| In this thesis we show how characteristic mode (CM) theory can be used to in the design of the antennas with the low envelope correlation coefficients preferred for MIMO antennas. This is due to the fact that the radiated fields of CMs are orthogonal over the radiation sphere. We show how a handheld device structure can be excited at various points (which form the multi-antenna ports) to ensure that each port excites a set of CMs such that the mode indices in each set are different for each antenna. It is this important result which causes the natural orthogonality properties of the CMs to be transferred to the overall radiation patterns of the multiple antennas. The CM analysis we have performed includes the effects of structure losses; it appears to be the first time that the computation of such modes have been reported. A logical and satisfying methodology for MIMO antenna design is the result. The methodology is extended to include arbitrary MIMO scattering environments using polarization sensitive generalized characteristic modes, which is the first of its kind from both the theory and methodology perspective. This design approach for MIMO antennas is not restricted to handheld devices, and can be used in a variety of applications. Lastly we discuss the various port mode theories that yield far-field orthogonality for perfect scattering environments, the application of which has already been discussed in the literature. We show succinctly that physical port symmetry yields frequency insensitive modes, but very minor symmetry breaking yields very narrow low envelope correlation bandwidth, something that is not discussed in the literature, but discussed in detail in this thesis. |
