Evaluation of MIMO handset antennas with decorative metal elements, using characteristic modes

The design of antennas for sub-1GHz frequency bands is particularly challenging, since the antenna becomes electrically small and the radiation is mainly provided by the handset chassis. For a MIMO antenna system, sharing the same chassis as radiator by multiple antennas, usually results in a very strong coupling between antennas, high correlation and decreased radiation efficiency. Recent work on application of the theory of characteristic modes to MIMO antenna has shown that the appropriate chassis modifications can be used to generate multiple characteristic modes below 1GHz. The decorative metal elements (handset surrounding rings or back-covers) often used in handsets, can be used for such purpose. The objective of this paper is to design a MIMO antenna, using the handset metal back-cover and obtain two sub-1GHz characteristic modes. The inherent orthogonality of the modes will provide good MIMO performance.