Mechanism of Capacitive Coupling of Proximal Electromagnetic Sources With Biological Bodies

This study investigates the absorption of the induced E-field in homogeneous biological tissue exposed to highly localized field sources in proximity of the body, such as the charged tips of antennas, where E-field coupling dominates. These conditions are relevant for compliance testing of modern mobile phones where exposure is evaluated at small separation between radiators and the body. We derive an approximation that characterizes the decay of the induced E-field in the tissue as a function of distance. The absorption is quantified in terms of the local specific absorption rate (SAR) at the tissue surface as a function of the charge at the antenna tip. The approximation is based on the analytical evaluation of the E-fields of a charged disk under quasi-static conditions. We validate this approximation using full-wave simulations of dipoles. We demonstrate that the coupling mechanism of the E-field is dominated by the perpendicular field component and that wave propagation need not be considered for the characterization of the exposure. The surface SAR decreases approximately with the fourth power of the distance and with the square of the ratio of the permittivities of the tissue and free-space. The approximation predicts the induced maximum E-field with an accuracy of better than 1.5 dB. © 2022 Bioelectromagnetics Society.