A Study Of Broadband Antennas And Its Applications In Base Station Antennas

With the rapid development of mobile communication systems in China, the co-system with 2G, 3G and recent TD-LTE and FDD-LTE makes a new demand on the base station antenna working as a receiver and a transmitter for the communication systems – namely, all-parameter wideband antenna. On the one hand, a wide impedance bandwidth is required for the all-parameter wideband antenna; on the other hand, the antenna should have a stable radiation pattern, a stable gain, a high cross polarization discrimination(XPD), etc, across the operating frequency band.Dual polarized base station antennas are widely applied to the mobile communication systems to reduce the effect of multipath fading and increase channel capacity. Consequently, the system performance is greatly improved. Also, the technique of antenna array is extensively used in the cellular mobile communication systems to improve the antenna gain. In order to reduce the interference of the co-frequency areas, the antenna array should have a beam down tilt. Mechanical down tilt is applied to the antenna array when the degree of the beam down tilt is small for a simple structure, a low cost, a high reliability and a high gain. However, electrical down tilt exhibits a better performance when the degree of the beam down tilt is big or when a network optimization is required for the complicated application occasions.In the recent years, differential-fed antennas and circularly polarized antennas have attracted more and more scholars’ attention. Compared with the single-fed antennas, differential-fed antennas reveal a major advantage i.e. the common mode signal elimination. Also, differential RF circuit designs are extensively used in modern systems and the differential-fed antennas are more compatible to them. The differential circuit cannot be directly connected to the conventional antenna which only has one port. As a result, a balun is usually needed to make a transition between the differential circuit and the conventional antenna. Many scholars also focus on the study of circularly polarized antennas for their advantages of resisting multipath fading.In this dissertation, wideband base station antennas covering the frequency band from 1.71 to 2.69 GHz, a wideband unidirectional differential-fed antenna and a wideband unidirectional circularly polarized antenna are investigated. In the process of research, some difficulties and problems appeared. How can we design a base station antenna to make sure it can meet the circuit performance parameters and the radiation performance parameters of the base station antenna at the same time? We were also confronted with a lot of new challenges to design a wideband unidirectional differential-fed antenna for an insufficient research on it. Another challenge is how to broaden the 3-d B axial ratio bandwidth of the circularly polarized antennas. This research exhibits a novel feed structure of base station antennas and some new methods to design wideband unidirectional differential-fed antennas and wideband unidirectional circularly polarized antennas.The research steps are shown as follows: Firstly, the methods to reach a wideband impedance matching and a wideband stable radiation pattern of antennas were studied and summarized and fundamental antenna theories were required before designing an antenna. Secondly, we began to design a base station antenna element for the 4G frequency band from 1.71 to 2.69 GHz. When the antenna element could meet the requirement of the base station antenna, we started to design antenna array. Afterwards, a wideband unidirectional differential-fed antenna and a wideband unidirectional circularly polarized antenna were developed using the original antenna model of the base station antenna element.