Study On Novel Frequency Selective Surfaces With High Performances

Frequency selective surfaces(FSSs)are periodic structures,which exhibit filtering responses for the electromagnetic fields and have been widely used as radomes,antennas reflectors,electromagnetic shields,artificial magnetic conductors,and so on.With the rapid development of these applications,FSSs are required to provide improved performances,such as miniaturized unit cell size,adjustable bandwidth,independently controllable polarization performance,high-order filtering response,ultra-wideband,low-profile,multi-band performance with arbitrary band number and ratio,and so on.Nevertheless,the traditional FSS structures can only be used for realizing a single one of the characteristics introduced above,and these FSSs usually suffer from complicated structure and time-consuming optimal design.To address these problems,research on the FSSs with high performances are performed.Then,several novel FSS structures are firstly proposed to provide multiple excellent characteristics simultaneously,and their equivalent circuits are established for achieving efficient design.Furthermore,the partial element equivalent circuit(PEEC)method based on magnetic currents is intensively investigated for establishing equivalent circuits of slot-type FSSs.The main innovations in this dissertation are as follows:1.Convoluted structures for miniaturized-element FSSs:A novel convoluted FSS element with simple geometry,miniaturized unit cell size,and stable filtering response is proposed.In this FSS element,the dipoles are convoluted and interweaved to provide increased inductance and capacitance simultaneously,thus leading to a unit cell size of 0.048.?0(?0 refers to the resonance wavelength in free space).Through the complementary structure,the proposed convoluted element can be extended for achieving dual-band bandpass performance.Unit cell size of this dual-band FSS is as small as 0.046?0 It is known that the traditional convoluted FSSs feature single-layered structure and thus suffers from restricted miniaturization performances.To address this problem,design concepts of using multi-layered convoluted elements are proposed for further miniaturizing the FSS unit cell size.In bandpass FSS design,the convoluted slot grid is introduced and cascaded with a convoluted slot dipole element to increase the FSS equivalent capacitance,thus leading to an ultra-miniaturized element size of 0.019?0.In the bandstop FSS design,the equivalent inductance and capacitance are increased by cascading multiple layers of convoluted dipoles.As a result,the FSS unit cell size is further reduced to 0.016 ?0.For all these FSS designs,the equivalent circuits are established for explain-ing the FSS resonance mechanisms and accurately calculate the FSS frequency performances.Furthermore,a set of closed-forms related to the circuit and dimension parameters for these FSSs are developed.They can serve as the substitute of full-wave solver to enhance the FSS design efficiency significantly.2.An FSS structure with geometrically separable inductors and capacitors:Conventionally,FSSs are constructed by resonant-type elements(e.g.patches,loops,or their complementary structures),of which the shapes and geometrical parameters determine the overall frequency response.They suffer from difficulties in achieving high performances and timeconsuming optimal design.To address these problems,an FSS structure with geometrically separable meander-line inductors(MLIs)and parallel-plate capacitors(PPCs)is proposed and investigated.For this FSS structure,the elements with desired frequency performances can be intentionally constructed based on the circuit theory,and their equivalent inductance and capacitance can be independently adjusted through MLI and PPC,respectively.Therefore,the proposed FSS structure can be readily extended for designing various high-performance FSSs,such as:? single-band FSSs with adjustable bandwidth and independently controllable polarization performance;? dual-band bandpass and bandstop FSSs with the band ratios ranging from 1.35 to 3.8 and 1.31 to 4.22,respectively;? a low-profile third-order FSS with a thickness of ?0/11,and a third-order FSS with ultra-wide bandwidth of 121%.For these FSSs,because MLI and PPC provide increased inductance and capacitance,respectively,their unit cell sizes are all smaller than 0.1 ?0.Therefore,frequency performances of these FSSs are stable with respect to different polarizations and incidence angles.Furthermore,the accurate equiva-lent circuits of these FSSs are established for explaining the FSS resonance mechanisms.,calculating the FSS frequency performances,and accelerating the design efficiency.3.An FSS structure based on parallel LC resonators for multi-band applications:The traditional multi-band FSSs usually suffer from limited transmission or reflection bands,parasitic resonance.,and difficult optimal design.To address these problems,an FSS structure based on parallel LC resonators is proposed to achieve multi-band filtering responses.The parallel LC resonator is implemented by a via-connected metallic line(VCML)and a PPC,which provides the inductance and capacitance performances,respectively.By introducing multiple resonators in a single unit cell and connecting them in series through metallic strip lines and vias,an FSS with multiband bandpass performance is obtained.Based on this design concept,the design examples with 1?5pass-bands are presented for validation.The pass-bands of these FSSs under TE and TM polarizations depend on separate resonators,which indicate that the frequency and polarization performance of these FSSs can be independently designed.Furthermore,by introducing an additional PPC in the FSS unit cell,these bandpass FSSs can be extended for achieving the multiband bandstop filtering response.Because of the increased capacitance from PPC,unit cell size of these multi-band FSSs are small,thus leading to the stable filtering responses under different polarizations and incidence angles.For all these FSSs,the equivalent circuits are established for not only precisely modeling the frequency behaviors but also providing the design guidelines.4.Magnetic current PEEC analysis for slottype FSSs with single-layered elements:A PEEC method based on magnetic currents is proposed to establish equivalent circuits for slot-type FSSs with various element shapes.In the derivation of this approach,only the equivalent magnetic currents located at the slot of a single unit cell are meshed,and the partial element equivalent circuits are defined accordingly.Therefore,the number of circuit components,compared with the traditional PEEC method based on electrical currents,are greatly reduced,thus leading to an improved calculating efficiency:To validate the effectiveness of this method,equivalent circuits for square slot loop,cross slot,and Y slot FSSs are established by the proposed PEEC method.Then,through these equivalent circuits,the FSS frequency performances can be calculated.The calculated results agree well with those obtained by CST within an ultrawide frequency band from 0 to 20GHz.Therefore,the PEEC method based on magnetic currents is a full-wave equivalent circuit analyzer for various slot-type FSSs.