Design of multi-antenna feeding for MIMO terminals based on characteristic modes

Conventional antennas in single-antenna terminals that resonate at frequencies lower than 1 GHz usually rely on the chassis as the main radiator. To effectively exploit chassis excitation for MIMO terminals, each of the multiple antennas is required to excite one distinct chassis mode. However, in today´s terminals, there is typically only one chassis mode that can radiate efficiently at frequencies below 1 GHz. Fortunately, it has been shown that minor modifications in the chassis structure can cause more than one mode to resonate at these frequencies. Nevertheless, proper antenna feeding methods are needed to practically tap into these modes. In this paper, we propose a general technique to feed orthogonal chassis modes of a given conducting structure using the theory of characteristic modes. By separating a radiating structure into individual modal currents, the near field radiating properties are exploited for capacitive or inductive feeding without significant coupling to other orthogonal modes of radiation. As a proof of concept, we apply the technique to feed a modified terminal chassis that has two significant characteristic modes at 0.89 GHz.