A reduction in mutual coupling (hereafter referred to as decoupling) is required to enhance the transmission performance of antennas in multiple-input multiple-output (MIMO) devices. In our previous study, a decoupling method was proposed that utilizes parasitic elements and a bridge line to reduce the mutual coupling between two planar inverted-F antennas; however, the decoupling principle of the proposed method was not quantitatively discussed. Therefore, in this study, we utilize a method called characteristic mode analysis (CMA) to quantitatively discuss the decoupling principle. Furthermore, based on the clarified decoupling principle, we propose another decoupling method using four inverted-L-shaped elements, which are placed on the ground plane of the PIFAs. As a result, the mutual coupling between two PIFAs can be reduced from −6.6 dB to −10.2 dB. Therefore, the total antenna efficiency can be improved from 77.4% to 86.1%.
