Metasurface-based Cylindrical Lenses And Their Application In Enhancement Of Antenna Gain

As a device used in the wireless communication system for radiating or receiving electromagnetic wave,antenna is an important part of the wireless communication link.By virtue of the characteristics of wave-front conversion and high gain,lens antenna is widely used in radar detection,satellite communication and other radio fields.The electromagnetic lens,which can effectively convert the non-planar wave-front into a planar wave-front,is an important component part of the lens antenna with improved boresight gain relative to the feeding antenna.For the traditional electromagnetic lens,the length of the ray-path is mainly adjusted through the gradual change of its medium parameters and shape,so that the phase compensation required by the wave-front transformation can be achieved.However,transitional lens is characterized by the big thickness along the propagation direction of electromagnetic wave,which may limit its conformal integration and application on the moving platforms.The metasurface,which exhibits a low-profile characteristic and flexibility of electromagnetic wave control,can be helpful to realize the low-profile design and conformal integration of the electromagnetic lens.Therefore,designing the electromagnetic lens based on metasurface is of practical importance.Based on the generalized laws of refraction and reflection,this thesis presents cylindrical lens designs based on the passive metasurface.Besides,cylindrical metasurface lens antennas with characteristics of low-profile and high-gain are studied.Specifically,the operating principle and design formula of the cylindrical lens based on the passive metasurface are obtained.Through comparisons between the transmission-or reflection-type cylindrical lens antenna and the feeding antenna,the gain enhancement effect is examined.The main work is described as follows,Firstly,the expressions of the transmission phase gradient and the discrete transmission phase distribution are derived for the transmission-type cylindrical metasurface lens.The simple slot structure is used as the basic unit cell of the metasurface and to realize the discrete values of the transmission phase.By adjusting the internal dimensions of the unit cell,the discrete values of the transmission phase varied within 360°can be achieved with the transmission magnitude larger than 0.7.Then the unit cells with determined dimensions are conformally constructed on a cylindrical surface to obtain the transmission-type cylindrical metasurface lens.Furthermore,the unit cell structure is modified to realize the transmission-type cylindrical metasurface lens with single-polarization or dual-polarization capability.For the plane electromagnetic wave illumination,the focused electric field intensity is observed at the designed focal point.In order to analyze the gain enhancement effect of the transmission-type cylindrical metasurface lens,a traditional microstrip patch antenna element,which functions as the feeding antenna,is placed at the focal point of the lens to obtain a cylindrical metasurface lens antenna.Compared with the feeding antenna,the single-polarized cylindrical lens antenna can increase the main lobe gain of transverse magnetic polarization by 11.4 d B at the operating frequency of 10 GHz.For the dual-polarized cylindrical lens antenna,the main lobe gain of transverse electric and transverse magnetic polarizations is increased by 11.23 d B and 11.44 d B,respectively.The effectiveness of the design is demonstrated by the experimental measurement.Based on the generalized reflection law,a reflection-type dual-polarized cylindrical metasurface lens is designed by using similar procedures.Different from the transmission-type lens,the cylindrical surface covered by the reflection-type lens is closely dependent on the position of the focal point.The simple loop structure is used to realize the reflection phase gradient along the circumference direction of the cylinder,focused electric field strength is observed at the objective focal point for the plane electromagnetic wave illumination with transverse magnetic or transverse electric polarization.With respect to the gain enhancement effect relative to the feeding antenna,full-wave simulation shows that the main lobe gain of transverse electric and transverse magnetic polarizations is enhanced by5.92 d B and 5.83 d B by the cylindrical metasurface lens antenna at 10 GHz,respectively.