MIMO antenna system based on orthogonality of the characteristic modes of a mobile device

A novel concept is introduced to realize a multiple antenna system on a small mobile device by coupling to its characteristic modes. According to the theory, the current density on the chassis of a mobile device is the superposition of characteristic mode current densities which have unique resonance frequencies. There are some interesting frequency ranges where more than a single mode is near resonance, i.e. may be appreciably excited and contributes to the overall radiation. For a selective excitation of the characteristic modes near to their resonant frequencies, an arrangement of 4 capacitive couplers and a feed network can be introduced. This type of excitation circumvents crosstalk between the input ports and ensures mutually orthogonal radiation patterns. The main problem to be solved is the simultaneous matching of all modes. The easiest approach is the method of a balanced mismatch (minimization of the maximum mode reflection coefficient). Advanced methods combine this idea with a systematic modification of the parameters of the characteristic modes.