A Multisurface Low-SAR MIMO Antenna for Mobile Phone Based on Collinear Reverse Currents and Multiple Hotspots

A multi-surfaces low-specific absorption rate (SAR) multiple-input-multiple-output (MIMO) antenna for mobile phone is proposed based on collinear reverse currents and multiple hotspots, and implemented by reactance loading. The proposed antenna element mainly consists of a common-mode (CM) dipole, a T-shaped power divider, and a center-loaded inductor. The T-shaped power divider is used to generate multiple peak points of tangential electric field and SAR by exciting the dual-end-fed CM dipole with collinear reverse current. The center-loaded inductor, analyzed by an equivalent circuit, further modifies the current distribution to uniform energy distribution, resulting in a multi-surface low-SAR antenna. Furthermore, by analyzing the position and the phase of the four antennas, the multi-surface low-SAR MIMO antenna is obtained. The fabricated antenna element and MIMO antenna can cover the -6dB bandwidth of 3.4 GHz-3.6 GHz, with the isolation of better than 17.5dB, envelope correlation coefficients (ECCs) of lower than 0.065 and efficiencies ranges from 85% to 95% and from 79.1% to 85.1%, respectively. The normalized measured SARs for the antenna element and MIMO antenna are 0.82 W/kg and 0.42 W/kg, respectively, which not only comply with the standards, but also ensure that SARs of the proposed antenna across multiple surfaces remain consistently low.