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Coupling of Electromagnetic Fields into Biological Systems
1.11.5 Summary of Radiofrequency Field Coupling
The coupling of RF radiation into biological tissues is influenced by the frequency and
configuration of the source and the size and composition of the exposed body, as well
as the body’s distance from the source. Both near-field inductive and far-field radiative interactions contribute significantly to fields induced inside the body. This is also
the case for cell phones and other wireless RF personal telecommunication systems.
The field coupling characteristics for the far field and the near field are summarized in
Sections 1.11.5.1 and 1.11.5.2.
1.11.5.1 Field Coupling in the Far Field
Field coupling characteristics for the far field are summarized as follows:
• Coupling is characterized by plane-wave RF field interaction.
• Interaction is independent of the source configuration.
• Electric and magnetic fields of the outgoing wave are in time phase with each other.
• Electric and magnetic fields are uniquely defined and are related through the
intrinsic impedance of the medium (120π Ω in air).
• Determination of electric field behavior is sufficient to characterize the interaction.
• Coupling of RF power from air into planar tissue models ranges from about 20%
to 60% at wireless communication frequencies.
• Enhanced coupling can occur at greater depths in bodies with curved surfaces
from the refraction of RF fields.
• Enhanced RF energy deposition can occur because of geometrically resonant
absorption by the head at 400–1500 MHz. The SAR maxima or hot spots may
occur inside the head.
• The coupling of RF energy depends on electric field polarization for elongated
bodies whose height-to-width ratio is large.
• Integrated SAR (or average SAR) in the head or the body is similar for both homogeneous and inhomogeneous models.
1.11.5.2 Field Coupling in the Near Field
Field coupling characteristics for the near field are summarized as follows:
• Radiofrequency electric and magnetic fields are decoupled, independent, and
nonuniform.
• Antenna beam width is divergent and small compared with the dimensions of the
human head for cellular mobile telephones.
• Wave impedance varies from point to point in the near field.
• The RF electric and magnetic fields are in time quadrature (90° out of time phase).
• Maxima of electric and magnetic fields occur at different locations as in a standing-wave field.
Coupling of Electromagnetic Fields into Biological Systems
1.11.5 Summary of Radiofrequency Field Coupling
The coupling of RF radiation into biological tissues is influenced by the frequency and
configuration of the source and the size and composition of the exposed body, as well
as the body’s distance from the source. Both near-field inductive and far-field radiative interactions contribute significantly to fields induced inside the body. This is also
the case for cell phones and other wireless RF personal telecommunication systems.
The field coupling characteristics for the far field and the near field are summarized in
Sections 1.11.5.1 and 1.11.5.2.
1.11.5.1 Field Coupling in the Far Field
Field coupling characteristics for the far field are summarized as follows:
• Coupling is characterized by plane-wave RF field interaction.
• Interaction is independent of the source configuration.
• Electric and magnetic fields of the outgoing wave are in time phase with each other.
• Electric and magnetic fields are uniquely defined and are related through the
intrinsic impedance of the medium (120π Ω in air).
• Determination of electric field behavior is sufficient to characterize the interaction.
• Coupling of RF power from air into planar tissue models ranges from about 20%
to 60% at wireless communication frequencies.
• Enhanced coupling can occur at greater depths in bodies with curved surfaces
from the refraction of RF fields.
• Enhanced RF energy deposition can occur because of geometrically resonant
absorption by the head at 400–1500 MHz. The SAR maxima or hot spots may
occur inside the head.
• The coupling of RF energy depends on electric field polarization for elongated
bodies whose height-to-width ratio is large.
• Integrated SAR (or average SAR) in the head or the body is similar for both homogeneous and inhomogeneous models.
1.11.5.2 Field Coupling in the Near Field
Field coupling characteristics for the near field are summarized as follows:
• Radiofrequency electric and magnetic fields are decoupled, independent, and
nonuniform.
• Antenna beam width is divergent and small compared with the dimensions of the
human head for cellular mobile telephones.
• Wave impedance varies from point to point in the near field.
• The RF electric and magnetic fields are in time quadrature (90° out of time phase).
• Maxima of electric and magnetic fields occur at different locations as in a standing-wave field.