| In this dissertation,the frequency independent planar spiral antenna and its feeding system is analyzed according to electromagnetic theory and simulation;the method of realizing the feature of unidirection and low profile is researched.Firstly,the definition and equations of the characteristic parameters such as gain,return loss,etc.,is discussed.The mechanism of radiation of equiangular spiral antenna and Archimedean spiral antenna is analyzed.With the help of electromagnetic simulation software,the simulation model of the spiral antennas are set within the frequency band of 2~18GHz,the return loss,total gain and axial ratio are compared.Planar spiral antenna radiates along both directions of the normal of antenna plane,in order to avoid the coupling between antenna and other parts of the system,the antennas applied in communication systems of military surveillance devices and commercial transportation equipments are required to radiate unidirectional wave.Based on the requirement above,a reflector is applied on one side of the antenna plane.However,the inner height of the typical copper cavity whose sub face either is plane structure or circular truncated cone is a quarter of the maximum wavelength,the height of the circular truncated cone's sub surface needs to be adjusted according to the efficiency radiation area.In order to reduce profile and improve total gain,the electromagnetic band-gap(EBG)structure is researched.Besides EBG structure,most of the artificial magnetic conductors(AMC)have the same property of reflection phase band-gap.The lumped parameter model of the EBG structure is analyzed.Since the EBG structure is constructed by unit cells periodically,it is hard to be analyzed.To simplify the analyzing process of EBG structure,the Floquet principle is essential;hence the Floquet principle is firstly analyzed.Then,the EBG structure is simulated in electromagnetic simulation software with the boundary condition of Master-Slave assigned.With the variable control method,simulate and research the influence of the width w,gap between unite cells g,permittivity of the dielectric board ?,the angle of input incident electromagnetic wave ?,and other parameters on the reflection phase band-gap.Finally the parameters of the unite cells are determined and a square shaped reflector with spiral antenna model is simulated and analyzed.In order to obtain more stable return loss,stronger normal total gain and smaller back lobe radiation,a circle shaped EBG structure reflector consists of arch shaped unite cells is employed.The effect of the number of metal patch on the reflector's characteristic is analyzed.The result shows that with the remove of the patches on the central part of the reflector,the normal total gain is increased,and the amplitude and width of the back lobe is reduced. |
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