Study On Absorption And Polarization Conversion Of Frequency Selective Surface

Frequency Selective Surface?FSS?is basically composed of identical patch or aperture elements periodically arranged in two-dimension infinite array.The unique spatial filtering characteristic of FSS to incident electromagnetic waves with different polarizations and angles of incidence activates its extensive use in electromagnetic domain.In recent years,it has been paid attention by international community already because of a great deal of research achievement and rapid development of engineering applications.So in-depth research on different applications of FSS will be studied in this dissertation,especially in absorbing material,refractive index sensors and circularly polarization convertors.The specific contents and innovations include:?1?Mobile communication and signal interchange systems have demand for absorbing materials operating in microwave band.Combined with the perfect absorption of FSS,three types of absorbers based on FSS have been designed in this dissertation.Firstly,a dual-band absorber is proposed based on resonant ring.It can not only have a large-angle absorption at5.68GHz and 11.36GHz,but also adjust the two resonant frequency independent.Secondly,a broadband absorber based on magnetic FSS is proposed.The magnetic FSS pattern is used to replace the ordinary copper,which improves the absorption and bandwidth greatly.By stacking several units,the absorber has brand operation bandwidth from 4.14GHz to12.74GHz.It also has an insensitivity to the polarization and incident angle of electromagnetic wave.Thirdly,a multi-layer broadband absorber is designed based on the interference cancellation principle of electromagnetic wave.The matching layer is used to optimize the structure to achieved a broadband from 3.95GHz to 10.4GHz with absorption over 90%.The absorbing principles of different absorbers have been explained from different aspects such as impedance matching,cell resonance,reflection interference and loss mechanism.The samples of dual-band absorber and multilayer broadband absorber have been fabricated and tested.The results show that the absorber based on FSS is feasible.?2?Refractive index sensors based on biological or optical have disadvantages such as complex,expensive and sensitive to the dielectric environment.Combined with the low cost and easy integration of FSS,two sensors based on FSS have been designed in this dissertation.Firstly,a square ring with four-leg is proposed to detect the refractive index of an unknown dielectric from 1.0 to 5^1/2.The resonant frequency of the sensor has a good linear relationship with the refractive index.The structure has high sensitivity and its quality factor is 3.73 RIU^-1.Secondly,a ring structure with T-shaped has been proposed,which expands the scope of detection to 10^1/2.The sensitivity of the sensor has also improved,and the quality factor has increased to 7.27 RIU^-1.The characteristic equation of the sensor is obtained by linear fitting method.The refractive index of the material under test can be calculated from the resonant frequency directly.The sample of the sensor is measured by the waveguide method,and the results show that the refractive index sensor based on FSS has high accuracy.?3?The circularly polarization convertor has requirement of broadband operation band,insensitive to incident angle and low insertion loss.FSS has different transmission characteristics of different linearly polarization.Four convertors based on FSS have been designed in this dissertation.Firstly,a multilayer structure based on right-angle FSS is proposed with the working bandwidth of 72.3%.Each layer is separated by an air gap,which replaces part of the dielectric layer to reduce the insertion loss.Secondly,a window type structure is proposed,which removes the air gap to avoid processing and application limitation.The window is derived from substation of different squares.The polarizer can achieve the bandwidth of 62.38%.It is insensitive to the incident angle,which could obtain the stable operation frequency with the incident angle increasing to 30°.Thirdly,a polarizer based on gradual FSS is presented.The dimensions decrease layer by layer with a reduction factor,resulting in an axial ration bandwidth of 66.69%.The insertion loss is reduced without air gap.Fourthly,a composite cross shape using frequency selective surfaces is applied to design a broadband polarizer.The structure is multilayer and the unit cell of each layer consists of a composite cross and a metal strip.This design takes advantages of both similar amplitude and stable phase difference close to 90°for the two orthogonal components of transmitted waves.The bandwidth reaches to 74%and the insertion loss is less than 1dB.In addition,it still has a bandwidth when the incident angle increases to 20°.In this dissertation,the reasons of polarization conversion are analyzed in detail.The samples of four polarizers are fabricated and tested,and the results show that the FSS can realize the broadband circularly polarization convertors.