Characteristic Mode-Based Neutralization Line Design for MIMO Antenna

Based on the theory of characteristic modes (TCMs), this paper proposes a systematic design procedure to guide the design of a neutralization line (NL) for a dual-band multiple-input-multiple-output (MIMO) antenna to achieve high isolation. According to the direction of the current flow of significant characteristic modes, coupling and uncoupling modes are identified and a metal NL is appropriately designed to cancel the coupling modes. More specifically, it introduces new currents opposite in direction to the original coupling characteristic currents, thus reducing mutual coupling. Compared with the traditional NL design method, the proposed approach simplifies the optimization process and saves lots of optimal design time. As an illustration of the proposed design method, we design another dual-band antenna operating in the WLAN bands with a compact size of 50 mm x 48 mm x 0.8 mm and minimum edge-to-edge space between the radiating elements with 0.033 lambda(0) of the lower band. The impedance bandwidths for S-11 <= -10 dB are 0.42 GHz (2.27-2.69 GHz) and 0.24 GHz (5.6-5.84 GHz). A fabricated prototype shows good agreement between the measured and simulated results. The introduced NL improves the isolation degree by about 18 dB for the 5.8 GHz band, and the total efficiency of the antenna was also improved. In addition, the proposed antenna also has a low envelop correlation coefficient (ECC < 0.3), high diversity gains, and low total active reflection coefficient (TARC) in the frequency band of interest. Measurement results show that the proposed MIMO antenna system is suitable for WLAN applications.