Research And Applications Of Microstrip Conformal Antenna Arrays

In recent decades, the research of planar antenna array has made great progress, and it has been widely used in many applications, but it also has some defects which are hard to overcome. For instance, the scan range is limited. Moreover, the gain of the array is gradually reduced and the beamwidth is gradually widened with the increasing of scanning angle. Conformal antenna array, as an extension of the concept of planar antenna array, can solve these problems well. On the other hand, microstrip antenna is well suited for conformal antenna applications, due to its favorable features of low profile, light weight, low cost and conformability. Microstrip conformal antenna, which combines the advantages of conformal antenna and microstrip antenna, has a broad application prospect.Despite these advantages, microstrip antenna has some deficiencies that need to be improved. For examples, the bandwidth of microstrip antenna is narrow, and the front-to-back ratio of microstrip antenna is subject to the size of the ground plane. In addition, surface wave excited by microstrip antenna intensifies the mutual coupling between microstrip antennas. The dissertation is aimed at researching some new methods and new mechanisms which can improve the performances of conventional microstrip antenna and expanding the applications of microstrip antenna in conformal antenna array. The main content and innovations of the dissertation are as follows:1. The research of the epsilon-negative transmission line(ENG TL)-based annular ring loaded(ARL) microstrip antenna element and array.A new method to improve the performances of conventional microstrip antenna is proposed, that is loading the microstrip antenna with a parasitic ENG TL-based ARL. Compared with conventional microstrip antenna, the proposed antenna exhibits improved impedance bandwidth, suppression of horizontal and backward radiation level, reduced surface wave excitation and lower mutual coupling in array environment. First, linearly polarized(LP) and circularly polarized(CP) microstip antenna elements based on the proposed concept are designed and performance comparisons between the proposed microstrip antenna and conventional microstrip antenna are made. The working mechanism of the new method is presented by dispersion analysis of the ENG TL unit and parametric study of the proposed antenna. And then, the effect of the variation of carrier radius on the performances of the proposed antenna is studied when conformed to a cylindrical platform. At last, the application of the proposed antenna in conformal antenna array is explored. Three conformal antenna arrays based on the proposed antenna are designed and analyzed, which are a uniform linear array when conformed to a conical platform along its generatrix, a uniform linear array when conformed to a conical platform along its circumferential direction and a thinned linear array when conformed to a cylindrical platform along its circumferential direction. To verify the design, prototypes of the proposed antenna element and array are fabricated and measured. Good agreements between simulated and measured results are achieved.2. The research of cavity-backed slot antenna based on higher-order modes.Conventional microstrip antenna array usually utilizes an external feed network, despite its high gain, the feed network increases the structure complexity and decreases antenna efficiency. To solve this problem, a new feed method using the higher-order modes in a cavity is proposed. Based on the new method, high gain can be achieved without using external feed network. First, two LP cavity-backed slot antennas based on higher-order mode fed by a single coaxial probe are designed, one is electrically excited, and another is magnetically excited. A linear-to-circular polarization converter is added in front of the LP antenna to generate circular polarization, and the polarization state depends on the orientation of the converter. And then, a wideband and high-gain slotted cavity antenna using higher-order modes for millimeter-wave application is designed by increasing the height of the substrate and etching out a circular area. The effect of the variation of carrier radius on the performances of the proposed antenna is studied when conformed to a cylindrical platform. At last the application of the proposed antenna in conformal antenna array is explored. Three conformal antenna arrays based on the proposed antenna are designed and analyzed, which are an omnidirectional antenna with vertical polarization, a cylindrical conformal phased array antenna based on TE150 mode and a quasi-spherical conformal beam switching antenna array with the ability of realizing half-space coverage with circular polarization.3. The research of a quasi-conformal microstrip antenna array with high isolationIn many applications of antenna array, high isolation between antenna elements is desirable. Based on practical application, conformal microstrip antenna arrays with high port isolation for WLAN MIMO application are designed. An aperture-coupled microstrip antenna fed by stripline is adopted here as the radiating element, since aperture-coupled feeding mechanism usually outperforms its edge- and probe-fed counterparts in terms of bandwidth and polarization purity. And then, antenna elements are mounted on the side faces of a regular hexagonal prism or pyramid platform. The antenna elements are backed with chokes and additionally, grooves are loaded between elements to improve the isolation performances.