Design And Application Of Curved Lens Based On Microwave Metasurface

The lens is an important wavefront conversion device in microwave band.Conventional lenses include the transmission-type focusing mirrors such as the dielectric lens and metalplate lens as well as the reflection-type focusing mirrors such as the parabolic reflector and grounded dielectric reflector.To achieve the wavefront conversion,conventional lenses mainly rely on their specifically tapered media parameters or geometrical shape to change the length of ray path,which indicates that the thickness along the wave propagation direction is usually big.As a two-dimensional artificial electromagnetic material with subwavelength thickness,metasurface can realize flexible manipulation of electromagnetic wave characteristics.In addition,the convenience of fabrication and integration facilitates the wide application of metasurface in the low-profile design of microwave devices including the lens and polarization converter.In the practical application environment,the carrier platform usually adopts curved surface shape,while most of the reported metasurface designs exhibit a flat profile,which limits the low-profile conformal integration with the carrier platform surface.In order to enhance the conformal integration of metasurface focusing devices with curved platform surfaces,design of the conformal focusing mirrors based on curved metasurfaces is of practical significance.Using the generalized Snell’s law as the main theoretical basis,a hemispherical-shellshaped transmissive focusing mirror design which possesses both the capability of being conformal to an approximated hemispherical surface and the characteristic of polarization conversion,and a reflective focusing mirror design which is conformal to an elliptical cylindrical surface,are proposed in this thesis based on passive curved metasurfaces.Besides,the application of the two focusing mirror designs in the circularly polarized antenna system and metallic particle detection are respectively investigated.The main work of this thesis is summarized as follows.For the hemispherical-shell-shaped transmission-type focusing mirror design,the functionality combination of transmissive focusing and linear-to-circular polarization conversion is realized on an approximated spherical surface.First,expressions for discrete distribution of the transmission phase of the hemispherical-shell-shaped transmission focusing mirror satisfying both functionality are derived with the approximated spherical surface taken into account.To achieve the calculated transmission phase values,an alternating structure of slot and patch is used to design the transmissive metasurface cells.Then,all cells of different internal dimensions are built on the hemispherical shell surface.The hemispherical-shell-shaped transmission-type focusing mirror with the polarization conversion characteristic is thus obtained.When an arbitrarily-polarized planar electromagnetic wave illuminates the outer surface side of the hemispherical shell,a focused field with enhanced amplitude is observed around the focal point.Second,due to the structural compatibility with the existing hemispherical-shell-shaped protective dome,a linearly-polarized antenna can be used as the primary radiation source and placed at the focal point of this focusing mirror,so as to form a circularly-polarized lens antenna and analyze the linkage improvement of ultra-long-range circularly polarized communication links.A half-wave dipole is used as the primary radiation source to verify the functionality of linearto-circular polarization conversion and transmissive focusing.To better evaluate the performance of this focusing mirror,a microstrip antenna with wider bandwidth is also used as the primary radiation source.It is observed that the main lobe gain can be improved by18.5 d B at the design frequency of 16 GHz,in comparison with the freestanding microstrip antenna.The 3 d B axial ratio bandwidth of the lens antenna is 15.2 GHz-20 GHz(28%).The proposed design is verified by the comparable analysis between measured and simulated results.For the design of reflection-type focusing mirror conformal to an elliptical cylindrical surface,expressions of the reflection phase gradient and discrete distribution of reflection phase are derived for both the one-dimensional focusing along the circumferential direction of the elliptical cylinder and the two-dimensional focusing along the circumferential and axial directions of the elliptical cylinder.Variation in the elliptical cylinder size as well as the focal point position are considered in the expressions.A patch structure is utilized to design the reflective metasurface cells which realize the calculated reflection phase values.Cells with different internal dimension are then constructed onto the cylindrical surface to obtain the one-dimensional and two-dimensional focusing mirrors.When the reflection-type focusing mirrors are illuminated by a planar electromagnetic wave,one-dimensional and two-dimensional focusing effects are respectively observed around the focal point.As a result of the electromagnetic spot beam formed by the two-dimensional focusing,detection and localization of metallic particles based on radar cross-section are investigated through analysis of the monostatic radar cross-section of metallic particles located in the vicinity of the focused beam.The full-wave simulation results show that the monostatic radar crosssection exhibits a peak characteristic if the metallic particle is located within the center of the focusing beam.Therefore,the reflection-type focusing mirror conformal to an elliptical cylindrical surface may achieve detection of metallic particles through the use of microwave frequency,and its curved shape facilitates integration onto the detection platform.