Compact UHF-band antennas for mobile terminals : focus on modelling, implementation, and user interaction

Abstract

The background of this thesis is the trend of ever decreasing space available for antennas embedded within mobile terminals. At the same time the antennas are increasingly required to cover a large number of separate frequency bands and/or have wideband operation. In addition, those antennas should perform sufficiently well in the vicinity of the user. This forms the motivation for the novel compact coupling-based antennas introduced and studied in this thesis. The operation of the compact coupling antennas is based on exploiting the separate wavemodes supported by the chassis of the mobile terminal. The antenna element itself functions mainly as a coupler which couples to those chassis wavemodes. That is the reason why they are called coupling-based antennas. This work concentrates on the modelling, implementation and design of such antennas in free space, and the effect of the user on the operation of the antenna. The understanding gained in this thesis can be exploited in the development of the antennas for mobile terminals of the future. In the first part of the thesis, an equivalent circuit model is derived for a capacitive coupling-based antenna. This model gives a helpful physical explanation for the operation of this type of antennas. Broadband small antennas operating in the lower part of the UHF band are also implemented and analysed in detail. These kinds of antennas could find applications, for example, in digital television reception, low-band LTE, or the spectrum sensing of cognitive radios. Furthermore, it is shown that very low-profile antennas can be implemented by exciting the chassis wavemodes galvanically with a direct feed. The implementation of frequency-tuneable antennas is also studied in order to cover broad virtual bandwidth or several separate non-simultaneous frequency bands. The second part of this thesis discusses issues concerning user interaction with the coupling-based antennas. The antenna-user interaction is first modelled with an equivalent circuit which provides an improved understanding of the phenomenon. Then, the effects of the user's hands on the operation of the broadband lower UHF-band antenna is studied in detail. In the end of the thesis, a method for reshaping the near fields of the mobile terminal antenna is also proposed, for example, for improved hearing-aid compatibility.