Design And Application Of Broadband Microwave Windows Characteristics Of Frequency Selective Surfaces

For low-visibility radar platforms such as military aircraft,the overall Radar Cross Section（RCS）is mainly contributed by the antenna scattering system.Therefore,antenna stealth has became the key technology for aircraft and attracted much attention.The electromagnetic absorbing structure of the Frequency Selective Surface（FSS）can be used not only as a bandpass radome,but also as backplane of the antenna to reduce the RCS.In this thesis,combining with the radiation characteristics of different antennas,we design a broadband absorber to reduce the RCS of Vivaldi array antenna,and a frequency absorption selective reflector（FASR）structure with broadband reflection to reduce the RCS of microstrip dipole antenna,respectively.In order to achieve the purpose of antenna stealth.Firstly,the reflection coefficient of the transmission line is derived from the perspective of the circuit and electromagnetic field,respectively,and the rationality of analyzing and designing FSS structures by the equivalent circuit method is explained.A segment distribution estimation algorithm is further investigated,by which circuit parameters satisfying specific performance specifications can be fitted.A FASR structure with narrow-band reflection is designed using equivalent circuit theory,and the effectiveness of the equivalent circuit is proved by the consistency of full-wave model and circuit results.And then,the square ring unit structure FSS broadband absorber with resistance loaded around is designed for the Vivaldi antenna array.And the simulation and measurement of absorber are carried out.The results show that the simulations and measurements are in good agreement.The antenna array is compounded with the FSS broadband absorber structure,and the radiation characteristics of the compound structure are simulated to study the effect of the electrical performance of the antenna array after compounding absorber structure.The scattering characteristics of the composite structure are further simulated,and the results show that the FSS broadband absorber structure can have a scaling effect on the antenna RCS in the range of 5 to 15GHz,which achieves the purpose of the Vivaldi antenna array stealth in a wider frequency band.Finally,a multi-layer frequency absorbing selective reflector structure with broadband reflection was designed for the F-type printed dipole antenna.The structure was divided into two parts:transmission-absorption（TA）layer and absorption-reflection（AR）layer.TA layer has the characteristics of transmitting in low frequency and absorbing in high frequency.And AR layer is absorbing in low frequency and reflecting in high frequency.The AR layer is a periodic array of metal square ring with resistors loaded all around and built-in metal square as a unit,which achieves the absorption of S₁₁<-10 d B in frequency band of 2.2 to 6.8 GHz and strong reflection of S₁₁>-1 d B in frequency band of 8 to 12 GHz.The TA layer is a metallic cross-shaped structure with four sides loaded with resistors and a periodic array of the lower two metal square layers as a unit,which achieves transmission of S₂₁>-1.15 d B in the frequency band of 2 to 12 GHz and absorption of S₁₁<-10 d B in the 13 to 18 GHz band.The AR layer and the TA layer are combined to obtain a broadband reflection FASR structure.The simulation analysis achieves the absorption of S₁₁<-10 d B in 2.2 to 5.2GHz and 13.5 to 17.1 GHz,the reflection of S₁₁>-1.5 d B in 7 to 12 GHz.The simulation and measurement results are in good consistent.Therefore,the designed broadband reflective FASR structure is compounded with an printed F-type dipole antenna to investigate its radiation characteristics.The results show that the broadband reflective FARS structure has little effect on the antenna port matching and improves the in-band gain of the antenna.The structure achieves the purpose of improving the in-band radiation performance of the antenna.Further,the composite scattering characteristics of the broadband reflection FASR structure and the antenna are calculated,and the analysis shows that the broadband reflection FASR structure has a certain reduction effect on the out-of-band RCS of the antenna.