Systematic design method of a mobile multiple antenna system using the theory of characteristic modes

In this study, the authors present a systematic design method of a three-port multiple-input-multiple-output antenna system for small terminals, such as smart phones based on the selective excitation of orthogonal chassis modes. The underlying idea is based on the theory of characteristic modes (TCM). Selective excitation of these modes is realised by sets of non-resonant inductive coupling elements integrated at predefined positions into the printed circuit board (PCB) layout. Stripline feed networks also integrated within the multi-layer PCB layout care for a good impedance match to 50 Ω. The design method of this antenna system is a straight forward application of the TCM and is therefore of general interest for the design of small terminal antennas. Furthermore, owing to the electrically small couplers that require only limited area at the outer edge of the PCB the method represents an interesting solution with the clear potential to be extended to more ports. The realised prototype resembles the predicted performance very well and therefore proves the applicability of the proposed method.