Welcome to the AMTA paper archive. Select a category, publication date or search by author.
(Note: Papers will always be listed by categories. To see ALL of the papers meeting your search criteria select the "AMTA Paper Archive" category after performing your search.)
The requirement to measure lower radar cross-section (RCS) levels within anechoic chambers has demonstrated the need to further analyze the performance of microwave absorbers. The interactions of the feed system, compact range reflector, target mount, and target/test body with the microwave absorber greatly effect both the measurement accuracy and ambient noise level within the anechoic chamber. Better absorber characterization and understanding leads to improved chamber performance analysis and chamber design modeling. Past absorber studies have evaluated the backscatter performance of most absorber types, however, bistatic performance characterizations have been limited. This paper will discuss a method of obtaining bistatic absorber data which offers the advantages of time gating and synthetic aperture imaging to improve measurement isolation and accuracy. The approach involves illuminating a large absorber test wall about several incidence angles with the plane wave generated by a compact range. A receive antenna is then moved about the test wall and bistatic scattering is observed. The technique provides improved measurement results over methods utilizing NRL arch type systems. Bistatic absorber data has been collected and analyzed over angles from normal to near grazing incidence. Test results will be demonstrated with different absorber shapes, sizes, orientations, and material transitions from wedge to pyramidal. Various bistatic conditions will be analyzed for both polarizations over a number of frequencies.
Evaluation methods for analyzing the performance of anechoic chambers have typically been limited to field probing, free space VSWR and pattern comparison techniques. These methods usually allow the users of such chambers to qualify or determine the amount of measurement accuracy achievable for a given test configuration. However, these methods in general do not allow the user to easily identify the reasons for limited or degraded performance. This paper presents a method based on synthetic aperture imagery which has been found usable for finding and identifying anechoic chamber performance problems. Photographs and illustrations of a working SAR imaging/mapping system are shown. Discussions are also given regarding the method's advantages and disadvantages, system requirements and limitations, focusing processing requirements, calibration techniques, and hardware setups. Both monostatic and bistatic configurations are considered and both RCS and antenna applications are discussed. The SAR system constructed to date makes use of a portable HP-8510 based radar placed on a hydraulic manlift for easy system maneuverability and flexibility. The radar antenna is mounted on an 8 foot mechanical scanner directed toward the area to be mapped. An image is processed after each scan of the receive antenna. Measured data and example results obtained using the mapping system are presented which demonstrate the system's capabilities.
Circularly polarized radar cross-section (RCS) measurements place stringent requirements on an RCS range. Indoor compact ranges without the problems of ground reflections have the potential of making accurate circular polarization (CP) measurements. A simple method for CP RCS measurements is described using broadband meander-line polarizers over the compact range feed horns. Axial ratio and differential phase measurements were performed to evaluate the polarizer fabrication accuracy. Basic scattering shapes were measured to test the performance of the CP measurement system. Comparison of CP measurements with analytical predictions demonstrated the success and limitations of the technique.
Compact range facilities designed for RCS measurements have exhibited a performance-limiting effect commonly referred to as "feed ringing". "Feed ringing" is a phenomenon in which energy is stored in or about the RF feed structure and is sustained for a sufficient period off time after the source is turned off such that its presence contaminates the true target return. This effect has placed severe constraints on the design of the RF feed for the compact range, particularly in regard to its operating bandwidth. This paper presents the design of a lossless, waveguide type RF feed suitable for compact range application with a demonstrated useful bandwidth approaching a full octave.
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.
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.
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.
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.
A unique RCS field probe system is described which determines: 1) the two way phase and amplitude field taper, and 2) the RCS measurement error within the quiet zone. The RCS of a suspended target is measured by the radar at selected locations or while moving in the quiet zone. The field taper is obtained from a time gated target return. The quiet zone RCS error for a target is obtained by comparing RCS measurements from anywhere in the quiet zone with the target RCS measured at the center of the quiet zone. A quiet zone containing a high quality illumination field was measured and found to have more than a 5 dB quiet zone RCS error. The RCS error magnitude is dependent upon the radar variables which are determined by the target size. There is a significant difference between the implied RCS error based on the illumination field quality and RCS measurement error caused by the additional contributions of multipath and target dependent clutter that are peculiar to each facility. Accurate RCS measurements require detailed knowledge of the test facility's multipath, target dependent clutter characteristics, and the target's bistatic signature.
To construct an image of a complex target, increasingly smaller range cells are desired. To decrease range cell size and improve resolution the bandwidth must be increased. The bandwidth of RCS measurements utilizing an HP8510 based collection system is limited to a maximum of 801 frequency points. This paper will present a technique to extend the bandwidth by using off-line processing to overcome this hardware limitation. Fully focused ISAR images formed at millimeter wave frequencies, with the addition of eternal mixers, will be demonstrated. Bandwidths of 5, 10, and 14 GHz, measured from 7-17 GHz and 26-40 GHz will be shown. The comparison of these focused images, with 3.2 cm, 1.6 cm, and 1.1 cm resolution will illustrate a powerful engineering tool to analyze closely spaced scatterers.
In the last few years, the interest in Radar Cross Section (RCS) measurements has increased rapidly. The development of high-performance Compact Ranges (CR) has made possible measurements on large targets down to very low RCS levels (below -70 dBsm). RCS imaging is a powerful tool to determine the location of scattering sources on a target. The response of the target is measured as a function of the frequency and aspect angle. A two-dimensional Fourier transform then gives the reflection density as a function of down-range and cross-range. If the response is measured vs. azimuth and elevation, even a complete 3-D image is possible. For high-resolution imaging (large bandwidth, wide aspect-angle span) a direct 2-dimensional Fourier transform gives rise to errors caused by the movement of the scatterers during the measurement. These errors can be corrected by applying a coordinate transformation to the measured data, prior to the Fourier transforms. This so called focused imaging allows further manipulation of measured data. However, the measurement accuracy can be a limiting factor in application of these techniques. It will be shown that the Compact Range performance as well as positioning accuracy can cause serious errors in high-resolution imaging and thus in interpretation of processed data.
We have reviewed the sampling-interval requirement associated with the algorithmic problem of extrapolating near-field radiation measurements to the far zone and concluded that the far-zone sampling rule (d?=?/D) works as well in the Fresnel portion of the near zone. In addition, we find that the angular window, W, over which the Fresnel-zone field must be measured is approximately W - 2D/R radians in width, where D is the nominal diameter of the antenna and R the range at which the near-field data are taken. This guideline is valid when one uses an integral extrapolation scheme, as opposed to a modal one, since the paraxial approximation gives some assurance that field contributions from points outside the sampling window will contribute negligibly to the far-zone amplitude. We have also looked at the sampling requirements associated with extrapolating near-field RCS measurements to the far zone and concluded that windowing techniques can reduce the magnitude of the bistatic scanning task dramatically.
This paper presents a simple and straightforward technique which significantly improves the performance of some anechoic absorbing materials. The method is easily applied to existing absorbers and chambers and does not change the basic design of the material. The technique involves the proper placement of additional absorbing materials between the shaped structures of the absorber to reduce major scattering contributions. These scattering mechanisms are demonstrated in the paper with measured evaluation data for various absorber types and sizes. The effectiveness of the technique has been best realized for pyramidal shaped absorbers 24 inches and longer and for normal plane-wave incidence. Improvements in the absorber's reflectivity of up to 30 dB have been demonstrated. An example illustrating the method for the reduction of the backwall RCS level of a compact-range chamber is presented.
A free-space RF absorber material (RAM) has been developed and optimized for frequencies above 30 GHz. It is particularly suited for use on equipment and fixtures for RCS, antenna, radiometric, and quasi-optical testing. The material has unique geometry which yields enhanced RF performance when compared with conventional wedge or pyramidal absorbers. Mechanically, the material is elastic, resists damage from flexing or repeated contact and is non-flammable and non-toxic. It offers advantages in size, durability, and mechanical uniformity over previously available products. Data describing RF and mechanical performance are presented.
Antenna and Radar Cross Section measurements require a large amount of data collection. Network Analyzers are often used to characterize these systems, and although these data ideally are collected automatically by computer it is not unusual for a single characterization to require many hours or even days to perform. We describe a technique for speeding up these measurements by at least an order of magnitude. Clearly making measurements in an hour that formerly took a day or making measurements in a day that formerly took two weeks is extremely appealing. The method we describe may be used for applications which require a large number of automatically performed measurements with sequentially swept frequencies, but which find lack of speed in tuning the network analyzer to be a limiting factor. Antenna, and Radar Cross Section measurements benefit substantially since frequency response measurements must be repeated many times to provide spatial characterization.
Recent advances in RCS measurement techniques, microwave hardware receiver technology and computer capability have drastically altered the price and performance considerations of turn key RCS measurement systems. Access to 'real time' data and processing improvements are a few of the issues addressed in lower cost and compute intensive configurations available in today's marketplace. This paper explores a systems approach to the wide variety of components configurable for 'state-of-the-art' RCS measurements. High performance, flexibility and productivity are emphasized.
The ISAR image is a domain that possesses many of the spatial physical characteristics of the target. Certain procedures can be performed in the image domain that are equivalent to physical operations on the target. These operations include the ability to modify the amplitude of the scatterers that are represented in the image and, after performing these modifications, subjecting the image to an inverse transformation that recovers RCS data of the whole body as a function of frequency and aspect angle. The RCS plots obtained by transforming the edited image are representative of similar modifications made to the physical body and are of value in eliminating the need for many model modifications and retests in low-observable model development. This paper describes, using simulated and actual target data, some of the procedures that can be fruitfully applied in this type of analysis.
In this paper a new system consisting of a single parabolic reflector and a point source will be presented. Such a system is capable of producing a cylindrical wavefront over a wide frequency range. Moreover, physically large text-zone dimensions can be realized. The principle of operation is identical to that of the near-field/far-field cylindrical scanning, however, the far-field antenna pattern or RCS response can be computed more efficiently by performing a simplified transformation procedure in one dimension only. It will be shown that such a system is suitable for both antenna and RCS measurements. Finally, experimental RCS data will be presented.