Research On Low-Profile High-Order Bandpass Frequency Selective Surface

Frequency selective surface（FSS）is a two-dimensional infinite periodic array,which acts as a spatial filter and exhibits the filtering characteristics for plane waves in free space.It is usually composed of periodically arranged metallic patch or aperture elements,and exhibits bandstop or bandpass filtering response.Based on these unique characteristics,FSSs have found wide applications in polarizers,radar radomes,reflective surface antennas,electromagnetic compatibility,and microwave absorbers.Among them,the polarizer is a special kind of FSS,in addition to its filtering characteristics,it can also change the polarization state or direction of the incoming waves and therefore also has broad application prospects.With the development of the electronics industry and information technology,high-order bandpass FSS with flatter top and faster roll-off has become a hot research area in recent years.However,conventional high-order bandpass FSS usually has the disadvantages of large thickness,complex structure,and low design efficiency.This thesis focuses on the high-order bandpass FSS,a class of FSS elements on aperture-coupled patch resonators（AC-PR）is systematically studied by combing the classical antenna and filter design theories,and the aperture-coupled dual-mode patch resonators（AC-DMPR）elements are then proposed to realize low-profile high-order bandpass FSS with simple topology and high design efficiency.Moreover,the design methodologies for a class of high-performance polarizers by using AC-PR elements are studied,which have the advantages of low-profile and high polarization conversion efficiency.Finally,the main work of this thesis can be summarized as follows:1.Study of AC-PR element:First,the resonant characteristics of the patch resonator are investigated based on the transmission line model and the cavity model,and the field distribution patterns of different resonant modes are analyzed.Then,the equivalent circuit model and coupling topology of the AC-PR element are developed based on the synthesis design theory of microwave filters.With the help of the extracted equivalent circuit parameters and corresponding coupling coefficients,the analysis method and design procedures of the AC-PR element are given,which improve the design efficiency and lays the theoretical foundation for the design and analysis of bandpass FSS or polarizers on AC-PR elements.2.Study of the high-order bandpass FSS based on AC-DMPR element under high-order modes:The conventional AC-PR element is based on the single-mode patch resonator.In order to increase the filtering order of AC-PR element without increasing the overall thickness of the structure,this thesis innovatively constructs the AC-DMPR element based on dual-mode patch resonator according to the classical multi-mode resonance theory.Firstly,an AC-DMPR element is constructed under the operation of TM₀₁ and TM₀₃ modes.The shorting pins are loaded in the patch resonator to reallocate the resonant frequencies of these resonant modes based on the cavity model.As a result,the filtering order is double compared with traditional AC-PR element.Then,for the purpose of a miniaturization design,an AC-DMPR element based on the TM₀₀ and TM₀₁ modes is proposed.Based on the electric field distribution of the resonant modes of the AC-DMPR element,the design methodology of the resonant frequencies and coupling mechanisms are studied,and several single-or dual-band high-order bandpass FSSs are designed.Simulated and measured results validate the feasibility of the design concept,and also demonstrate that,the proposed high-order bandpass FSSs have the merit of low-profile,high frequency selectivity and few numbers of substrate layers.3.Study of the high-order bandpass FSS based on AC-DMPR element under degenerate modes:In order to address the problem of structural asymmetries in the AC-DMPR element under the operation of high-order modes,this thesis innovatively constructs the AC-DMPR element under the operation of degenerate modes.First,the patch resonator with diagonal corner truncations is studied so as to be transformed into a dual-mode resonator based on the degenerate modes.Then,to eliminate the unwanted cross-polarization produced by the degenerate modes while still maintaining the dual-mode characteristic,four such patch resonators are arranged together with 90°rotation between each two adjacent ones,and a new composite element with symmetric structure is constructed to form the AC-DMPR element.The operation mechanism and equivalent circuit model of the AC-DMPR element are studied and analyzed,which allows for the flexible design of single-and dual-band high-order bandpass FSSs.Finally,simulation and experimental results show that the proposed high-order bandpass FSSs have the advantages of low-profile,high frequency selectivity,good angular stability,and dual-polarized response.4.Study of the low-profile polarizer based on AC-PR element:The polarizers can be classified into two types:（a）The circular polarizer,which can convert a linearly polarized plane wave to a circularly polarized wave.（b）The linear polarization rotator,which can rotate the incoming linearly polarized wave by 90°.Based on the concept of polarization separation design,the phase difference of the two orthogonal transmission coefficients of the circular polarizer and linear polarization rotator is 90°and 180°.In order to achieve such phase difference,it is difficult for the conventional polarizers to simultaneously achieve the properties of thin thickness,high polarization conversion efficiency,simple structure,and multi-band response.To address this problem,three novel design methodologies have been proposed based on the AC-PR element:（a）First,a dual-band dual-polarized bandpass FSS is proposed and designed by using the composite dual-band AC-PR element which composed of two AC-PR elements.Then,the feasibility of loading slots in the patch to introduce 90°phase difference between the two orthogonal transmission coefficients is studied.Finally,two dual-band circular polarizers with different frequency ratio,and different polarization conversions in each operation band are designed.（b）Second,the orthogonal transmission paths of the AC-PR element with electric and magnetic coupling respectively are constructed and analyzed.Based on the out-of-phase properties between electric coupling and magnetic coupling,the thickness of the linear polarization rotator is significantly reduced without decreasing the polarization conversion efficiency.Then,the composite dual-band AC-PR element is also adopted to obtain a dual-band frequency response.（c）Third,the orthogonal degenerate modes of the patch resonators are studied to construct the multi-layer AC-PR element with polarization rotation properties,where low-profile,high-order linear polarization rotator is obtained.Finally,the simulated and experimental results show that the above proposed structures have the advantages of low-profile,low axial ratio or low insertion loss,and high design freedom.