Investigation of Thin Films for Frequency Selective Surface Based Radar Absorbing Structures

Abstract

Modern day stealth aircraft uses microwave absorbers for absorption of incident Radar signals transmitted by rival aircraft. Recently, several attempts were made to fabricate radar absorbing structures (RAS) of various configurations and embedding frequency selective surfaces (FSS), in the form of periodic 2-D patterns, is one of the approaches. The FSS is fabricated from resistive thin films which are usually coated on a dielectric substate. The use of thin films with optimum properties is essential for achieving broadband microwave absorption. For stealth aircraft structures, there exists a need to synthesize and investigate resistive thin films having properties ranging from good optical transparency, electrical conductivity coupled with good mechanical properties and structural integrity. In this work, sputter coating of thin films of Gold, Indium Tin Oxide (ITO), Silver Nanowire were attempted on Polymethyl Methacrylate (PMMA), a difficult substrate (PMMA) to process in vacuum, and characterized for their optical & electrical properties and environmental stability. Thin film of ITO emerged as the suitable candidate and the film properties were further tailored such that it can be employed as top FSS layer and backplane of the optically transparent RAS. The RAS was designed using CST microwave studio by employing Jerusalem Cross as FSS element. The absorber configuration was optimized for wide bandwidth performance, good angular stability and polarization insensitiveness. Similarly, Nickel thin films were deposited on E-Glass fabric and electromagnetic and mechanical properties were characterized to examine their suitability for load bearing structural members. Integral RAS for both optically transparent and structural applications were fabricated for the optimized configurations. The optically transparent microwave absorber was shown to exhibit polarization independent absorptivity with a bandwidth of 8.5 GHz and good angular stability up to 45o. The absorber for structural applications was shown to possess good absorptivity with a bandwidth of 6.5 GHz in comparison to the bandwidth of 4.5 GHz exhibited by a conventional particle loaded absorber which was also fabricated in this work