Investigation of Criterion for Applying Incident Power Density in the Near-Field Region for EMF Assessment at Quasi-Millimeter and Millimeter-Wave Frequencies

In the international electromagnetic field (EMF) exposure guidelines revised in the year 2020, the absorbed power density (APD) and the incident power density (IPD) are the exposure quantities that can be used to determine compliance above 6 GHz for local exposure. It is indicated in the guidelines that the IPD, a quantity to be assessed in free space, cannot be used for compliance assessment in the reactive near field of the antenna of the equipment under the test. In such a condition, the APD, the quantity to be assessed inside the human body, should be used. The reason stated in the guidelines is that the IPD does not appropriately correlate with APD in the reactive near field. However, the guidelines do not explicitly specify the criteria or computation method that can be applied to determine such boundary. Although the rough guide of $\lambda$/2$\pi$ is mentioned, it may be too small and not suitable for EMF assessment of practical user equipment under test. This article investigates a criterion and method for determining the boundary for applying the IPD in the near field. It has been demonstrated by the analytical solution for an infinitesimal dipole antenna, simulated and measured results for practical antenna arrays. For the studied antennas, the results show that the boundary determined by the proposed method could better represent the transition from the reactive near field to the radiation near field compared to the classical reactive near-field boundary when examining the correlation between IPD and APD for EMF assessment.