Key Technology Of Reconfigurable Absorptive Frequency-selective Transmission Structures

The stealth radomes with in-band wave transmission and out-of-band wave absorption characteristics can effectively improve the out-of-band multi-station stealth capability of antennas in military equipment.As a possible design solution of this kind of radomes,the absorptive frequency-selective transmission structures(or called rasorber)with both wave transmission and absorption characteristics have attracted wide attention in recent years.Furthermore,the switchable or tunable transmission windows of rasorbers can also be achieved through loading diodes,graphene or applying adjustable mechanical structures.Hence,the in-band stealth ability can be further improved,which is of great theoretical and application value in the field of antenna stealth.However,there are few researches on reconfigurable rasorbers,and the existing designs still have problems such as insufficient transmission bandwidth,poor selectivity and high insertion loss,which limit the application range of reconfigurable rasorbers.The two-dimensional rasorbers are usually composed of frequency selective surface(FSS)and impedance layers.In this dissertation,based on the principle of FSS and rasorber,the influencing factors and design methods of number and bandwidth of transmission windows are analyzed.Later,the switching method of wide-band FSS and the instability of absorption band in the design of wide-band rasorber are analyzed.Then,the design method of switchable wide-band rasorber is proposed.Finally,the stability of tunable FSS is studied,which provides a basis for the design of tunable rasorber with high selectivity and low insertion loss.The main research content can be summarized as follows.At first the relationship between the transmission bandwidth of rasorber and the parameters of impedance layer is studied,and an bandwidth estimation method is proposed.The correction coefficient is also given to improve the estimation accuracy.This estimation method provides a basis for the design of wide-band reconfigurable rasorbers.In addition,the design method of multi-band FSS with second-order response is proposed to expand the transmission bandwidth of multi-band rasorbers.Secondly,the switching mechanism of high-order FSS is theoretically derived.By loading PIN diode on the top layer of FSS to change the resonant frequency of the corresponding resonant structure,the FSS can be switched between band-passing and total reflecting states.Combining the reconfigurable FSS and the switchable parallel resonant structure in the impedance layer,a design method of switchable rasorber is proposed,which exhibits the extended transmission bandwidth compared with the existing designs.To further expand the transmission bandwidth,the absorbing stability of the wideband rasorber in the switching process is studied.Based on the general absorbing conditions of the circuit analog absorber,the influence of the impedance mismatch between the impedance layer and air is analyzed when the rasorber is switched between the rasorbing and absorbing states.The necessary condition for stable absorption of the switchable rasorber is given.Later on,the design method of switchable rasorber with double impedance layer is proposed using the low-Q double spiral resonant structure,and the relative transmission bandwidth is improved up to 30%.By embedding the bias network in the unit-cell of rasorber,the proposed design is also extended to dual polarization.Finally,based on periodic metal gate structure,a tunable FSS using liquid crystal is proposed,and the corresponding equivalent circuit model is given to explain its working mechanism.The method of extending the first-order transmission window to the secondorder response is also studied to improve the transmission bandwidth of the FSS.In addition,the tunable FSS with two-dimensional structure is proposed using slot coupling structure to reduce the thickness.The proposed FSSs have the advantages of high selectivity,low insertion loss and high tuning stability,which provides a basis for the design of tunable rasorbers.