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Coupling of Electromagnetic Fields into Biological Systems
and 20% higher at 835 MHz compared with the values for adult head models. It was
suggested that the shielding effect of the pinna was larger at the higher frequency of
1900 MHz. Moreover, the observation was reported as the result of a greater extent
of absorbed energy distribution for the child models at both 835 and 1900 MHz and
the thinner ears of the smaller models, resulting in the antenna being located closer
to the brain region. In contrast, Schoenborn, Burkhardt, and Kuster (1998) concluded
that there were no differences in the absolute extent of SAR distribution between adult
and child head models. It should be noted that the SAR values were normalized to the
antenna feed-point current in this case. The effect of normalization by antenna current
versus output power was demonstrated in a paper by Wang and Fujiwara (2003).
The influence of anatomical features on SAR was studied by Hadjem et al. (2005). The
result of their study supported the conclusion that since the brain is closer to the cell phone
in the heads of children, the absorption of RF energy in child brain models is slightly
higher than that in adults. Another study showed that 1- and 10-g average SAR values
are significantly influenced by the SAR averaging procedure. Adoption of a fixed-cube
averaging procedure yielded a marked increase in average SAR with decreasing head size
(Bit-Babik et al. 2005). However, the results of the study conducted by Beard et al. (2006)
showed that for 1900 MHz cell phones, the peak 1- and 10-g SAR values in the head, pinna,
and average tissue of the adult model are consistently higher than those in the child model,
whether normalized to the antenna current or radiated power for the cheek and tilt positions. Nevertheless, a majority of the SARs were higher in the child than in the adult model,
especially for the 835 MHz phone in tilt position when normalized to the antenna current.
1.11.3 Fields from Body-Worn Devices
A hands-free wireless device for a cell phone including the headset can be positioned
at different body locations. Evaluation of SAR for these body-worn devices can be
accomplished numerically in a similar manner as described in Section 1.11.1 using a
heterogeneous model that correctly reproduces the human anatomy. In addition, for the
regulatory compliance of body-worn devices several flat phantoms have been suggested
for use in experimental SAR measurement (FCC 1997; Christ et al. 2006; IEC 2010),
because the part of the body where the phone is located is often flat.
It has been shown that when the cell phone is placed with the antenna close to the
body, SARs in the chest can be up to 2.1 times higher at 835 MHz and up to 5.8 times
higher at 1900 MHz compared to the SARs found when the antennas are placed away
from the body (Gandhi and Kang 2002). This is expected since turning the phone with
the antenna away from the body can cause the antenna to be 12–16 mm closer to the
body than in the opposite placement. Moreover, both the peak 1- and 10-g SARs are
reduced monotonically with increasing separation between the cell phone and the body.
1.11.4 Whole-Body Exposure from Cell-Phone Base Stations
Most public and some occupational exposures to the electromagnetic field radiated
by wireless communication base stations take place in the far field of the antenna and
involve the whole body. The coupling of fields from a fixed base station may be assessed