| High-gain antennas(HGA)play a key role in wireless applications such as radar,satellite communication,radio astronomy,remote sensing,millimeter-wave imaging and terahertz exploration,etc.As a novel type of HGA,lens antenna attracts growing interests of many researchers by virtue of its advantages of low cost,lightweight,high radiation efficiency,no feed blockage and multibeam.However,there still exist some crucial challenges in lens antenna design in terms of bandwidth and beam scanning.Based on the practical application background,the bandwidth enhancement and beam scanning techniques of lens antennas are investigated in this dissertation.The main contents are summarized as follows:1.Researches on ultra-wideband(UWB)high-gain lens antennas using receivertransmitter(R-T)configurationTo resolve the inherent narrowband limitations of planar lens antenna,a R-T scheme combining UWB array elements and true-time-delay(TTD)transmission lines is used to design two practical UWB lens antennas.The first single-polarzied lens element employs vertically-oriented tightly coupled dipoles as the receivers/transmitters and integrated grounded coplanar lines as phase shift structure,and achieves low transmission loss and TTD phase shift within 4.2～15.8 GHz.The overall element structure is solderless,and all metallic components except the ground can be directly printed on a single-layer substrate,which extremely reduces the assembly difficulty of the lens aperture.The measured and simulated results of the fabricated 21 × 21-element lens prototype agree reasonably well,indicating that stable high-gain beams are achieved over an UWB range.The second is UWB dual-polarized lens antenna using single-layer Vivaldi element.The research focuses on the topology of the dual-polarized Vivaldi lens element,the design of highefficient high-impedance microstrip phase delay line,the implementation of highisolation grid ground in the dual-polarized lens aperture and the solutions to the resulting resonance phenomenons.The designed dual-polarized lens element achieves high polarization purity,high transmitting efficiency and large TTD phase range over 5～16GHz.The aforementioned design method is verified by the experiments of a small-scale lens prototype with 22 × 23 elements and a full-scale one including three 39 × 39-element sub-lenses.Furthermore,the proposed two lenses are characterized by low weight and high anti-seismic ability,showing great application potential in UWB wireless communications and radars.2.Researches on low-cost beam scanning using rotatable broadband planar phase gradient lensFor the beam alighment in long-distance point-to-point communication,inspired by the concept of Risley prism beam steering in optical systems,a low-cost beam scanning method using a pair of rotatable broadband planar phase gradient lens is proposed.Based on the antenna array theory,the linear phase progression over the planar lens is determined firstly according to the required maximum scan angle.Subsequently,to meet the bandwidth requirement of the lens and minimize its quantization phase error,a set of broadband subwavelength non-resonant M-FSS lens elements are efficiently designed under the guidance of an equivalent circuit model.All elements of different sizes can achieve less than 0.6 d B transmission losses and linear transmission phases within the required bandwidth.Then,based on the proposed M-FSS element,two identical circular phase gradient lenses with a diameter of 400 mm are fabricated and placed in front of a high-gain reflector with a diameter of 360 mm for beam steering.Numerical and experimental results indicate that by independently rotating two planar lenses,the reflector antenna can achieve a two-dimensional(2-D)beam scanning within ±15° over14.4～15.35 GHz,and the total insertion losses of two lenses and the scanning gain variations are within 0.8 and 1.4 d B,respectively,which verifies the feasibility of the design.This low-cost mechanical scanning provides a solution to the beam alignment in point-to-point communications.3.Researches on wideband wide-angle scanning technique of phased array-fed lens antennasFor the limited scanning capabilities of planar phased array,a hybrid approach placing a curved lens over a planar array is used to design two phased array-fed lens antennas for wideband wide-angle beam scanning.The first three-dimensional(3-D)printed dielectric dome array(DDA)antenna is proposed to resolve the two-dimensional(2-D)ultra-wide-angle beam scanning of planar phased array,and a general design flowchart is presented.Firstly,a phase delay gradient equation of an ideal dome lens with arbitrary curve function for scan range expansion is derived using ray-tracing method,and a simplified approach is proposed to determine the inner and outer contours of the dielectric dome lens(DDL).Then,two finite-by-infinite one-dimensional(1-D)DDA models are simulated to provide the preliminary evaluation of scanning performances in the E and H planes,respectively,the array excitation phases are optimized by particle swarm optimization(PSO)method with the goal of the maximum gain at each scan angle.As a proof of concept,a 3-D printed DDL fed by an 8 × 8-element E-shaped patch array at 20 GHz is manufactured and tested.The measurements agree reasonably well with fullwave simulations,showing that the designed 3-D printed DDL extends the ±60° scan range of the planar array to ±80° with smaller scan gain variation.The second is X-band phased cylindrical lens antenna using polarization-rotation lens element,which resolves wideband ultra-wide-angle beam scanning of linear phased array.The functions of the designed lens are divided into two parts: on the scanning surface,the semicircular lens can extend the scanning range of the linear array feed;on the non-scanning surface,the linear array and the cylindrical lens are simplified as 1-D single-focus lens antenna model,which can converge beams and improve scan gains.In view of the lightweight,miniaturization and manufacturing difficulty of the cylindrical lens,a wideband polarization-rotation lens element using the flexible polyimide film is designed,and its arrangement is optimized to minimize the total phase error of the constructed 13 × 21-element lens in X-band.Full-wave simulations show that when combined with the polarization-rotation cylindrical lens,the ±60° scan range of the linear 8-element array feed is extended to ±90° in X-band.Moreover,when scanning to different angles at different frequencies,the active VSWR of each array element does not deteriorate significantly,the scanning patterns have no significant distortion and the scan gains are also improved,which verifies the design of the wideband polarization-rotation cylindrical lens.This low-cost hybrid scheme is suitable for radar detection and tracking applications that require ultra-wide-angle scanning performance. |
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