Coupling element antennas for small terminals based on the characteristic modes
Future mobile communication systems use the broadband and multiple antenna technology (MIMO) to increase the spatial efficiency of the transmission channel and therefore provide higher data rates over a particular frequency range. In this thesis, the Theory of Characteristic Modes (TCM) was followed, which allows principally the use of the orthogonal chassis modes, decomposed out of the current density distribution, in order to design broadband and multiple antenna systems. A major challenge was the development of suitable coupling structures to excite several modes independently. Here, the capacitive, the inductive and the hybrid coupling concepts were discussed and compared for the selective coupling of the characteristic modes. A reconstruction of the resulting radiation patterns and the calculation of the power coefficient realized the specific coupling of the modes, the respective power budget and antenna matching as well. In particular, the results indicated a direct relationship between the characteristic modes and the low correlation of the transfer functions in multi-antenna systems. In order to show a direct coupling of the several concepts, different kind of feeding networks at the feed-point of the investigated coupling element have been developed and evaluated. Moreover, they include the power splitter and the matching networks as well. On the development of multi-band and broadband antennas, different coupling concepts for the common coupling of several modes have been investigated. This kind of excitation could be realized by using the coupling elements itself as a tuning element or by using space efficient matching networks at the feeding port. The results of this thesis reveals the utilization of the theory of characteristic modes as a synthesis tool for the design of the space efficient multi-antenna systems, as well as multi-band and broadband antennas by direct coupling of the chassis itself.
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