SAR evaluation of MIMO antennas with a wide tunable range power divider

Characterizing specific absorption rate (SAR) of multi-antenna system is a facing challenge for implementing large-scale multiple-input multiple-output (MIMO) communications in mobile devices. For body-worn antennas, active circuit components capable of overcoming dynamic body area network (BAN) channels make MIMO-SAR complex in compliance testing. This study is the first to present the SAR of a body-worn multi-antenna integrated by 2-way microwave power dividers (PDs) with a wide tunable range over 20 dB. Dosimetry analysis was performed using a multi-layer human body model, which was exposed by orthogonally fed antenna arrays with tunable PDs at 2.45 GHz. The dependence of MIMO-SAR on the weight function, i.e., power ratio that is determined by considering the variation in cross-polarization power ratio (XPR) and antenna tilt angle, was investigated. It is found that over a wide tunable power ratio range from 0.1 to 10, the local average MIMO-SAR is dependent on weight function combinations. When the human body is very close to the antenna, the coupling between the microstrip structure of the PD and the human body will affect the local average SAR. The results also show that by increasing the number of array elements, the dependence of the ratio of the average SAR to MIMO channel capacity on the power ratio of the PD will be significantly reduced. The findings of this study provide guidelines for designing integrated structures of wearable antennas and active microwave circuits with low SARs.