Safety Evaluation of EM Absorption in the Human Head for Portable Devices above 3 GHz

This study was motivated by in-harmonization among various safety guidelines for the peak specific absorption rate (SAR) restriction for portable devices at frequencies above 3 GHz. Based on the fact that the SAR in the user body concentrates only on superficial tissue at such a high frequency band, the feasibility with the freespace power density as a reference level of the safety evaluation was investigated numerically at 3 - 6 GHz. From the numerical results obtained with a multi-grid finite-difference time-domain (FDTD) method in conjunction with an anatomically based human head model, a relationship between the spatial peak SAR in the human head and the free-space power density in a far field region for a typical half-wavelength (λ/2) dipole antenna was derived. As a result, it was found that when the freespace power density meets 1 mW/cm² at a distance of 0.5λ from the antenna, the one-gram averaged spatial peak SAR would not exceed 1.6 W/kg, and the ten-gram averaged spatial peak SAR would not exceed 2 W/kg. Application of this reference level to a 5 GHz portable device demonstrated its feasibility.