Computational Physics GroupKarel Matous |
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Optimization of Electromagnetic Scattering in Laminated Composite PlatesK. Matous and George J. Dvorak Department of Mechanical Engineering, Aeronautical Engineering and Mechanics Rensselaer Polytechnic Institute 110 8th Street, Troy, NY 12180 AbstractAn electromagnetic model
of radar absorbing layered structures is analyzed for
several stacking sequences of woven glass/vinyl ester
laminate and foam layers and resistive sheets.
Configurations that are either deposited on different
backing materials or embedded in a laminated sandwich
plate are considered. Through-the-thickness layer
dimensions and sheet resistances offering best signal
absorption over a specified frequency range are found
for each configuration by minimizing an objective
functions with a modified genetic algorithm. The
objective functions include selected values of minimum
reflection coefficients and novel weight function
distributions. In contrast to other optimization
methods, this approach works with a population of
initially selected values of the objective function and
explores in parallel new areas in the search space, thus
reducing the probability of being trapped in a local
minimum. Minimum reflection coefficient of -38.9 dB was
found for a 0-degree incident wave passing through an
optimized Jaumann absorber deposited on a metallic
backing in the 7.5-18 GHz range, a 22.7 % improvement
over a patented design (U.S. Patent No. 4,038,660). Two
additional surface-mounted designs and three sandwich
plate configurations were analyzed in a frequency band
used by marine radars. In general, the surface-mounted
designs have much lower reflection coefficients.
ConclusionA novel approach is outlined to optimization of radar absorbing structures applied either on structural surfaces or within sandwich plate structures. The Jaumann absorber consisting of six dielectric foam layers and six resistive sheets of optimized thicknesses and resistivities, built on a metal backing, provides the best absorption performance, with minimum reflection coefficient of R(min) = -38.9 dB over the frequency range 7.5-18 GHz. This is a 22.7 % improvement over the performance of the patented configuration, based on optimized resistivities and constant spacer thicknesses. Simpler variants of surface arrangements are also very effective, with R(min) = -30.6 dB in the 8-18 GHz bandwidth, as illustrated by the Type Db design. Since the optimized design parameters are very similar for both free and metallic backing, these design should be effective regardless of substrate properties. For applications that require placing of the absorbing layers within a sandwich plate with glass/epoxy laminate faces, the present results suggest three design alternatives involving different numbers of absorbing layers with optimized resistivity values and spacer thicknesses. However, the high permittivity and thickness of the laminate surface layer impairs absorbing capacity of the embedded designs. Although the design optimization was constrained within the narrower 8-12 GHz bandwidth preferred by some marine radars, we found the minimum reflection coefficient of only R(min) = -15.0 dB. Even higher values would obtain in a broader frequency range. In contrast, R(min) = -30.6 and -36.2 dB was reached in the same narrower bandwidth with the Type D designs. AcknowledgmentThe authors appreciate financial support of this work by the Ship Structures and Systems S&T Division of the Office of Naval Research. Dr. Yapa D.S. Rajapakse served as program monitor. © 2009 Notre Dame and Dr.
Karel Matous
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