Design and Analysis of Nonmagnetic Specular Radar Absorbing Materials
Abstract: The design and analysis of specular radar absorbing materials are investigated throughout the dissertation. Although the topic has been investigated for more than 60 years by many researchers and numerous designs have been proposed, still many technical challenges are remained unanswered. The basic questions are, e.g., the optimal thickness of a design, the possibility of utilization of unconventional materials in an absorber to meet more design requirements, e.g. thermal or mechanical, than solely the electrical properties, the maximum possible bandwidth achievable with a low profile nonmagnetic coating and the feasibility of designing wideband multilayered absorbers for large scan angles. The thesis proposes systematic solutions for these challenging problems. The dissertation is composed of six peer-reviewed papers. A new versatile design method is proposed in the first paper with outstanding capabilities and remarkable applications. The method is named ''capacitive circuit absorber'' (CCA) and is demonstrated with different design examples to verify its superiority in comparison to the other design approaches. For example, the possibility of utilizing high permittivity dielectric spacers in conjunction with frequency selective surfaces (FSS) in wideband designs is illustrated. The second paper deals with ultra thin absorbers. Recently different proposals based on meta-materials or electromagnetic band-gap structures were suggested for low profile absorbers. The thesis shows that the absorption mechanisms in ultra thin structures are due to excitation of longitude electric field component and no meta-material effect is involved in the absorption process. It is demonstrated that TM cavity modes of patch antennas can approximate fairly accurately the absorption frequency in both periodic and finite extent absorbers. The design of multilayered Jaumann and FSS based absorbers for large scan angles are presented in the third paper. The possibility of extension of the scan and frequency compensation techniques, formerly formulated for single resistive layer designs, to multilayered absorbers is illustrated. It is shown that in contrast to single resistive layer designs, there are some degrees of freedom in the selection of the dielectric layers. Design of ultra wideband absorbers, bandwidth ratios in order of 10:1, with optimal thickness is studied in the fourth paper. It is shown that for achieving an ultra wideband design with optimal thickness, utilization of different spatial periodicities for the periodic layers is essential. By the aid of the physical bound for absorbers it is verified that our design approach leads to optimal total thicknesses. Design of thin wideband absorbers is the topic of the fifth paper, where the challenging problem of reasonable tradeoff between bandwidth and thickness is addressed. The effect of mutual coupling between periodic layers and the ground plane on the frequency response of a thin design is investigated and practical methods for minimizing the couplings are introduced. A thin design is proposed for the X-band which has a total thickness very close to the theoretical limit. Finally in the last paper the physical bound on the absorbers, originally published for normal angle of incidence, is extended to arbitrary angle of incidence for different polarizations. Applicability of the new bounds is examined with different design examples.
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