Limitations of Incident Power Density as a Proxy for Induced Electromagnetic Fields

Andreas Christ, Theodoros Samaras, Esra Neufeld, and Niels Kuster, Bioelectromagnetics 2020, Volume 41, Issue 5, online 13 June 2020; doi: 10.1002/bem.22268

The most recent safety guidelines define basic restrictions for electromagnetic field exposure at frequencies more than 6  GHz in terms of spatial- and time-averaged transmitted power density inside the body. To enable easy-to-perform evaluations in situ, the reference levels for the incident power density were derived. In this study, we examined whether compliance with the reference levels always ensures compliance with basic restrictions. This was evaluated at several distances from different antennas (dipole, loop, slot, patch, and helix). Three power density definitions based on integration of the perpendicular real part of the Poynting vector, the real part of its three vector components, and its modulus were compared for averaging areas of λ2/16, 4 cm2 (<30 GHz) and 1 cm2 (30 GHz). In the reactive near-field (d<λ/(2π)), the transmitted power density can be underestimated if an antenna is operating at the free space exposure limit. This underestimation may exceed 6 dB (4 times) and depends on the field source due to different coupling mechanisms; it is further frequency-dependent for fixed-size averaging areas (4 and 1 cm2). At larger distances, transmission can be larger than the theoretical plane-wave transmission coefficient due to backscattering between the body and field source. The use of the modulus of the incident Poynting vector yields the smallest underestimation of the transmitted power density.

The scientific and technical impact of the study can be summarized as:

  • In the reactive near-field at distances <λ/(2π), the evaluation of the Poynting vector does not always yield a conservative estimate of the transmitted power density
  • The extent to which the transmitted power density is under- and overestimated depends on the antenna type
  • The assessment of the transmitted power density using the modulus of the Poynting vector substantially reduces its underestimation
  • Only a dosimetric approach could overcome the above mentioned issues, including the correct assessment of the observed enhancement due to scattering and interference of the source with the exposed body