| Radar absorbing material(RAM),a kind of functional materials,can effectively absorb the incident radar electromagnetic wave emitted by the radars and reduce the echo intensity of targets,which is a key factor in stealth technology.In stealth technology,the specular scattering can be effectively suppressed by a good shape design,as has already reported.In comparison,the contribution of the non-specular scattering caused by the surface wave to radar cross section(RCS)remains a non-negligible factor in stealth technology.Thus,it is very important to take account of the surface wave presence in testing the electromagnetic property of RAM as it is often critical to achieving the suppression of surface wave scattering.This dissertation mainly studies the measurement of surface wave suppression of RAM.By distinguishing the surface wave coupling from the coupling of other types of the electromagnetic wave in the transmit-receive system,this dissertation expounds the surface wave propagation equation and analyzes the associated propagation coefficient,which provides the foundation and premise for studying the measurement system of surface wave suppression.Subsequently,the dissertation discusses the setup of the RAM’s surface wave suppression test system and calculates the attenuation rate of different RAMs.In addition,based on different test systems,the dissertation proposes two kinds of ultra-wideband antennas as the excitation sources,which can generate the surface wave that is necessary for the measurement condition.The designs of the slow-wave structure based on microstrip parallel plates and grooves are also presented,which significantly improve the excitation efficiency of the surface wave.Furthermore,the influence of the groove depth of the slow-wave structure on the surface wave propagation is analyzed,and the design of wideband soft impedance surface structure is presented.Finally,the RAMs are measured at different frequencies,and the associated measurement results and errors are analyzed.The results of the surface wave attenuation rate test system of two kinds of RAM are compared and analyzed,providing the guideline for the establishment of material surface wave attenuation rate test standard.The main contributions and research contents of this dissertation are summarized as follows:1.Background on the surface wave theory is introduced first.Then,the development of various testing methods of surface wave inhibition performance of RAM is briefly discussed,such as the surface current test method,the electromagnetic surface wave testing method based on light modulation and so on.As such,the history and the research status of measurement systems of surface wave inhibition performance of RAM are reviewed systematically.2.The propagation mechanism of surface wave is analyzed,and three different forms of surface wave field distribution are discussed.Then,the characteristic parameters of the surface wave,such as the propagation constant,impedance,equiphase surface,equi-amplitude surface,phase velocity,energy velocity,attenuation,and loss are deduced.Finally,the surface wave propagation equation is proposed,and its validity is demonstrated theoretically.This proposed equation provides a powerful theoretical support for the measurement of surface wave suppression performance of RAM.3.A variety of surface wave excitation approaches are briefly discussed,and an excitation approach based on slow-wave structure groove is proposed,which can not only provide efficient surface waves but also can be integrated into the measurement system.In addition,the excitation efficiency of surface wave is studied theoretically,and the way to increase the excitation efficiency of surface wave is put forward.For the analysis of the mutual coupling of electromagnetic energy among antennas,different electromagnetic wave modes that are involved are separated.Based on this idea,the coupling between different surface wave modes,and between the surface wave mode and the free space mode are discussed,respectively.Finally,this dissertation analyzes the transmission directivity of electromagnetic wave coupling and calculates the coupling coefficient between the free-space wave and surface wave.4.Based on the structure consisting of a thin layer dielectric on a metal plate,the principle of the surface wave attenuation rate test is theoretically analyzed.Then,the designs of ultra-wideband double-ridged horn antenna and ultra-wideband antipode Vivaldi antenna are presented.The receiving device and the testing platform of the surface wave are also analyzed and designed.Accordingly,the measurement system is established.Moreover,the procedure of testing RAM samples is presented in detail.Eventually,the surface wave attenuation rate of RAM is tested and the measurement results and measurement errors are analyzed.5.This dissertation designs another measurement system of surface wave inhibition performance based on slow-wave structure.In this system,the microstrip parallel plates and grooves are acted as the excitation source of the surface wave.Then,the measurement principle of attenuation rate of RAM is analyzed,and the excitation source components,i.e.,the exponential taper,parallel plate waveguide and groove are designed respectively.The receiving device,which has the same design as the excitation source due to the symmetrical structure of the measurement system,and the testing platform of the surface wave are also analyzed.As such,a complete measurement system is established.Meanwhile,by analyzing the operating principle of the groove,this dissertation designs the slow-wave structure guiding the electromagnetic wave along the metallic surface and the wideband soft impedance structure suppressing the electromagnetic wave propagation.This constitutes the measurement system of surface wave inhibition performance.Finally,the author introduces the procedure of testing attenuation rate of RAM sample and analyzes the corresponding measurement results and measurement errors.The results of the surface wave attenuation rate test system of two kinds of RAM are compared and analyzed,providing the guidance for the establishment of material surface wave attenuation rate test standard.To conclude,the research of this dissertation is summarized and the direction for future research is given. |
PDF Full Text Request