Investigations On Techniques Of Novel Multi-functional Microwave Absorbers

Microwave absorber is one kind of material or structure,which can absorb the electromagnetic wave.Through some special physical mechanism,the absorber can transform the electromagnetic energy into the heat,thus achieving small reflection.Owing to this special performance,the absorber plays an important role both in the civilian and national defense fields.On the one hand,the absorber can eliminate the mutual interference between electronic devices,realizing electromagnetic shielding;on the other hand,the absorber can absorb the radar signals,which reduces the radar cross sections of the friendly targets,thus achieving electromagnetic stealth.With the rapid development of radar detection technology,to satisfy the requirements of the advanced stealth technology,such as high-efficiency and intelligence,etc.Multiple functions are becoming the development trend of modern microwave absorbers.Therefore,it has great academic and application values to investigate absorbers with special physical properties or functions.In this thesis,aiming at the requirements of the antenna stealth and aircraft stealth applications,based on the printed circuit board technologiey and the paperbased composites technology respectively,in-depth researches on the frequency-selective absorbers and the paper-based honeycomb absorber have been conducted and concluded.The main content of this work can be summarized as follows:1.Design method of frequency-selective absorber with dual passband based on multimode resonance: the design methods of the polarization-insensitive band-pass frequency-selective absorbers are investigated.(a)Based on the equivalent circuit and the impedance matching analysis,the design method of frequency-selective absorber with multiple passband is proposed;(b)By loading slot resonators on periodic structures with capacitive impedance,one broadband low-profile frequency-selective absorber is proposed;(c)Based on the band-pass resonators with broadband resonances,one frequency-selective absorber with enhanced passband is proposed;(d)Based on the periodic structures with multimode resonance,one frequency-selective absorber with dual passband is proposed.2.Design method of highly-selective and switchable frequency-selective absorbers:the design methods of frequency-selective absorbers with highly-selective passband and stopband are investigated and presented,respectively.On this basis,by loading switching diodes and bias circuits,absorbers with switchable functions are proposed.(a)Based on the second-order band-pass resonance and band-stop resonance,the design methods of frequency-selective absorbers with highly-selective passband and stopband are proposed,respectively;(b)By switching the diodes to change the frequency response of the lossless FSS,one absorber with switchable stopband/passband is proposed;(c)By switching the diodes to change the band-pass resonance of the FSSs,one absorber with switchable absorption band/passband is proposed.3.Investigation of the paper-based absorbing structure aiming at aircraft stealth application: by loading the absorbing agent(carbon fiber)on the honeycomb frame directly,one novel technology to implement the honeycomb absorber is proposed.Compared with conventional approaches,the proposed honeycomb absorber avoids the additional coating layers,incorporating the functions of reinforcement and radar absorption in one unit.(a)One novel paper-based composites loaded with carbon fiber is presented and investigated.The absorption mechanism of the material is studied by analyzing the electrical and dielectric properties;(b)One broadband paper-based honeycomb absorber with both absorbing and supporting functions is proposed.By using the equivalent medium theory and electromagnetic simulation,the electromagnetic property and absorbing performance of the honeycomb absorber are analyzed;(c)One reverse electromagnetic parameters analysis method of the honeycomb absorber is proposed.According to the requirements of absorbing performance,the range of the complex permittivity of the honeycomb frame(paper-based material)can be well estimated,which assists to select the proper dimension and content of the absorbing agent.4.Four-dimensional(4-D)paper-based honeycomb absorber based on electromagnetic periodic structure: by loading electromagnetic periodic structures on the inner walls of the honeycomb frame,four-dimensional honeycomb absorber technology is investigated and proposed.Attributed to the resonances of the periodic structures,the performances of the 4-D honeycomb can be well improved in comparision with the traditional structure.(a)Taper-shaped resistive periodic structures are presented to improve the impedance with the free-space.Based on this impedance gradient structure,one ultrawideband honeycomb absorber is designed;(b)Periodic split-ring resonators are loaded in the inner walls of the honeycomb.Due to the regulating effect of the resonantors on the polarization of the electromagnetic waves,honeycomb polarizers with dual-band and broadband characteristics are propposed;(c)The double split-ring resonators are used in the honeycomb to produce magnetic resonance,which results in 4-D honeycomb with designable permeability.