57
Coupling of Electromagnetic Fields into Biological Systems
sequences of a large number of pulses. Experimental and theoretical studies have shown
that auditory responses can be evoked when the head of human subjects is irradiated
with radarlike pulses (Lin 1978, 1980, 1990). Auditory responses to RF pulses have been
reported and predicted for humans undergoing MRI procedures (Lin and Wang 2010;
Roschmann 1991). However, this topic will not be covered in this chapter; the interested
reader can find additional details on this topic in the cited reference works.
The topics of high power electromagnetic pulses are discussed include two types
of pulses for a variety of exposure conditions. Specifically, EMPs with electric field
strengths up to 500 KV/m or higher and pulse widths in the microseconds range with
frequency spectra up to 80 MHz and short nanosecond UWB pulses have spectra that
extend up to 10,000 MHz. Their salient features in field coupling into biological matrials
are summarized below.
1.12.6.1 Gaussian Pulse in Planar Medium
• The transmitted Gaussian pulse characteristics are independent of location inside
a planar tissue layer.
• For an incident pulse of given strength, the transmitted pulse strength is related
inversely to the pulse width, that is, the narrower the pulse the higher the transmitted pulse strength.
• Pulse widths that are sufficiently narrow will not be altered by the medium.
However, at τ = 50 μs the broadening is about 20% of the initial width, that is, a
considerable amount of broadening occurs.
• The transmitted pulse strengths are at least two orders of magnitude smaller than
incident pulse.
• For an incident pulse of 50 kV/m with a pulse width of 1 μs, the transmitted pulse
strength will reach 221 V/m, whereas that for the 50-μs pulse will reach 188 V/m.
• Shorter pulses have higher spectral content relative to longer ones and the higher
frequency components are more readily transmitted into the biological medium
and have greater strength than the lower frequency components.
1.12.6.2 Gaussian Pulse in a Spherical Body
For dispersive permittivity:
• The maxima of transmitted EMP amplitude always occur along the direction of
propagation and through the center of the brain sphere.
• Transmitted pulse amplitude is asymmetric about the center of the spherical
model.
• Pulse amplitude approaches a zero or minimum value near the center.
• Width of the transmitted EMP is about the same as the width of the incident
pulse.
• Transmitted EMP amplitude is inversely proportional to pulse width.
• Peak transmitted pulse amplitude is relatively small, about four orders of magnitude lower than the incident value.
Coupling of Electromagnetic Fields into Biological Systems
sequences of a large number of pulses. Experimental and theoretical studies have shown
that auditory responses can be evoked when the head of human subjects is irradiated
with radarlike pulses (Lin 1978, 1980, 1990). Auditory responses to RF pulses have been
reported and predicted for humans undergoing MRI procedures (Lin and Wang 2010;
Roschmann 1991). However, this topic will not be covered in this chapter; the interested
reader can find additional details on this topic in the cited reference works.
The topics of high power electromagnetic pulses are discussed include two types
of pulses for a variety of exposure conditions. Specifically, EMPs with electric field
strengths up to 500 KV/m or higher and pulse widths in the microseconds range with
frequency spectra up to 80 MHz and short nanosecond UWB pulses have spectra that
extend up to 10,000 MHz. Their salient features in field coupling into biological matrials
are summarized below.
1.12.6.1 Gaussian Pulse in Planar Medium
• The transmitted Gaussian pulse characteristics are independent of location inside
a planar tissue layer.
• For an incident pulse of given strength, the transmitted pulse strength is related
inversely to the pulse width, that is, the narrower the pulse the higher the transmitted pulse strength.
• Pulse widths that are sufficiently narrow will not be altered by the medium.
However, at τ = 50 μs the broadening is about 20% of the initial width, that is, a
considerable amount of broadening occurs.
• The transmitted pulse strengths are at least two orders of magnitude smaller than
incident pulse.
• For an incident pulse of 50 kV/m with a pulse width of 1 μs, the transmitted pulse
strength will reach 221 V/m, whereas that for the 50-μs pulse will reach 188 V/m.
• Shorter pulses have higher spectral content relative to longer ones and the higher
frequency components are more readily transmitted into the biological medium
and have greater strength than the lower frequency components.
1.12.6.2 Gaussian Pulse in a Spherical Body
For dispersive permittivity:
• The maxima of transmitted EMP amplitude always occur along the direction of
propagation and through the center of the brain sphere.
• Transmitted pulse amplitude is asymmetric about the center of the spherical
model.
• Pulse amplitude approaches a zero or minimum value near the center.
• Width of the transmitted EMP is about the same as the width of the incident
pulse.
• Transmitted EMP amplitude is inversely proportional to pulse width.
• Peak transmitted pulse amplitude is relatively small, about four orders of magnitude lower than the incident value.