Design and implementation techniques of wideband mobile communications antennas (2004)

Wireless communications has been a major motivator of small antenna research during the last decade. New communication systems with wider system bands have been introduced, single-band terminals have evolved into multiband and multimode terminals, the average terminal size has decreased drastically, and internal antennas have been developed into standard solutions. All this combined with strict limitations set for the energy absorbed by the users of mobile terminals has created needs for improved antenna solutions and better understanding of small antennas on small complex platforms. In response to these challenges, it is studied in this thesis how the frequency band, over which the combination of an electrically small antenna and a small radio device efficiently transmit and receive radiowaves, can be systematically maximized.\n\nWhen a small antenna is attached to a small metal object, like the metal chassis of a mobile phone, the size and shape of the object and the position of the antenna on it can have a strong effect on the antenna performance. The thesis shows that the behavior of a radiating system formed by a small antenna and the metal chassis of a small radio device can be studied by approximating the system as a combination of the separate resonant wavemodes of its components. The modes of the antenna and the chassis are described with resonant circuits that are combined into one dual-resonant equivalent circuit model. It is shown with the model that the characteristics of the antenna-chassis combination depend on the unloaded quality factors and the relative amplitudes of the resonant modes of the antenna and the chassis. Based on the results obtained with the circuit model, several important conclusions on the significant properties of the system can be drawn.\n\nThe effect of the metal chassis on the bandwidth, radiation efficiency, and SAR (specific absorption rate) of internal mobile phone antennas is also studied with electromagnetic simulations in the thesis. The results support those of the resonator-based analysis and show that in addition to the impedance bandwidth, the radiation efficiency in talk position and SAR depend strongly on the parameters of the phone chassis.\n\nWhen the size and efficiency are fixed, making a small antenna dual-resonant or multiresonant is a very effective method of increasing its bandwidth. The method is extensively studied in this thesis, which presents for the first time a unified theory for the impedance bandwidth optimization of small antennas comprising two coupled resonators with arbitrary unloaded quality factors (Q01 and Q02). Simulated and measured results are presented to support the theory. In addition, results for novel antenna designs that were developed based on the theory are presented. An example of the results is the first published single-feed internal mobile phone antenna that covers the frequencies of E-GSM900, GSM1800, GSM1900, and UMTS with a return loss of at least 6 dB and high radiation efficiency.\n\nOne of the main problems of increasing the effective bandwidth of a small resonant antenna with electrical frequency tuning is power loss in the tuning circuit. A systematic method for the minimization of RF power loss in certain frequency-tuning circuits of small resonant antennas is developed and demonstrated in the thesis. The design principles are also adapted to the design of a novel frequency-tuning circuit for internal mobile phone antennas. It enables adding a new band of operation to an existing dual-band antenna structure.