AMTA Paper Archive
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Applications of Multilayer Resistive Strips (R-Card) in EM Measurements
A single tapered resistive strip (R-Card) has been used in the past in several applications related to antenna designs and ground bounce reduction for far-field ranges. Several antenna designs use single tapered R-Card to significantly reduce the diffracted fields from the antenna to achieve low side lobe performance and also maintain stable phase center location across wide frequency bandwidth. Single layer R-Card fences have also been successfully designed and used to reduce the ground bounce stray signal in far field ranges. Recently, a multilayer tapered R-Card concept has been investigated and implemented in two different applications for interaction reduction due to performance requirements. One of the applications is to use multilayer R-Card fences to reduce the groundbounce effect between two antennas for GPS applications. The second application is to embed the multilayer R-Card with the Styrofoam target support column used in RCS measurements to reduce the interaction between the target-under-test and the metallic azimuth rotator underneath the Styrofoam column. In both applications, the multilayer R-Card concept, with different resistance distributions and proper spacing, has been designed and evaluated such that it behaves as an absorber to reduce the interference/interaction between two antennas or two scattering objects. The design and evaluation of this new multilayer R-Card concept will be presented in this paper.
Design of Target Support Columns Using EPS Foam: Predictions vs. Measurements
When making large scale RCS measurements on a ground bounce range, EPS foam columns are frequently used as target support structures for test bodies and air vehicles. Thus, the design of foam columns is a key part in preparing for a large-scale outdoor test. Range engineers require foam column design methods and tools that are both efficient and reliable. This paper describes effective foam column design methods and shows comparisons of predicted column RCS to column measurements performed at NRTF. These comparisons give credibility to the concept of foam column modeling and ground bounce range scattering simulations, and give range engineers confidence in their foam column design process.
Time Domain Near Field Focusing to Map Stray Signals in Spherical Ranges
A novel technique for mapping stray signal sources in spherical test ranges is presented. The technique is based on near field focusing. However, instead of the phase information, the time of arrival information is used for focusing. Thus, the technique uses field probe data over a frequency band, and provides good down range resolution. The technique is applied to the field probe data of an experimental outdoor spherical test range. The test range uses R-card fences to suppress ground bounce term in the quiet zone. From the stray signal maps obtained using the proposed technique it is clear that the test range is free of the ground bounce term.
Field Control on a Ground Bounce Range Using Array Technology
When making large scale RCS measurements at outdoor ground bounce ranges, vector background subtraction is often not performed. To get a clean measurement, range engineers must control the backscatter from the target supports that mainly dictate the background level. Presently, nearly all ranges use single high gain antennas to concentrate incident field energy onto the target, but single antennas have physical limitations for controlling the incident field energy in the target support region. To improve the incident field distribution, an array of transmitting elements can be used instead of a single radiator. With an array, engineers can control the illumination in both vertical and cross range dimensions, making it possible to concentrate the incident field energy on the target while reducing the field level over the target supports. This paper describes ground bounce range and incident field modeling, shows beamforming applied to foam column scattering, and demonstrates that a 2-dimensional array can improve the cross range phase taper. It also discusses design sensitivity issues.
Performance of an Experimental Outdoor RCS Range With R-Card Fences
A 30-meter experimental outdoor RCS range designed to operate from 6-18 GHz is described. In the range, the radar antenna height is 60 cm; whereas the center of the quiet zone is 3 meters above ground. The test range, therefore, has features of many real world outdoor RCS ranges. The test range uses six R-card fences with edge taper to eliminate the ground bounce term. Using the quiet zone field probe data and backscatter measurements, it is demonstrated that the R-card fences are very effective in eliminating the ground bounce term.
Design of Target Support Columns Using EPS Foam
When making RCS measurements on a ground bounce range, EPS foam columns are frequently used as target supports for testbodies and air vehicles. Since background subtraction is rarely used to suppress foam column scattering in large scale RCS measurements, the columns must be structurally sound while maintaining a minimized RCS signature over the aspect angles and radar frequency band of interest. The goal is to devise a column that is unnoticeable in the measured data yet strong enough to support a specified weight. The major factor that contributes to EPS foam column scattering is shaping, and finding the optimal shape for a particular test is not trivial. This paper describes methods in the design and construction of EPS foam columns. Subjects include determination of EPS material properties, mathematical specification of column geometries, accurate and efficient computation of column mechanics and scattering, and effective optimization of column parameters.
Ground Bounce Reduction Using a Tapered Resistive Sheet Fence
One problem in a RCS ground bounce range is that the direct signal can be interfered with by the ground reflected signal. The undesired ground bounce signal will cause errors in the RCS measurement. This paper presents a study of ground bounce reduction using a tapered and stepped resistive sheet fence. In order to show that the proposed R-card fence technique can reduce the ground reflected signal significantly, both experimental and theoretical studies are performed. The resistance of the R-card varies based on a Kaiser-Bessel taper function. The experimentall results with and without the R-card fence show that the ground reflected signal can be attenuated by about 20dB. Both vertical and horizontal polarization cases are considered. This paper also the results of a simulation using NEC-BSC (Numerical Electromagnetic Code - Basic Scattering Code, developed at The Ohio State University ElectroScience Laboratory). Comparison of the results between measurements and simulations will be shown in this paper.
Pulsed Transmission Used for Improved Antenna Pattern Measurements
Pulsed systems have been used for many years to eliminate unwanted clutter in RCS measurements, but have not been used much for antenna measurements, even though similar clutter problems are common to both. There are many reasons for this, such as cost, increased bandwidth requirements, lack of necessary hardware, etc. However, with the development of modern pin diode switches, one can construct a low cost pulsed measurement system that simply adds to existing CW equipment. Using the system design presented in this paper, one can eliminate unwanted clutter from antenna measurements simply by adjusting the transmit and receive pulse widths and the delay between them. For example, it can be used to range gate out the ground bounce for outdoor measurements or the backwall for an indoor facility so that one can accurately measure the backlobe of a high gain antenna. The pulsed system is presented along with several measured examples of its use.
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