High Selectivity And Bandwidth Extension Of Frequency Selective Surface Rasorbers

With the development of radar detective technology and the complex electromagnetic environment,the stealth of antenna system is an urgent and difficult to solve the key problem.Owing to the frequency selective characteristics of electromagnetic waves,frequency selective surface(FSS)microwave stealth structure has been used in stealth field to obtain the perfect absorption and transmission performance.Hence,this kind of structure can improve the survivability of weapon systems.Traditional stealth radomes adopt the conformal bandpass filter as curved radome structure to realize the low detectable performance.It exhibits transparent in the operating band while absorptive for the out-of-band detection signal.Nevertheless,bistatic radar and netted radar make this structure unable to be apply to stealth platform.In order to find solution to above problem,a kind of novel structure named rasorber could be used in stealth radome to reduce the radar cross section of the antenna systems.Therefore,Frequency selective surface stealth radome was systematically investigated in this thesis.The equivalent circuit model was given to analysis the rasorber structure and pushed to accelerate the design process.Some FSS rasorbers are optimized and designed for the highselectivity,miniaturization and wideband demand.Firstly,the design method of high-selectivity rasorber was extensively studied,which is based on the low-profile bandpass filter.The LC series circuit of FSS structure is employed to replace capacitor component,which introduces a transmission zero in the right of working band.Consequently,a high-selectivity transmission response has been obtained with the insertion loss less than 1 d B.Transperant bandwidth is from 7.9-9.0 GHz,and the-10 d B absorption bandwidth covers 3.3 to 7.1 GHz at low frequency and 9.4 to 12.0 GHz at high frequency.In addition,the result shows that the relative transition bandwidth at the low band and the high band is 9.24% and 3.89%,respectively.And then,the miniaturization design of the frequency selective surface rasorber was futher investigated and analyzed.The double-sided FSS lossy layer is designed to obtain the interlayer plate capacitance,which realizes a large value of equivalent capacitance.Accordingly,the large equivalent capacitance can obtain the series resonance with the equivalent inductance of lossy layer in the lower frequency.Simultaneously,the helical-type choke is applied to realize the wideband transmission response.Futhermore,a miniaturization rasorber structure was designed to achieve the transmission band between 7.4 and 9.9 GHz,the insertion loss in the band is less than 1 d B,the-10 d B absorption bandwidth covers 1.3 to4.3 GHz at the low band.The size of structure relative to the lowest operating frequency is0.26,which avoid the grating lobe arise in the interested band.As a result,under the TE and TM polarization,the rasorber could gain stable performance within 30 degree oblique incidence.Lastly,the wideband absorption and transmission design of the frequency-selective surface rasorber was carried out in detail.The four functional frequency selective surface layers replace the traditional method that employs two functional layers.The whole structure consists of two lossy layers and two lossless layers.The loss characteristics of two lossy layers can control the absorption performance in the high band and low band respectively,and the tramsmission characteristics of four functional layers in the same frequency guarantee the transmission performance with low insertion loss.Therefore,the four functional layers could obtain the ultra-wideband absorption and transmission behavior.The results show that an ultra-wideband window absorber structure was designed to achieve the transmission band between 7.8 and 11.0 GHz,the insertion loss in the band is less than 1 d B,and the-10 d B absorbing bandwidth covers 1.8 to 6.6 GHz in the low frequency and covers 13.3 to 18.2 GHz in the high frequency.