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.
The scientific and technical impact of the study can be summarized as:
- Capacitive coupling is dominant for electrically small antenna elements located close to the exposed tissue, whereas inductive coupling dominates at distances of a few millimeters or more
- The attenuation of the capacitively coupled E-fields in the tissue reaches the 4th power of the distance and is significantly higher than that of inductive coupling
- In the tissue, the amplitude of the perpendicular incident E-field is reduced by up to 60 dB in the extremely low frequency range because of the large contrast in dielectric properties of the tissue compared to free space
- For applications operating in the low MHz range, such as wireless chargers, exposure due to capacitive coupling is negligible due to the high dielectric contrast, and inductive coupling is dominant
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