Research On Miniaturized And Dual-band Frequency Selective Surfaces With Low Frequency Ratios

Frequency selective surfaces(FSSs)is periodic arrays which is mainly composed of metal patches or slots.It is widely used in various fields because of the selectivity characteristic to incident waves with different frequencies and different polarizations.With the rapid development of modern wireless communication,FSS with miniaturization characteristic and multiple independent bands has gradually become the focus of research these years.For the multi-band FSSs,it also involves the spacing between adjacent bands.Especially in some cases where the frequency bands are closely spaced,there is a great demand for dual-band FSS with closely spaced responses.This thesis focuses on FSSs with low frequency ratio and miniaturization characteristics and the specific research contents are as follows:Firstly,this thesis introduces the implementation methods and research status of miniaturized FSS,multi-band FSS and low frequency ratio FSS.Then,the basic resonance modes and resonance principles of FSS are introduced from the equivalent circuit.Some important parameters that affect frequency responses are also analyzed,mainly including the element structure,layer spacing,incident angles and polarizations,which provide theoretical guidance for the design and optimization of the subsequent FSSs.In this thesis,the problem of resonant frequenciey fusion that may occur during the design of dual band-pass FSS is proposed.Namely,in the process of transforming from the basic circuit model to the actual model,one of the two resonant points which are originally independent of each other obviously shrinks or almost disappears.By comparing different equivalent circuit models,it is deeply analyzed that the reason for this phenomenon is that the coupling effect caused by the adjacent metal strips of two resonant circuits changes the original circuit structure.As a result,the current flow is blocked and different frequency responses are produced.In order to verify the correctness of the above theoretical analysis,a simple model is further established in the full wave simulation software,and the surface current distribution is observed and compared to confirm the previous conclusions.In view of this phenomenon,two tentative solutions are provided.One is to use the triangular unit cell and the other is to invert one of the resonant circuits.The purpose is to isolate the adjacent inductive metal strips in the two circuits as far as possible.A frequency selective surface composed of parallel patches and meandering stripes is proposed in this thesis.By placing the two resonant circuits on the diagonal of the element,the coupling effect can be avoided as much as possible,thereby achieving the independent dual pass-band performances.The design utilizes parallel metal patches with different area but similar size to control the high and low resonant frequencies respectively,which not only achieves a lower frequency ratio,but also produces a larger interlayer capacitance,achieves low profile and improves the miniaturization.In addition,through the parameter simulation of the patch side length,the independence between two pass-bands is proved.By observing the distribution of surface current and electric field,an equivalent circuit model corresponding to the element is established.The simulation results show that the structure achieves good dual pass-band characteristic within 1.05GHz-1.3GHz and 1.84GHz-2.29 GHz,with a frequency ratio as low as 1.25.In addition,there is no redundant resonant frequencies outside the main working band,which shows good reflection characteristics in the out of band.Then,a nested zigzag strip frequency selective surface is proposed.The structure uses two zigzag and nested metal strips arranged in the upper right triangle of the element to control the high and low resonant frequencies of a polarization,and the physical lengths of the two strips are similar to obtain a low frequency ratio.The metal strips in the upper and lower plane are connected through the metallic holes to make full use of the space,thus greatly improving its miniaturization.A similar patch-type FSS is proposed by the same dual-polarization method with respect to the diagonal symmetry of the element.The single polarization model is established to explain the use of the isolation of the middle strip to avoid the occurrence of resonant frequencies fusion.The simulation results show that the former structure resonates at 1.45 GHz and 1.78 GHz,and the latter structure resonantes at 1.69 GHz and 2.12 GHz.The frequency ratio is 1.22 and 1.25 respectively.