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In this paper we present concepts for the selective excitation of orthogonal modes of small terminals by inductive and capacitive coupling elements. The concepts can be used to implement an efficient MIMO (Multiple Input Multiple Output) antenna system based on the TCM (Theory of Characteristic Modes). Both coupling concepts are discussed with respect to their ability to excite specific modes separately. An important result obtained is that inductive couplers offer advantages over capacitive couplers in exciting the desired modes purely even when they are not close to their modal resonance.

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.

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This letter introduces a novel 2.6-GHz multiple-input-multiple-output (MIMO) antenna system for mobile terminals. The antenna structures consist of a broadband main antenna covering most of the LTE-A bands and a narrowband second antenna operating at the 2.6-GHz band. The main antenna is a traditional monopole-type capacitive coupling element (CCE) placed on the short edge of the terminal. The second antenna consists of two out-of-phase fed inductive coupling elements (ICEs) placed on the long edges of the chassis. The main purpose of the letter is to demonstrate that this kind of antenna system offers good performance in terms of electromagnetic (EM) isolation and envelope correlation between the antennas. This has been experimentally verified and is originated on the fact that both antennas excite effectively different orthogonal wavemodes, which is studied with the help of the theory of characteristic wavemodes.