Electromagnetic wave absorber for leading edge shape

Abstract

In this study, the development of radar absorbing structures for the application to the leading edge of wing-shaped structure was presented. Recently, as electromagnetic wave absorbers are used to reduce radar cross section (RCS) strength, research for radar absorbing structures (RAS) using composite materials have been actively pursued. Composite materials have been the most attractive and promising solution with de-mand for lightweight and high performance structures. Since composite materials have high specific strength and stiffness compared with bulk materials, their applications range is rapidly expanding from aerospace and military use to commercial areas. Using these merits of composite materials, radar absorbing structures functioning simultaneously as radar absorbers and load bearing structures have been widely studied to reduce RCS. Since the system performance and efficiency is closely related to the weight increase of the structure, such as stealth fighter and UCAV, research on a thin and lightweight RAS was conducted. To satisfy such requirements, firstly, a radar absorbing structure using periodic structure on the surface was intensively studied. A CA absorber using periodic pattern surface was designed in a flat-plate shape through a parametric study that was performed to optimize the device in the X-band (8.2 GHz ~ 12.4 GHz). The measured reflection loss showed excellent performance in X-band with satisfying the -10 dB in whole band. When the CA absorber was applied to the leading edge, the measured echo RCS level showed a 15 dB reduction near 10 GHz and a 10 dB reduction in most of the X-band in both polarizations. Additionally, the circuit analog (CA) absorbing structure compatible with $180^\circ C$ autoclave process was developed. The designed CA absorber with carbon based conducting material showed a 15 dB reduction near 10 GHz and a 10 dB reduction in most of the X-band in both polarizations. From this study, the fabrication process compatible with $180^\circ C$ autoclave process was established. Furthermore, as for improving research, a thin hybrid circuit-analogue (CA) microwave composite absorbing double slab structure was presented. Unlike conventional CA absorbers, the composite spacer in the proposed structure is consisted of a glass/epoxy and high permittivity layer of glass/epoxy-MWCNT composite, which contributes to the stable absorption performance in the curved surface while maintaining the thin thickness of the composite spacer. The total thickness of the designed double slab CA absorber was very thin thickness of 2.37 mm. Although the designed absorber has very thin thickness, it could satisfy the -10 dB absorption in most of X-band. Since the thickness is directly related to the impedance matching, the key design parameters of radar absorbing structure are the thickness of layers for given material properties, permittivities and permeabilities. In most reported literatures, a planar interface between free space and a semi-infinite medium was used for the design of radar absorbing structures, in the context of electromagnetic wave reflections. However, although the design approach is simple, it does not take into account curved surfaces and wave polarizations. Therefore, another design approach is necessary to design curved surfaces like the leading edge of an airfoil. The planar surface approach and cylindrical layer approach to reduce the monostatic RCS of an airfoil was investigated by replacing the leading edge with RAS. Although the planar-surface approach is simple, it could not control both polarizations, and was only effective on one polarization. However, the cylindrical layer approach could control both polarizations and was more effective in controlling the absorbing performance for the wave polarizations. Recently, researches on neutralizing or decreasing the advantages of stealth technologies, referred to as counter stealth or anti-stealth, are actively progressed in military area. Since stealth technology mostly had been focused on defeating monostatic radar systems of narrow frequency bands, broadband or low frequency radar becomes significant threat for conventional stealth aircraft. Usually, most of microwave absorbers are only effective over a narrowband of frequencies. If frequency band is changed to the other frequency bands, a matching point will be deviated from the optimal design, as a result, the absorbing performance is greatly degraded. Therefore, in chapter 5, since the narrowband microwave absorbing structures are of limited use in stealth applications, several broadband microwave absorbing structures to overcome the bandwidth were proposed. Firstly, a novel microwave absorber which consists of a structural part, a resistive sheet, and a low dielectric layer was presented. Unlike the conventional Salisbury screen, a newly proposed absorber is capable of a range of absorbing performance, from narrow to broadband. Secondly, a new design concept of broadband radar absorbing structure consisting of load-bearing glass/epoxy-MWCNT composite and having low density/lightweight characteristics for C- to X-band and X- to Ku-band radar was presented. The presented microwave absorbing structures have triple layers which are the low density and light weight layer to reduce the total weight, covering layers to protect the foam layer from humidity absorption, and load-bearing layer as the actual structural layer. The presented microwave absorbing structure satisfied -10 dB absorption in the C-band to X-band and X-band to Ku-band using only two different materials. Lastly, a broadband microwave absorber design concept using a honeycomb sandwich structure was presented. Unlike the conventional microwave absorbing honeycomb sandwich structure, the newly presented design concept uses the transverse direction of a honeycomb structure with a coated lossy material. When the incident waves reach the inside of the honeycomb coated with the lossy material, multiple scattering occurs inside the honeycomb structure due to the two different refractive indices. Then, the trapped EM waves lose energy due to the coated lossy walls. In that the honeycomb structure can be used in the transverse direction, the effective thickness in terms of the incident EM waves becomes very large. This considerable thickness represents a very effective way to sufficiently attenuate the trapped waves. In this way, a lightweight and broadband absorber could be implemented without the use of a magnetic material and without limitations on the thickness.