A new design idea of MIMO beamforming antenna array for compact and thin handheld devices is investigated, where the beamforming function is used for transmitting and the MIMO function for receiving. The new design idea is illustrated by an antenna array consisting of eight printed planar inverted-F elements operating at GSM1900 (1880-1920 MHz) and LTE2300 (2300-2400 MHz). The 8-element antenna array is printed on an FR4 substrate of dimensions 136 mm × 68.8 mm × 1 mm. By using the radiation pattern diversity, good isolations, envelope correlation coefficients and mean effective gains are achieved for MIMO receiving. To realize the beamforming function when the antenna is used for transmitting, an optimal feeding mechanism is introduced by the method of maximum power transmission efficiency, which is then implemented by a continuously adjustable feeding circuit board. With the optimized feeding mechanism, the gain of the antenna array in the desired direction can be significantly enhanced. The effects of the human body on the performance of antenna array are also examined, and the results indicate that the proposed design still exhibits good MIMO and beamforming performances in a practical scenario.
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