Multiband and environment insensitive handset antennas

The number of antennas in small handsets has been increasing, while the volume reserved for the antennas has remained the same or even reduced. Consequently, antennas become easily inefficient, thereby lowering the data rate and coverage. An additional challenge is that the antenna should be insensitive to the way the handset is held by the user. The results of this doctoral thesis can be used to develop small-sized, multiband and efficient handset antennas with good robustness against environmental changes caused for instance by the user's hand. The first part of the thesis introduces novel handset antennas that have state-of-the-art performance in terms of antenna volume, impedance bandwidth and efficiency. A design strategy for 4th generation (4G) handset antennas and a new method to analyze the suitability of an antenna geometry for frequency tuning are introduced. Furthermore, a novel tunable handset antenna with Multiple-Input Multiple-Output (MIMO) capability is presented. The total efficiency of the proposed tunable MIMO antenna is shown to be better than 49% across the frequencies of 698–960 MHz and better than 56% across the frequencies of 1430–2690 MHz when MicroElectroMechanical System (MEMS)-based digitally tunable capacitors are used. Finally, an effective method to achieve up to 100 dB port-to-port isolation is proposed. Different user-related challenges of handset antennas are discussed in the second part of this thesis. It is shown that a part of the hand losses can be avoided with a clever antenna design. This part also discusses the benefits and drawbacks of using either compact or large-sized antennas. A small-sized antenna can be more tolerant to user-originated losses than a largesized antenna when an effective frequency tuning method is used. Furthermore, design aspects for gaining a good multi-antenna performance with the user are discussed. Novel methods to reduce the electromagnetic exposure and the interaction between a user and a mobile handset antenna are discussed in the third part. The thesis gives the first analysis of feasibility of balanced antennas for handsets. Moreover, a novel shielding method to decrease the specific absorption rate (SAR) in the head by 81% and to improve the total efficiency by 5 dB is introduced. In the end of the thesis, a method is proposed to weaken the near-fields of handset antenna: wavetraps can be used to reduce, for example, electric field by up to 75%, resulting in improved hearing-aid compatibility (HAC) and SAR.