Low-band MIMO antenna for smartphones with robust performance to user interaction

This letter proposes a two-port MIMO smartphone antenna for frequency bands below 1 GHz, which is robust to user effects. The design is achieved by first analyzing the characteristic modes of a chassis that includes the large screen. Two modes predicted to be less affected by the user than other commonly used modes are selected. The modal currents and near-fields of the two desired modes then guide the design: The monopole-like mode introduced by the screen is tuned to resonance using shorting pins and selectively excited using the center feed location. The non-resonant loop mode is selectively excited for the first time by four inductive feeds added along the longer sides of the chassis, with proper phase shifts provided by a feeding network. The proposed antenna features isolation of above 19 dB and envelope correlation coefficient of below 0.12 in the considered scenarios. The measured bandwidth is above 15% for both ports, and the average radiation efficiency is 2 dB and 4.57 dB higher for two user scenarios with respect to a reference design. Moreover, no adaptive matching is needed as the impedance matching is robust to the user hand/head.