| With the development of satellite navigation,ship communications,radar ranging and 5G wireless communication systems,the requirements for multi-beam antennas are becoming increasingly stringent when the multi-beam antennas are widely used in many areas.There are three categories of traditional multi-beam antennas:phased array antennas(PAAs),reflector antennas(or reflectarray antennas)and lens antennas(or transmitarray antennas).Among them,phased array antennas usually have the inherent shortcomings of narrow operating band,high power consumption,complicated feeding network and high cost,while reflector antennas usually have limited beam coverage and poor scanning beam uniformity.Thus,it is very hard to meet the challenging demands of the new generation of electronic systems which require for low-cost and high-performance antennas.Lens antennas are expected to serve a wider range of application scenarios due to their attractive characterstics such as low cost,wide band and wide-scanning.The engineering applications of lens antennas can be extended to a large extent in many areas,as long as the traditional shortcomings of lens antennas can be solved,such as the large volume and weight in low frequency bands,large dielectric loss in millimeter wave frequency bands,and low integration with the system.To meet the aforementioned requirements of lens antennas,studies on the miniaturization,planarization and ease of integration of lens antennas are carried out in this dissertation,while maintaining the excellent focusing performance and wide-scanning characteristics of lens antennas at the same time.The main contents or novelties of this dissertation are summarized as follows:1.Studies on modified horizontal omnidirectional multi-beam cylindrical Luneburg lens antennaIn view of poor isolation between the adjacent feeding ports and low overall structural strength of traditional cylindrical Luneburg lens antenna,several methods are proposed to improve the isolation between the adjacent feeding ports and the overall structural strength of the original lens antenna without reducing the original antenna performance,and thus improved engineering practicability of the cylindrical Luneburg lens antenna.For verification,a three-layer multi-beam cylindrical Luneburg lens antenna prototype operating from 4.4 to 5 GHz was fabricated and measured.The measured results of the lens antenna prototype show that the isolation between the adjacent ports of the same lens layer is over 15 dB,the 360° omnidirectional beam scanning is obtained in the horizontal plane and the 3 dB beam coverage in the vertical plane is over ±20°,and the higher strength of overall lens antennas structure is achieved.2.Studies on wide-scanning modified hemispherical lens antenna fed by phased array antennaIn order to solve the issue of the limited scanning angle of the traditional hemispherical dielectric lens antenna,the shapes of the traditional hemispherical dilelectric lens are modified based on the geometrical optics and conjugate field matching method.By employing the PAA as the feeding antenna,a novel methodology with high design freedom and flexibility is proposed to broaden the beam scanning range of the hemispherical lens antenna,which greatly expands the scan angle of the hemispherical lens antenna while reducing the focal length of the lens antenna simultaneously.To verify the feasibility of this design methodology,a modified hemispherical dielectric lens antenna prototype operating at 28 GHz band was fabricated and assembled,based on 3D printing technology and CNC(computer numerical control)process.The experimental results show that the lens antenna is able to scan up to ±60° in one dimension and the aperture efficiency is over 50%.3.Studies on the two-dimensional flat Luneburg lens antennas based on transformation opticsAimed at the issues of the large volume and the arc-shaped feeding antenna arrangement of the two-dimensional cylindrical Luneburg lens antenna,a design method of the two-dimensional Luneburg lens antenna based on transformation optics is proposed to compress the volume and flatten the feeding antenna arrangement of the lens antenna.The technical improvements based on geometrical optics have been taken to solve the limited scanning angle of the transformed flat Luneburg lens antenna on the basis of the above design.For verification,the initial two-dimensional flat Luneburg lens antenna,the wide-angle scanning two-dimensional flat Luneburg lens antenna based on beam switching mechanism and the phased two-dimensional wide-scanning flat Luneburg lens antenna operating at 28 GHz band were fabircated and measured respectively,based on 3D printing technology and CNC process.The experimental results show that the three types of flat Luneburg lens antennas achieve the beam coverage of-31°?30°,-50°?48° and ±60° in the azimuth plane respectively,which veriied the feasibility of the designed two-dimensional flat Luneburg lens antenna and the effectiveness of proposed wide-scanning improvement approach.4.Studies on fully metallic multi-beam lens antennas at millimeter wave bandTraditional dielectric lens antennas usually suffer from low aperture efficiency at millimeter wave band due to the high dielectric loss of the lens material,and the assembling errors have great impact on the performance of the lens antenna.Therefore,two new structures of fully metallic multi-beam lens antennas are proposed to improve the engineering practicability of the lens antenna in the 5G millimeter wave band.The first proposed lens antenna is a low-profile fully metallic multi-beam lens antenna,based on the theory of geodesic Luneburg lens antenna and the air-filled lens antenna is obtained with a lower lens profile and a simpler lens curve.The second proposed lens antenna is a compact fully metallic lens antenna based on quasi-conformal transformation optics,where the quasi-conformal transformation optics is employed to reduce the volume of the lens antenna and the required permittivity of the transformed lens is realized by loading the metallic periodic structure.Due to the CNC technology,the integration processing of the fully metallic lens antenna can be achieved through proper design procedure,which greatly reduces the assembling error.For verification purpose,the aforementioned two lens antennas working at 28 GHz band have been processed and measured,and the measured results show that the proposed two lens antennas achieve the wide-angle beam coverage of ±77° and ±52° in the azimuth plane,respectively,while the aperture efficiencies of the two lenses are all over 60%in the operating bands. |
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