SAR assessment of miniaturized wideband MIMO antenna structure for millimeter wave 5G smartphones

This work introduces a wideband two-element Multiple-Input Multiple-Output (MIMO) antenna array that covers the desired frequencies of 28/38 GHz for millimeter-wave (mmW) 5G smartphones. The antenna array demonstrates significant isolation and gain increase based on dual-mode planar dipole antennas. The proposed structure was designed using CST Microwave Studio 2019. The design is implemented on a Rogers RT 4003 substrate measuring 14.76 × 8.38 mm² with a dielectric constant of 3.55. It features two planar dipole antennas positioned at the corners in a perpendicular arrangement to each other. To achieve the desired wideband performance, each element consists of a dipole antenna and a partial ground plane. The spacing between elements, including the parasitic element (PE), is set to 0.5λ₀ to increase isolation between the MIMO antenna elements while keeping complexity and cost to a minimum. Simulation results demonstrate an improvement in mutual coupling between array members, with measured values ranging from −55 dB to −75 dB. The envelope correlation coefficient (ECC) is also improved. Furthermore, enhancements are observed in the total active reflection coefficient (TARC), mean effective gain (MEG), and diversity gain (DG). The measured gains of the proposed designs range from 6 dBi across the entire band to 10 dBi at 40 GHz, with a radiation efficiency close to 95%. The antenna performs well in the presence of the handset and human head model during simulation. The study identifies a safe and acceptable specific absorption rate (SAR) value, which provides a low SAR10g of about 0.963 W/Kg at 28 GHz and 0.583 W/Kg at 38 GHz. while maintaining superior efficiency and radiation patterns. When the antennas are constructed and tested, the experimental results surpass the modeling results. The simulation and test findings demonstrate a satisfactory fit within the target band, suggesting that the proposed structure could be applied to millimeter-wave 5G smartphones.