Design of high-efficiency antennas for mobile communications devices

Abstract

This thesis deals with the design of high-efficiency antennas for small mobile communications devices. Owing to the continuously stricter requirements set for multisystem mobile terminals, the ongoing need for efficient antennas in personal mobile communications is evident. In this work, the entire system consisting of the antenna; the mobile terminal working actually as part of the antenna; and the user of the terminal is considered. The ratio between the power radiated into the free space and the antenna input power, i.e. the total efficiency of this system, forms a general concept for the studies. The total efficiency is partly affected by the losses in the antenna element. As the antenna efficiency, bandwidth, and volume are strongly interrelated exchangeable quantities, it is essential to find other approaches for enhancing the antenna efficiency than simply sacrificing other performance. Further, the metal chassis of a mobile terminal has to be part of the antenna element design because of its considerable effect on antenna performance. In addition, the total efficiency of the entire system is partly affected by the losses owing to the user. Thus, the evaluation of antenna performance is equally important when the mobile terminal is located near a user or when it is in free space. The main goal of this work is to provide novel and useful information for the design of mobile terminal antennas with special emphasis placed on the maximization of the total efficiency.

To obtain necessary background understanding for the design of antennas with minimized user interaction, the general energy-absorption mechanism in the human tissue is studied in this thesis. It is shown that the peak SAR (specific absorption rate) is not actually related to the antenna current, as has been commonly believed. Instead, the SAR maximums can be explained by inspecting the antenna's quasi-static electric near field components perpendicular and parallel to the surface of the tissue at the air-tissue interface and utilizing the boundary conditions of quasi-static fields at the interface. As SAR is directly proportional to the total electric field in the tissue, the SAR distributions caused by a certain antenna differ considerably in tissues with different permittivity values, e.g. brain and fat.

The bandwidth, efficiency in talk position, and SAR performance of a typical monoblock handset antenna-chassis combination is comprehensively investigated in this work for clarifying the roles of different parts of the radiating system. The system is treated as a combination of the separate wavemodes of the antenna element and the chassis. Based on the results, guidelines are given to control or analyze the combined performance both in the sense of radiation properties (bandwidth, efficiency) and user interaction (SAR). It is also demonstrated that there is a connection between the studied three performance parameters: a local maximum in SAR values and a local minimum in radiation efficiency occur when the bandwidth reaches its maximum and the resonant frequency of the chassis equals that of the antenna.

The suitability of dielectric resonator antennas (DRA) for mobile terminals is studied theoretically and experimentally with the main attention paid to the loss characteristics. It is observed that DRAs are appropriate for this purpose especially when very small antenna elements are needed. As an application example, a novel means to realize a high-performance dual-resonant antenna design for mobile terminals is presented.

In addition, losses in the frequency-tuning circuits of small resonant antennas are systematically investigated. Design guidelines for tuning circuits with minimized losses with respect to the achievable tuning range are given. Based on the proposed theory, a low-loss tuning circuit with suitable characteristics for mobile terminal antennas is introduced.