Andreas Christ, Arya Fallahi, Esra Neufeld, Quirino Balzano, and Niels Kuster, Bioelectromagnetics 2022, Volume 43, Issue 7, pp. 404-412, online 06 November 2022; doi: 10.1002/bem.22422
In this study, we investigate the absorption of the electric (E-) field induced in homogeneous biological tissue exposed to highly localized field sources, such as the charged tips of antennas, in proximity to the body, where E-field coupling dominates. These conditions are relevant for compliance testing of modern mobile phones, for which exposure is evaluated under conditions of a small separation between the radiator 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 4th power of the distance and with the square of the ratio of the permittivities of the tissue and free-space. The approximation allows the induced maximum E-field to be predicted with an accuracy of better than 1.5 dB.
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