A Highly Integrated MIMO Antenna Unit: Differential/Common Mode Design

A novel concept of antenna design, termed differential mode/common mode (DM/CM) design, is proposed to achieve a highly integrated multi-input multi-output (MIMO) antenna unit in mobile terminals. The inspiration comes from a dipole fed by a differential line which can be considered a DM feed. What will happen if the DM feed is transformed into a CM feed? Some interesting features are provided in this article. By symmetrically placing one DM antenna and one CM antenna together, a DM/CM antenna can be achieved. Benefitting from the coupling cancellation of antiphase currents and the different distributions of the radiation currents, a DM/CM antenna can obtain high isolation and complementary patterns, even if the radiators of the DM and CM antennas are overlapped. Therefore, good MIMO performance can be realized in a very compact volume. To validate the concept, a miniaturized DM/CM antenna unit is designed for mobile phones. The 24.2 dB isolation and complementary patterns are achieved in the dimension of $0.330 \lambda _{0} \times 0.058\lambda _{0} \times 0.019 \lambda _{0}$ at 3.5 GHz. One $8 \times 8$ MIMO antenna array is constructed by using four DM/CM antenna units and shows good overall performance. The proposed concept of DM/CM design may also be promising for other applications that need high isolation between closely packed antenna elements and wide-angle pattern coverage.