Periodic Structures And Their Applications In Antennas

With the development of novel electromagnetic materials,researches and applications on antennas of artificial magnetic conductors(AMCs)and frequency selective surfaces(FSSs)have recently been one of the hotpots in microwave field.This dissertation focuses mainly on AMC to reduce the profile of planar antennas and to reduce the radar cross section(RCS),and on FSS to be as subreflector of multi-band reflector antennas.A wideband low-profile slot antenna,two wideband low-profile bowtie antennas,an AMC-based slot array,an AMC-based bowtie antenna array,two low-RCS AMC structures,and a multi-band reflector antenna using a novel multi-band FSS as subreflector are designed in this dissertation.At the same time,those antennas mentioned above have been manufactured in order to verify the feasibility and accuracy of the designed antennas.The main aspects of this dissertation can be concluded as follows:1.The in-phase reflection characteristics of AMC is studied and a wideband low-profile slot antenna based on AMC is designed.Firstly,by loading with envloved C-shaped brabches,a wideband planar CPW(Co-Plane Waveguide)-fed slot antenna is proposed.Secondly,a wideband AMC surface is designed,the in-phase reflection band of which covers the operation frequency band of the slot antenna discussed above.Then the AMC surface is located under the slot antenna as ground plane,which enables the combined antenna to achieve better impedance matching and radiation properties as well as low profile.The measured results show that the designed antenna operates in the band ranging from 7.64 GHz to 14.58 GHz(62.47%)and the maximum gain reaches 10.26 d Bi,and higher front-to-back ratios and lower cross polarizations can be obtained.Based on the slot antenna proposed above,a 1×8 slot array antenna is designed,which operates in X band.The designed array operates in the band ranging from 8 GHz to 11 GHz,and it has good electrical perfermences.2.Two wideband low-profile gain-enhanced bowtie antennas based on AMC are presented based on the in-phase reflection property of AMC.Firstly,two broadband bowtie antennas operating in different bands are designed by loading different branches.Then,two kinds of AMC structures,the in-phase bands of which cover the two frequency bands of the two priviously discussed bowtie antennas respectively,are presented.In addition,to furtherly enhance the gain and decrease the sidelobes and backlobes of the two low-profile bowtie antennas,two copper walls are added around the two AMC ground planes.The measured results show that the two low-profile bowtie antennas can achieve broadband and high gain properties.Based on the bowtie antenna proposed above,a 4×4 bowtie array antenna is designed,which operates in Wi MAX band.The designed array operates in the band ranging from 2.4 GHz to 3.6 GHz,in which the array has good electrical performances.3.The technique of AMC surface to reduce RCS is studied based on the phase difference property of different AMC unit cells and two kinds of AMC surfaces are proposed for RCS reduction.Firstly,a single-layer AMC structure is presented,the two unit cells of which contribute to destructive phase difference(180°±37°)over a broad frequency ranging from 11.6 to 28.1 GHz.By fan-type arrangement,the number of interfaces between two unit cells can be increased,which,in turn,serves to scatter the energy more effectively in non-specular directions.Secondly,a two-layer AMC chessboard is designed.The air between the upper substrate layer and the bottom PEC layer enables the phase curve of the two unit cells to become gently,which brings in a 94.06% phase difference bandwidth.And then,by chessboard arrangement of the two unit cells,the two-layer AMC surface achieves low RCS property in ultra-wide band.The consistence between the measures and simulated results proves that the two designed AMC surfaces can be utilized in objects to achieve stealth in a ultra-wideband.4.The design methods of multi-band FSS multi-band reflector antenna are studied.By using FSS as subreflector,the reflector antenna can make two feed horns in different bands operate simultaneously,and then achieves a number of different operation frequency bands.Firstly,a tri-band FSS in Ku/K/Ka band is designed to be the subreflector of reflector antennas,which reflects electromagnetic waves in Ku band but transmits electromagnetis waves in K and Ka band.Using a two-metal-layer of a square loop combined with a hexagonal ring and a hexagon as unit cell,the FSS has the merit of broad band in each band.And multi-layer structure enables the proposed FSS to achieve low loss property and stability under large incident angles for both TE and TM polarizations.Secondly,an axial corrugated horn in Ku band and an axial corrugated horn in K/Ka band are designed as the two feed horns of the reflector,which achieve low level of cross polarization and good rotational symmetry of the main polarization patterns.Finally,design method of multi-band reflector antenna is studied.The K/Ka feed horn is located at the focal of the reflector,and the FSS is placed at between the reflector and the focal,and the Ku feed horn is located at the symmetry position of the K/Ka feed horn with regard to FSS.Therefore,the electromagnetic waves from the K/Ka feed horn transmit on the reflector by passing through FSS,and the electromagnetic waves from the Ku feed horn transmit on the reflector through reflection by FSS.Then the electromagnetic waves by the two feed horns are reflected by the reflector and radiated to the far field.The radiation patterns in far field of the multi-band reflector system are simulated by GRASP sofeware.The simulations prove that,the multi-band reflector antenna has an aperture efficiency larger than 55% in Ku band,and more than 64% in K and Ka bands,which meets the requirements in principle.