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.

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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.

The influence of the antenna impedance loading on the characteristic wave modes is investigated and evaluated. It is shown that the eigenvalue behavior of the characteristic modes can be modified by impedance loading at specific positions on the antenna. Thereby, it is possible to manipulate certain modes such that they resonate in the desired frequency band. These modes can then be effectively excited to achieve the desired antenna performance. It is found that using characteristic wave modes leads to better intuitive insight compared to the entire current distribution. Furthermore, we present a definition of the modal efficiency of the characteristic modes which can be used to take into account the effect of losses in the matching components or the load elements. Index Terms -Antenna theory, Eigenvalues and eigenfunctions, Impedance loading0018-926X (c)