| With the rapid development of wireless communication, the miniaturization and integration of communication systems has become the trend of the development and attracted more and more attention. Antenna is a crucial part at the front end of communication equipments. The performance of antenna directly affects the overall quality of communication. As a result, the research of antenna for mobile terminal has become widely involved and great importance has been attached to it. The focus of related research is on the miniaturization and multi-band of the antenna. For the current 3G and the forthcoming 4G telecommunication system, the physical layer structure for MIMO (Multiple Input Multiple Output) multi-antenna array which enables high channel capacity, high transmission rate and excellent communication quality has definitely become a key technology.Due to the development of wireless communication, the growing popularity of mobile communication products has also give rise to the increasing concern about the threaten of electromagnetic radiation on human health. Generally, the amount of human exposure to EM energy is measured by specific absorption rate (SAR).In this paper, several methods and techniques to achieve minimization and multi-band application for mobile terminals are summarized. By combining the advantages of above methods, a novel miniaturized antenna in terminal and a two-port MIMO antenna are designed which are practical in use, easy for manufacturing and miniaturized in size. Besides, a whole set of evaluation model is setup to enable precise and effective SAR estimation for miniaturized mobile terminal. The main contribution of this paper is listed as follows. Firstly, the antenna's basic working mechanisms and parameters for electrical properties are analyzed. Secondly, by combining the above theories and the techniques to achieve miniaturization and multi-band application properly, a miniaturized antenna is designed which the characteristics of unique structure and excellent performance. This miniaturized antenna is used in mobile terminal with the size of 24 mm * 9.5 mm * 1.8 mm and covers the frequency band of GSM (900M), DCS, PCS, UMTS, WLAN (2.4G) and DMB. A technique using the network cancelling is applied to the above antenna so as to improve the isolation between MIMO antenna. Using this technique, two types of two-port MIMO antenna are designed and validated by measurement. One antenna is working for the band of GSM (900M) and the isolation over the whole frequency band of GSM (900M) is better than -12 dB. The other antenna improves the isolation for DCS, PCS and UMTS, with the isolation better than -20 dB.Last but not least, a whole set of SAR evaluation model is setup for miniaturized antenna in mobile terminal to enable precise and effective SAR estimations. SAR evaluation model is established in two stages, one for antenna model and the other for systematic model. Then, by applying transmission line theory, the systematic model can be simplified without the sacrifice of accuracy. |
