Design of MIMO Antenna for a Realistic Mobile Phone Platform Using Characteristic Modes

Abstract: The Theory of Characteristic Modes (TCM) provides an alternative means of antenna synthesis in compact mobile handsets by taking into account the chassis. This makes it possible to create dual-element Multiple-Input Multiple-Output (MIMO) antennas in mobile handsets with very little coupling and correlation between the two antennas. But apart from the antennas, modern mobile phones are each equipped with internal components such as a touch screen, a battery, a camera and a microphone that are capable of affecting the characteristic mode behavior of the chassis. Until now, very little work has been done to study the impact of these internal components on MIMO antenna design using TCM. In this thesis, adetailed evaluation was performed on the effects that the internal components of a mobile phone have on the characteristic modes of the antenna chassis and how these effects can be mitigated. In particular, we compared the individual eigenmodes obtained in a plain chassis structure with their corresponding (or equivalent) modes obtained after addition of the components (individually and as complete unit) and quantify the correlation between the corresponding modes. The results show that, at frequencies lower than 1 GHz, the fundamental mode of the plain chassis structure is largely unaffected by the introduction of the internal components. On the other hand, the two modes attributed to the two T-strips on the plain chassis experienced a shift in their resonant frequencies when the internal components were introduced. However, it was found that their original resonant frequencies could be restored using minor structural changes. At higher frequencies, the introduction of the components was found to have very little impact on the characteristic modes. Finally, the chassis with all the components was employed to design a dual-band T-strip antenna, which relied on the restoration of the modal resonant frequencies to provide a broadband resonance at below 1 GHz, highlighting the usefulness of TCM analysis in antenna design.