24
Electromagnetic Fields in Biological Systems
The coupling of plane RF radiation into more complex models of the head structure
in which a spherical core of the brain is surrounded by five concentric shells of other
tissues shows that the effect of skin, fat, skull, dura, and cerebrospinal fluid on specific
absorption rate (SAR) distribution is to increase SAR in the skin. Absorptions in fat
and skull are the lowest among tissue layers (Lin 1986). Moreover, the peak and average
SAR values may be several times greater than the values for homogeneous models. The
enhancement is apparently due to resonant coupling of plane-wave RF into the brain
sphere by the outer tissue layers.
Although a spherical model can provide some results that are relatively close to those
obtained with more realistic head models, the SAR values obtained with spherical or
simplified head models that do not include the ear tend to be greater than those obtained
with head models that include the ear. A hand holding a handset absorbs a significant
amount of the cell phone’s output power (see Section 1.11.1).
1.10.3 Orientation and Polarization Dependence
For elongated bodies such as a human body or the prolate spheroid, shown in Figure 1.3,
for which the height-to-width ratio is large, the coupling of RF energy is influenced by
the orientation of the electric field vector (polarization) with respect to the body. The
three principal polarizations of the impinging plane wave to be distinguished are as
follows: (1) E-polarization, in which the electric vector is parallel to the long axis of the
body; (2) H-polarization, in which the magnetic vector is parallel to the long axis of the
body; and (3) K-polarization, in which neither the electric vector nor the magnetic vector is parallel to the long axis of the body. The frequency at which the highest (resonant)
absorption occurs is a function of both polarization and the exposed subject. In general, the shorter the subject the higher the resonance frequency and vice versa. Further,
E-polarization couples RF energy most efficiently into the body in a plane-wave field for
frequencies up to and slightly above the resonance region, where the body dimension
and wavelength are approximately equal.
For RFs well below resonance, such that the ratio of long body dimension (L) to free
space wavelength (λ) is less than 0.2, the average SAR is characterized by an f 2 dependence. The SAR goes through the resonance region in which 0.2 < L/λ < 1.0. Specifically,
the SAR rapidly increases to a maximum near L/λ = 0.4 and then falls off as 1/f. At frequencies for which L/λ > 1.0, whole-body absorption decreases slightly but approaches
asymptotically about one-half of the incident power, that is, 1-power reflection coefficient is transmitted into biological tissue. The resonances are not nearly as well defined
for H-polarization as for E-polarization. The average SAR for H-polarization gradually
reaches a plateau throughout the RF spectrum (Lin and Gandhi 1996).
1.10.4 Radiofrequency Coupling in the Near Field
The accelerated use of RF radiation for wireless communication in recent years has generated considerable attention on the amount of RF energy coupled into human bodies
and on the potential biological effects of radiation. The antenna of a cellular mobile telephone is typically located next to the user’s head, thereby creating an exposure situation
Electromagnetic Fields in Biological Systems
The coupling of plane RF radiation into more complex models of the head structure
in which a spherical core of the brain is surrounded by five concentric shells of other
tissues shows that the effect of skin, fat, skull, dura, and cerebrospinal fluid on specific
absorption rate (SAR) distribution is to increase SAR in the skin. Absorptions in fat
and skull are the lowest among tissue layers (Lin 1986). Moreover, the peak and average
SAR values may be several times greater than the values for homogeneous models. The
enhancement is apparently due to resonant coupling of plane-wave RF into the brain
sphere by the outer tissue layers.
Although a spherical model can provide some results that are relatively close to those
obtained with more realistic head models, the SAR values obtained with spherical or
simplified head models that do not include the ear tend to be greater than those obtained
with head models that include the ear. A hand holding a handset absorbs a significant
amount of the cell phone’s output power (see Section 1.11.1).
1.10.3 Orientation and Polarization Dependence
For elongated bodies such as a human body or the prolate spheroid, shown in Figure 1.3,
for which the height-to-width ratio is large, the coupling of RF energy is influenced by
the orientation of the electric field vector (polarization) with respect to the body. The
three principal polarizations of the impinging plane wave to be distinguished are as
follows: (1) E-polarization, in which the electric vector is parallel to the long axis of the
body; (2) H-polarization, in which the magnetic vector is parallel to the long axis of the
body; and (3) K-polarization, in which neither the electric vector nor the magnetic vector is parallel to the long axis of the body. The frequency at which the highest (resonant)
absorption occurs is a function of both polarization and the exposed subject. In general, the shorter the subject the higher the resonance frequency and vice versa. Further,
E-polarization couples RF energy most efficiently into the body in a plane-wave field for
frequencies up to and slightly above the resonance region, where the body dimension
and wavelength are approximately equal.
For RFs well below resonance, such that the ratio of long body dimension (L) to free
space wavelength (λ) is less than 0.2, the average SAR is characterized by an f 2 dependence. The SAR goes through the resonance region in which 0.2 < L/λ < 1.0. Specifically,
the SAR rapidly increases to a maximum near L/λ = 0.4 and then falls off as 1/f. At frequencies for which L/λ > 1.0, whole-body absorption decreases slightly but approaches
asymptotically about one-half of the incident power, that is, 1-power reflection coefficient is transmitted into biological tissue. The resonances are not nearly as well defined
for H-polarization as for E-polarization. The average SAR for H-polarization gradually
reaches a plateau throughout the RF spectrum (Lin and Gandhi 1996).
1.10.4 Radiofrequency Coupling in the Near Field
The accelerated use of RF radiation for wireless communication in recent years has generated considerable attention on the amount of RF energy coupled into human bodies
and on the potential biological effects of radiation. The antenna of a cellular mobile telephone is typically located next to the user’s head, thereby creating an exposure situation