Filtering Characteristics Of Active Frequency Selective Surfaces Based On Coupled Structure

Active frequency selective surfaces(AFSS, called active FSS) is a kind of artificial electromagnetic materials whose resonance could be tuned or switched under the control of external excitation. Active FSS is usually composed of filtering structure and excitation source. Compared to Passive FSS, active FSS has the advantages when it is used in complicated electromagnetic environment. And it is the research hotspot in areas such as aircraft stealth, satellite communications and electromagnetic compatibility of advanced equipment. Up to now, there are three problems of active FSS: the first is the serious loss caused by the excitation source;the second is the filtering structure limit the excitation’s loading way; the third is further enhancement of the active FSS’s incident angular stability.The active FSS should not only have the tunable or switchable property, but also have good incident angular stability and high transmission. In this dissertation,the filtering Characteristics and mechanism of the passive FSSs, coupled FSSs, and active FSS are analyzed. The conclusion that the coupled FSS structure has the advantages in terms of transmission, excitation’s loading way and incident angular stability is drawn by theoretical analysis. Therefore, some theoretical and experimental research is carried out around the design theory and physical mechanism of the active FSS based on coupled structure, according to therequirements in polarization selectivity, tunable and switchable passband. Such work can provide technical reserves to active FSS applications.The main research work is as follows:The traditional passive FSS is analyzed first because it is the basic theory of coupled active FSS. The mechanism of different active FSS is studied based on the analysis of PIN diodes and controllable media. The coupling mechanism of coupled FSS is explored in the near-field interaction point of view. The geometry of unit cell,the relative position and distance between two FSS screens are all the main factors that influence the filtering characteristics of coupled active FSS. The coupled structure has the advantages of good incident angular stability and low insertion loss.This part of work is the theoretical basis for further analysis and design of coupled active FSS.A coupled FSS structure with polarization selective property is set up based on the coupling mechanism by introducing the lateral displacement operation. This model verifies again that the filtering characteristics of coupled structure could be influenced by the relative position between the two FSS screens. Full-wave numerical analysis and experimental tests show that the proposed coupled structure has stable polarization selective property and low insertion loss. The largest incident angle could be 60°, and the passband transmission reaches-0.3d B. The structure can be rotated 90° to switch polarization filtering state, so the proposed structure has the reconfigurable property. This work provides the theoretical basis for further study of active FSS based on the coupled structure.According to the influence of relative position on the filtering characteristics of coupled structure, the Y-shaped slot and patch which have rotational symmetry are adopted. The coupled active FSS structure with tunable resonance is proposed by introducing the continuous rotation operation. Compared to the active FSS loaded with varactors, the outstanding advantage of the proposed model is the large tunable resonance range, and that of the simulated model is up to 8.6GHz. The proposed structure also has good angular stability(50°) and polarization stability, as well aslow loss. The establishment of the proposed model can provide a feasible scheme for solving the design problem of active FSS such as resonance range, angular stability,and insertion loss, and also provide a new technological approache for realizing tunable active FSS.The coupled active FSS model is established by combining PIN diodes and coupled structure. Its switchable property is realized by the controllable impedance of PIN diodes. The modified ELC element is used to integrate the filtering structure and feeding line, and such combination can effectively reduce the influence of via hole and unnecessary feeding line on active FSS’s filtering characteristics. This proposed active FSS has steady switchable property, and its incident angle could reach to 45°. According to the simulation result, increasing the shunt resistance of the reversely biased PIN diode can increase the transmission is put forward. And such conclusion can provide feasible scheme for solving the low transmission of active FSS. Fabrication scheme and processing parameters of active FSS are summarized, which provides reference for the automatic production of active FSS.