25
Coupling of Electromagnetic Fields into Biological Systems
in the near zone of the RF antenna. As mentioned in Section 1.8.2, the distance to the
near field is on the order of a few wavelengths or less in most antenna systems. Even for
an elementary dipole, the reactive near-field distance is approximately λ/(2π), which is
about 5 cm in air at 900 MHz or 2 cm at 2400 MHz, 900 and 2400 MHz being the typical
frequencies used for wireless mobile communication.
As discussed in Section 1.8.2, the near fields of an antenna are quasistatic (see Equations
1.30 through 1.35). In contrast to a plane wave in the far field, near-field RF electric and
magnetic fields are in time quadrature and their magnitudes vary with location or distance.
Wave impedance is no longer the same as intrinsic impedance and varies from point to
point in the near zone. The maxima of electric and magnetic fields also do not occur at the
same location in space. These are precisely the characteristics of a standing wave, and thus
RF radiation in the near field behaves like a standing-wave field. The RF energy will be
transferred back and forth between the radiating antenna and the human body.
Typical wireless RF antennas for personal communication are tiny compared to the
size of a human head (Balanis 2008). The beam width in the near field is also smaller
than the head. As shown in Figure 1.6, the field is diverging in the near zone. In addition to the reactive induction field, there is a radiative component that is outgoing and is
proportional to the product of the electric and magnetic field components. Since RF currents produce magnetic fields and time-varying magnetic fields generate electric fields in
tissue, RF coupling in biological tissues can be properly expressed in terms of induced
electric fields.
1.11 Radiofrequency Dosimetry and Energy
Absorption in Anatomical Models
Electromagnetic fields must be coupled into tissues and energy must be absorbed or
deposited in the biological systems in order for biological system to respond in some
manner. Thus to establish any biological response, the electric, magnetic, or electromagnetic field that is effective in exerting its influence must be quantified and correlated with the observed effect. The commonly employed metrics or dosimetric quantities
include incident field, induced field, SAR, and SA in biological systems or tissue media.
The metric SAR (in watt per kilogram) is a derived quantity and is defined as the time
derivative of the incremental energy absorbed by (or dissipated in) an incremental mass
contained in a volume of a given density (NCRP 1981). This definition allows SAR to
be used as a metric for RF in both near and far fields. Specific absorption (in joules
per kilogram) is the total amount of energy deposited or absorbed and is given by the
integral of SAR over a finite interval of time. Information on SA and SAR is of interest
because it can serve as an index for the extrapolation of experimental results from cell to
animal, animal to animal, and animal to human exposures. It is also useful in analyzing
relationships among various observed biological effects in different experimental models and subjects. Indeed, SAR has been adopted worldwide as the dosimetric quantity in
regulations established for human exposure to cellular mobile telephone RF fields.
The induced field is of primary interest because it relates the RF field to specific
responses of the body, facilitates an understanding of biological phenomena, and
is independent of the mechanisms of interaction. Once the induced field is known,
Coupling of Electromagnetic Fields into Biological Systems
in the near zone of the RF antenna. As mentioned in Section 1.8.2, the distance to the
near field is on the order of a few wavelengths or less in most antenna systems. Even for
an elementary dipole, the reactive near-field distance is approximately λ/(2π), which is
about 5 cm in air at 900 MHz or 2 cm at 2400 MHz, 900 and 2400 MHz being the typical
frequencies used for wireless mobile communication.
As discussed in Section 1.8.2, the near fields of an antenna are quasistatic (see Equations
1.30 through 1.35). In contrast to a plane wave in the far field, near-field RF electric and
magnetic fields are in time quadrature and their magnitudes vary with location or distance.
Wave impedance is no longer the same as intrinsic impedance and varies from point to
point in the near zone. The maxima of electric and magnetic fields also do not occur at the
same location in space. These are precisely the characteristics of a standing wave, and thus
RF radiation in the near field behaves like a standing-wave field. The RF energy will be
transferred back and forth between the radiating antenna and the human body.
Typical wireless RF antennas for personal communication are tiny compared to the
size of a human head (Balanis 2008). The beam width in the near field is also smaller
than the head. As shown in Figure 1.6, the field is diverging in the near zone. In addition to the reactive induction field, there is a radiative component that is outgoing and is
proportional to the product of the electric and magnetic field components. Since RF currents produce magnetic fields and time-varying magnetic fields generate electric fields in
tissue, RF coupling in biological tissues can be properly expressed in terms of induced
electric fields.
1.11 Radiofrequency Dosimetry and Energy
Absorption in Anatomical Models
Electromagnetic fields must be coupled into tissues and energy must be absorbed or
deposited in the biological systems in order for biological system to respond in some
manner. Thus to establish any biological response, the electric, magnetic, or electromagnetic field that is effective in exerting its influence must be quantified and correlated with the observed effect. The commonly employed metrics or dosimetric quantities
include incident field, induced field, SAR, and SA in biological systems or tissue media.
The metric SAR (in watt per kilogram) is a derived quantity and is defined as the time
derivative of the incremental energy absorbed by (or dissipated in) an incremental mass
contained in a volume of a given density (NCRP 1981). This definition allows SAR to
be used as a metric for RF in both near and far fields. Specific absorption (in joules
per kilogram) is the total amount of energy deposited or absorbed and is given by the
integral of SAR over a finite interval of time. Information on SA and SAR is of interest
because it can serve as an index for the extrapolation of experimental results from cell to
animal, animal to animal, and animal to human exposures. It is also useful in analyzing
relationships among various observed biological effects in different experimental models and subjects. Indeed, SAR has been adopted worldwide as the dosimetric quantity in
regulations established for human exposure to cellular mobile telephone RF fields.
The induced field is of primary interest because it relates the RF field to specific
responses of the body, facilitates an understanding of biological phenomena, and
is independent of the mechanisms of interaction. Once the induced field is known,