| The antenna is an indispensable key component of wireless communication systems. Its main role is to receive and transmit electromagnetic energy in a particular direction. Therefore, the performance of the antenna plays a decisive role in the whole wireless communication systems. As the volume of wireless communication equipments reducing and the working bandwidth broadening, all kinds of the methods used for broadening the antenna impedance bandwidth and reducing the size of antenna have been extensively concerned and studied in-depth. Thus, the development of the antenna has been put forward to miniaturization and wideband performance. In this dissertation, the commonly method used for broadening the antenna impedance bandwidth and reducing the antenna size is firstly introduced. Then the research status quo at home and abroad of ultra-wideband antennas is discussed. On this basis, this dissertation mainly explores the research of miniaturization, bandwidth broadening and unidirectional radiation for planar printed antenna. The thesis mainly consists of following parts:1. Bandwidth broadening technology based on the loading theory.For the loading theory, the bandwidth broadening technology based on the principle of closely spaced loading is firstly studied. The principle of closely spaced loading is based on that the antenna input impedance will be reduced when the near field of antenna is loaded. By introducing a metal strip element to the main radiation direction of a rectangular loop with two gaps, the input impedance of the rectangular loop antenna is reduced at high frequencies by the effect of electromagnetic coupling. Then the antenna can be easily matched with the feeding port in a wider frequency band. Measured results show that the input impedance of the rectangular loop at high frequencies is significantly reduced and then the impedance matching of the antenna is greatly improved. The measured impedance bandwidth of the rectangular loop antenna has been significantly increased from 8% to 40.7%, ranging from 2.29 GHz to 3.46 GHz. In addition, the performance of the rectangular loop antenna with two loading strips is also studied. The measured impedance bandwidth of the rectangular loop with two loading strips is about 62.6%, ranging from 2.29 GHz to 4.38 GHz.The bandwidth broadening technology based on terminal loading is then studied. A compact planar printed dipole antenna based on terminal loading technology is proposed. At the end of the feeding slotted line, by designing a beveled slot with stepped connection structure, the impedance matching of the dipole antenna is significantly improved and then a wider impedance bandwidth is obtained. Measured result of an antenna prototype demonstrates the effectiveness of terminal loading technology used for broadening antenna impedance bandwidth. The measured result shows that the fabricated dipole antenna achieves a wide impedance bandwidth ranging from 2.65 GHz to 17.5 GHz with a compact size(33 mm×16 mm). The proposed dipole antenna is also compared with some similar printed dipole antennas with respect to impedance bandwidth, substrate dielectric constant, antenna gain and the overall size of antenna for demonstrating its advantage.2. Bandwidth broadening and miniaturization technology of printed Yagi antenna.Firstly, a compact planar printed Yagi antenna with wideband performance is presented. The impedance matching of the antenna at high frequencies is improved by using a parasitic strip and then the impedance bandwidth is broadening. A stepped connection structure between the CPS and the feeding slotted line is adopted to improve the overall impedance matching of the antenna. And then the wideband performance is achieved. The experimental results show that the fabricated antenna can provides a 92.2% impedance bandwidth, which is ranging from 3.8 to 10.3 GHz. Within the effective bandwidth, a moderate gain, which is better than 4 dBi, is also obtained.Secondly, the miniaturization of printed Yagi antenna is also studied. When the lateral size of the ground plane is reduced and two stubs are symmetrically extended from the ground plane, the effective electrical length of ground plane is extended and the miniaturization of antenna is achieved. Compared with the conventional printed Yagi antennas, the low-end bandwidth limitation of the antenna is extended from 3.9 GHz to 3.6 GHz. The width of the Yagi antenna is also reduced by approximately 16.7%. Antenna miniaturization and wideband performance is achieved simultaneously. The measured bandwidth is ranging from 3.6 to 11.6 GHz. In addition, the measured group delay shows that this printed Yagi antenna has a good time-domain characteristics.3. Bandwidth broadening and miniaturization technology of open slot antenna.First of all, a printed stepped open slot antenna with wideband performance is presented. By introducing a parasitic metal strip and modifying the ground plane with a pair of L-shaped slits, the bandwidth of the open slot antenna is significantly extended and the radiation directivity of the proposed antenna is also enhanced. Experimental results show that the fabricated antenna provides a fractional bandwidth of 101%, ranging from 3.65 to 11.1 GHz with a compact size(30 mm×30 mm). Secondly, the miniaturization technology of a typical kind of tapered slot antenna(Vivaldi antenna) has been studied. A Vivaldi antenna based on microstrip to slotline transition structure is proposed. By using the modified ground plane, the effective electrical length of the Vivaldi antenna is extended and the miniaturization of antenna is achieved. At the same time, by adopting the stepped connection structure between the slotline and the tapered slot, the overall impedance matching of the antenna is improved. The measured results of a fabricated prototype verified the effectiveness of the proposed design. The fabricated antenna can realize a 144.5% impedance bandwidth ranging from 2.45 to 15.2 GHz within a compact size(40 mm×45 mm). In addition, good time domain characteristics with nearly constant group time delay are also achieved.4. The radiation mechanism and equivalent feeding structure of double-dipole antenna with anti-phase feeding(W8JK array antenna).In the practical applications, it is difficult to provide a pair of anti-phase signals with equal amplitude for feeding W8 JK array antenna in planar structure. Therefore, a new equivalent feeding mechanism with left and right feeding structures is presented to solve the difficulty of realizing a pair of anti-phase feeding signals in the planar structure. Firstly, the radiation mechanism of W8 JK array antenna is investigated. And then the unidirectional radiation characteristics of W8 JK array antenna are studied in theory by designing the two dipole elements with different lengths. Secondly, the realized structure of equivalent feeding mechanism realized by using a pair of parallel striplines is discussed. Based on microstrip balun, which is used to provide a pair of anti-phase feeding signals with equal amplitude, a W8 JK array antenna is designed. Simulation and measured results verified the correctness and effectiveness of the proposed equivalent feeding mechanism. At last, based on in-phase power splitters, an anti-phase feeding network is first time proposed. To demonstrate its effectiveness, a W8 JK array antenna is designed and fabricated. Measured results of the fabricated prototype verified the effectiveness of the proposed anti-phase feeding network.
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