To provide large channel capacity and high spectral efficiency in mobile communications systems, multiple antennas can be used within a small device. In a multiple-input multiple-output (MIMO) system, however, it is difficult to reduce the mutual coupling between antenna elements. At Seoul's Hanyang University, Jaewon Lee, Seokjin Hong, and Jaehoon Choi have proposed a solution to this problem with a low-profile, ultra-wideband (UWB) MIMO antenna for a personal digital assistant (PDA).
To improve the impedance bandwidth, a 2-x-1- mm connecting strip is used on every antenna element. The isolation characteristic between those elements is improved by inserting two T-shaped stubs. From 2.2 to 11 GHz, the antenna offers a 9-GHz bandwidth with return loss of 10 dB or more and isolation of 20 dB or less. It provides an envelope correlation coefficient below 0.12. See "Design of an Ultra-Wideband MIMO Antenna for PDA Applications," Microwave And Optical Technology Letters, October 2010, p. 2165.