Miniaturization and Evaluation Methods of Mobile Terminal Antenna Structures

In recent years, the trend in the mobile communications market has been towards thinner and mechanically more complex terminal devices, which are able to operate in several wireless systems. Due to this development, miniaturization and performance enhancement of internal mobile terminal antennas have become major challenges for the industry of the field. Not only should an internal antenna be small in size and broadband, it must also be able to ensure reliable power transmission in a real multipath propagation environment. A well-known way to improve the reliability of a radio connection is to use multi-antenna reception in the mobile terminal. In multiple-input multiple-output (MIMO) systems, multiple antennas are additionally used at the base station. In both cases, fast and reliable methods are needed for the performance evaluation of multi-antenna terminals. In the first part of this thesis, the main benefits, drawbacks and applications of coupling element based mobile terminal antenna structures are studied, with special emphasis on antenna miniaturization. Optimum shaping and placement of capacitive coupling elements are first studied in this work. Next, it is demonstrated by simulations and measurements that the bandwidth-to-volume ratio of a mobile terminal antenna structure can be improved significantly by using optimized coupling elements instead of traditional self-resonant antenna elements. To facilitate the implementation of coupling element based multi-resonant antenna structures, a theoretical study on the dual-resonant impedance matching of non-resonant coupling elements is also presented. The feasibility of coupling elements for multi-band terminals is demonstrated with a novel quad-band GSM antenna, which consumes a total volume of only 0.7 cm. Finally, a novel frequency tunable matching circuitry designed for non-resonant coupling elements is introduced. In this work, also the bandwidth, efficiency in talk position, and SAR (specific absorption rate) of mobile phone antennas are studied as a function of frequency over wide frequency band (0.6 GHz 6 GHz) by applying the idea of coupling elements. The results show that below 3 GHz the three parameters are strongly affected by the resonant wavemodes of the chassis, whereas above 3 GHz, the wavemodes of the coupling element dominate. In addition to the above, the resonant wavemodes of the chassis of a clamshell phone are investigated in this work by using coupling elements. The results bring out several challenges, such as a nonradiating resonance, that an antenna designer may face with clamshell phones. The second part of this thesis concentrates on the performance evaluation of mobile terminal multi-antenna configurations. First, the accuracy of a novel measurement based antenna test bed (MEBAT) is thoroughly studied. After this, the performance of several mobile terminal multi-antenna configurations is systematically investigated using the MEBAT. The emphasis is kept on the power reception properties (effective array gain or EAG) of the multi-antenna configurations. An accurate and fast theoretical way of predicting the median EAG of an antenna configuration is proposed in the work. Based on a comprehensive analysis of the theoretical and empirical EAG results, guidelines for optimum radiation pattern characteristics of a multi-antenna configuration are given. Based on the eigenvalue dispersion and capacity results obtained in the studied MIMO environments, it is concluded that it may be difficult to affect the spatial multiplexing properties of a MIMO system by means of handset antenna design. The presented results indicate that a handset antenna designer should mainly focus on maximizing the EAG.