Computational Physics Group

An 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.

A 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.

The 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.

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