Broadband Base Station Antennas For Mobile Communication Applications

Base station antennas are important parts of mobile communication systems, which are communication interfaces between the mobile terminals such as mobile phones and the base stations. Also, base station antennas are necessary carriers for communication systems to realize frequency reuse, diversity reception and other technologies. The advent of 4G era coexisted with the 2G, 3G systems gives rise to a huge demand for LTE base station antennas. In order to solve the problem of site scarcity and meet the demands of integrated, multi-function base station antennas, the newly designed base station antennas are required to be broadband with good performance and can be applied in the band of LTE(the low frequency band covers 698-824 MHz and the high frequency band covers 2.3-2.7 GHz). Thus, the investigation on broadband LTE base station antennas has important research and engineering value. The major works are as follow: 1. A broadband dual-polarized base station antenna is presented. The antenna consists of two similar linearly polarized dipole antennas and a shaped reflector. Two linearly polarized dipole antennas are orthogonally nested together and are fixed on the reflector. Each dipole antenna is composed of a half-wave dipole, a coplanar microstrip line and a L-shaped microstrip feed line, which are all printed on the same substrate. The configurations of the two dipole antennas are basically the same, except that the bending direction of the L-shaped microstrip feed line and the slotting direction of the substrates are opposite. The dipole antenna has wide arms, which adopts a broadband balun to achieve broadband performance. The shaped reflector is used to adjust the half power beam width for the horizontal plane. Measured results show that the antenna has a 37% relative bandwidth with the 15-d B return loss bandwidth covering 0.69-1 GHz band. The isolation is better than 30 d B. The radiation patterns are stable. The antenna gain is about 7 d Bi. 2. A broadband circularly polarized base station antenna is presented. The antenna consists of two circularly polarized antenna elements, each of which is composed of an antenna radiator, a feeding structure and a reflector. The antenna radiator is a square loop. Circular polarization performance is achieved by cutting two slots on the opposite sides of the loop. A pair of smaller parasitic loops are introduced to improve the axial ratio bandwidth. The antenna element adopts a broadband balun to achieve broadband impedance bandwidth. When the circularly polarized antennas are arrayed to be a two-element antenna array, the axial ratio of the two-element antenna array becomes worse. By appropriately selecting the element spacing between the antenna elements and thinning the short arms of the square loops, the axial ratio of the antenna array is largly improved. Measured results show that the circularly polarized two-element antenna array has a 42% relative bandwidth, the 15-d B return loss bandwidth covers 1.7-2.6 GHz. The axial ratio of the antenna array over 1.7-2.6 GHz is less than 3 d B. The gain is about 11 d Bi, and the radiation patterns are stable. 3. Based on previous investigations, a four-element broadband dual-polarized antenna array and an eight-element broadband circularly polarized antenna array are proposed. The element spacing of the dual-polarized antenna array is 300 mm(~λ0.98GHz). The measured results show that the dual-polarized antenna array has a 37% relative bandwidth with the 15-d B return loss bandwidth covering 0.7-1.02 GHz. No grating lobes appear. The gain of the dual-polarized antenna array is 14 d Bi. The antenna radiation patterns for the two polarizations are similar and the radiation patterns are stable.The element spacing of the circularly polarized antenna array is 115 mm(1.04λ2.7GHz). The measured results show that the antenna array obtains a 40% relative bandwidth, the 15-d B return loss bandwidth covers 1.7-2.55 GHz. Within the operation bandwidth, the axial ratio is less than 3 d B. No grating lobes appear. The antenna gain is about 16 d Bi. The radiation pattern are stable over the frequency bandwith of 1.7-2.7 GHz. The antenna arrays are suitable to be used as LTE base station antenna which are backward compatible with 2G and 3G systems. This thesis systematically investigates the operating principles and design methods of different types of broadband LTE antennas; thus enriches the antenna theory. This thesis also provides an exploration on the application of the broadband circularly polarized antenna in mobile communication base stations.