bandwidth have proven useful. This paper analytically examines the differences between narrowband and matched filter instrumentation receivers and describes typical conditions under which gated CW measurements are made. Useful relationships between PRF and IF bandwidth are derived.