| With the rapid development of wide band communication system and the increasing requirements on portable mobile communications, compact and multifunctional wireless communication systems have spurred the development of multi-band and wideband antennas with small size. With focus on those new demands, this dissertation is mainly concerned with the VHF/UHF dual-wideband compact antennas, mobile terminal antennas built on planar system ground plane and some antennas with wide radiation pattern bandwidth. The author's major contributions are outlined as follows:1,The basic principle on electronically small antennas over finite ground is investigated. Begins with the relation between size, Q value and bandwidth of electrically small antenna, the lower bounds of the radiation Q for electrically small antennas above conductive hemispheres and flat disks are first calculated. It is found that the Q limitation variations with the radius of ground plane according to the Bessel functions. A conjecture is proposed: For an antenna enclosed in a hemisphere over a rotational symmetric conductor, when the lowest Q amplitude is achieved, its field outside the hemisphere should correspond to that of an infinitesimally small dipole.2,Optimization methods for antenna design are investigated. After a quick review on some well known optimization design methods for antenna design, a modified Particle Swarm Optimization (PSO), named as Gold Rush PSO is proposed by combining PSO with standard Simplex method. Its performance is verified by several benchmark functions. The combination of optimization methods and the CST software is also considered, where the CST works as a subprogram that can do a full-wave analysis and return results to the main program. As examples, both a multiple circumferential slotted coaxial antenna and a microstrip slot array are designed using this method to achieve shaped radiation pattern. The shaped beam is realized by adjusting the feed line that composed of stepped impedance lines. The slotted coaxial antenna operating at 900 MHz radiates omnidirectional in its H plane, while has a shaped beam in its E plane, which makes it a good candidate for base station antenna. The microstrip slot array antenna has dual-side shaped beam with elevation coverage of 35°, a gain of 10.86dBi and side lobe levels less than -19dB, which make it suit for used in a narrow and long zone such as highways or tunnels. Experimental results demonstrate the validity of this design.3,VHF/UHF dual-wideband compact antennas are studied. Two very compact wideband antennas are designed and prototypes are fabricated and measured. One antenna is planar and has a compact size of only 42cm×20cm, while it covers 30-86 MHz and 110-600MHz with VSWR <3 and band notched in 86-110 MHz. This antenna has gain values larger than -21dBi in the lower frequency band and larger than -5 dBi in the upper band, respectively. Besides, the design of sharing aperture with an L band antenna is also considered. The design idea of the other antenna that has multiple-arm folded wire structure comes from the Q value theory, which makes the antenna has a high utilization efficiency of antenna mounting room. A reasonable tradeoff between radiation efficiency and VSWR bandwidth is achieved by folded multiple-arm structure and lumped elements loaded between arms. In addition, the theoretical performance limits of the two antennas and the influence of ground plane size on the antenna bandwidth are both analyzed, which give decisive guidance to electronically small antenna design.4,Wide band and dual band onboard antennas are investigated. A compact wideband antenna for digital television (DTV), a dual band antenna for WLAN and a tri-band antenna for WLAN/WiMax are proposed. The influence of system ground board on antenna performance is discussed. Methods to mitigate the strong dependence of impedance bandwidth on the ground plane size for onboard antenna are investigated; a novel compact current chocker and a wide band chocker are proposed. Based on those techniques, a ground plane independent dual band antenna is proposed. Measured results of the aforementioned antennas are given.5,Two Antennas with wide radiation pattern bandwidth are designed. A wideband antenna that has omnidirectional radiation in its E plane is proposed. It consists of four modified T-dipoles, with the coupling structure between dipoles the current distribution on the antenna is improved to radiate in omnidirectional E patterns. A novel low profile high gain cavity backed and metallic superstrate covered slot antenna that operates in the 2.4GHz WLAN or ISM band is described. The proposed antenna has a bandwidth of 12% and a gain of 12.3dBi at its center frequency. Moreover, its radiation mechanism is investigated and the design guidelines of this kind of antenna are given.
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