33
Coupling of Electromagnetic Fields into Biological Systems
The bulk of power deposition is on the side of the head nearest to the radiating structure of the cellular telephone. The SAR distribution follows an exponential trend away
from the side closest to the antenna and is considerably lower elsewhere in the head.
The maximum SAR value and its distribution in the head are related to the distance
of the radiating element from the skin surface and the electric current distribution
on the antenna. For example, the highest SAR is typically located near the feed point
of the antenna. Also, the peak SAR is lower for longer antennas compared to shorter
antennas, coinciding with the fact that the high-current region for the longer antenna
is higher up along the antenna and thus further away from the surface of the head.
At the feed point of the antenna, the current density is the highest, which gives rise to
a strong magnetic field. Because of the dominance of quasistatic magnetic field coupling
in the near zone of the antenna, the induced electric field is closely correlated with the
current distribution on the antenna. The maximum SAR and its distribution in the head
follow an eddy current course. Since eddy currents circulating in the ear (pinna) and the
head must go through a narrow appendage, the current density and SAR value are the
highest at this location. Also, RF electric and magnetic fields are decoupled in the near
zone. The effect of the electric field is weaker since the dielectric permittivity of muscle
and brain tissues is relatively high at these frequencies. Thus, inductive coupling of the
magnetic field generated by the antenna current dominates power deposition in the near
zone of a cellular telephone antenna.
Questions have been raised concerning the intercomparability of computational
results using different models of the human head and the mobile telephone transceiver. A comparative dosimetry study under the acronym of CEPHOS—cell phones
standards—was conducted between 1997 and 1999 by investigators from 15 organizations in Europe. The project compared different implementations of the FDTD scheme
and differences in the modeling of the antenna and the head on SAR (Nikita et al.
2000). The project found that the computed results were not sensitive to the ABCs used,
although the variability in peak local SAR and 10-g averaged SAR was mostly related to
the modeling of the head and antenna. For example, the uncertainty related to antenna
modeling in predicting the averaged SAR values was on the order of 12%–15%. The position of peak local SAR was independent of both antenna modeling and the ABCs. The
total uncertainty in computing the power absorbed by the head was mainly related to
the model for the head. It concluded that large differences in numerical results could be
obtained from different numerical models of the head or antenna structure. Another
study evaluated SARs in 14 head models based on different anatomical characteristics,
although the comparison among the different head models focused on differences in
modeling the pinna (Kainz et al. 2005).
As an illustration, Figure 1.12 presents the SAR distribution inside the head with a
cell phone on the right side of a male user with an output power of 0.25 W. The computer
modeling employed a basic voxel size of 1 mm × 1 mm × 1 mm and the voxel size was
reduced to 0.2 mm in head regions close to the phone. In this case, the 0-dB SAR equals
1.25 W/kg. It can be seen that most of the power deposition is on the side of the head
nearest to the radiating structure of the cellular telephone. The SARs are considerably
lower elsewhere in the head.
Coupling of Electromagnetic Fields into Biological Systems
The bulk of power deposition is on the side of the head nearest to the radiating structure of the cellular telephone. The SAR distribution follows an exponential trend away
from the side closest to the antenna and is considerably lower elsewhere in the head.
The maximum SAR value and its distribution in the head are related to the distance
of the radiating element from the skin surface and the electric current distribution
on the antenna. For example, the highest SAR is typically located near the feed point
of the antenna. Also, the peak SAR is lower for longer antennas compared to shorter
antennas, coinciding with the fact that the high-current region for the longer antenna
is higher up along the antenna and thus further away from the surface of the head.
At the feed point of the antenna, the current density is the highest, which gives rise to
a strong magnetic field. Because of the dominance of quasistatic magnetic field coupling
in the near zone of the antenna, the induced electric field is closely correlated with the
current distribution on the antenna. The maximum SAR and its distribution in the head
follow an eddy current course. Since eddy currents circulating in the ear (pinna) and the
head must go through a narrow appendage, the current density and SAR value are the
highest at this location. Also, RF electric and magnetic fields are decoupled in the near
zone. The effect of the electric field is weaker since the dielectric permittivity of muscle
and brain tissues is relatively high at these frequencies. Thus, inductive coupling of the
magnetic field generated by the antenna current dominates power deposition in the near
zone of a cellular telephone antenna.
Questions have been raised concerning the intercomparability of computational
results using different models of the human head and the mobile telephone transceiver. A comparative dosimetry study under the acronym of CEPHOS—cell phones
standards—was conducted between 1997 and 1999 by investigators from 15 organizations in Europe. The project compared different implementations of the FDTD scheme
and differences in the modeling of the antenna and the head on SAR (Nikita et al.
2000). The project found that the computed results were not sensitive to the ABCs used,
although the variability in peak local SAR and 10-g averaged SAR was mostly related to
the modeling of the head and antenna. For example, the uncertainty related to antenna
modeling in predicting the averaged SAR values was on the order of 12%–15%. The position of peak local SAR was independent of both antenna modeling and the ABCs. The
total uncertainty in computing the power absorbed by the head was mainly related to
the model for the head. It concluded that large differences in numerical results could be
obtained from different numerical models of the head or antenna structure. Another
study evaluated SARs in 14 head models based on different anatomical characteristics,
although the comparison among the different head models focused on differences in
modeling the pinna (Kainz et al. 2005).
As an illustration, Figure 1.12 presents the SAR distribution inside the head with a
cell phone on the right side of a male user with an output power of 0.25 W. The computer
modeling employed a basic voxel size of 1 mm × 1 mm × 1 mm and the voxel size was
reduced to 0.2 mm in head regions close to the phone. In this case, the 0-dB SAR equals
1.25 W/kg. It can be seen that most of the power deposition is on the side of the head
nearest to the radiating structure of the cellular telephone. The SARs are considerably
lower elsewhere in the head.