41
Coupling of Electromagnetic Fields into Biological Systems
The U.S. FCC defines an UWB device or system as any device or system in which
the fractional bandwidth is greater than 0.20% or occupies 0.5 GHz of spectrum (FCC
2002). This definition requires that UWB systems with a center frequency greater than
2.5 GHz must have a −10-dB bandwidth of at least 500 MHz, whereas UWB systems
operating with a center frequency below 2.5 GHz must have a fractional bandwidth
of at least 0.20%. The FCC based its definition of a UWB device on the −10-dB bandwidth rather than the −20-dB bandwidth used earlier, because UWB devices operate
so close to the noise floor that in many cases it is not possible to measure the −20-dB
bandwidth. The FCC also requires that the bandwidth be determined using the antenna
that is designed to be used with a UWB device. The emission limits for UWB systems
are specified in terms of effective isotropic radiated power (EIRP) measured in dBm
(dB referenced to 1 mW) with 1 MHz resolution bandwidth. The highest value is −41.3
dBm/MHz, which corresponds to 75 nW/MHz. A UWB system offers a powerful combination of low power, high throughput, wide range, and good inherent security, using
nanosecond pulses.
Earlier investigations of induced fields and power depositions and their interactions
with biological systems relied on mathematical analyses of canonical shapes of dielectric
equivalent bodies using models such as planar tissue layers and spherical objects (Lin
1975, 1976a; Lin, Wu, and Lam 1975; Lin and Lam 1976) The well-known effect of microwave hearing from pulse-induced thermoelastic pressure in the human head has been
investigated both analytically (Lin 1976b,c; 1977a,b,c; 1978; 1980; 1990) and numerically
(Lin and Wang 2010; Watanabe et al. 2000); detailed information can be found in the
works mentioned in the reference list and it is not discussed here. Section 1.12.1 presents
induced fields and power depositions for EMP and UWB fields in models of biological
bodies. Note that a significant difference between UWB and EMP is the duration of
the pulse; also, their shapes may be different. A typical UWB has a duration measured
between the half values in a few nanoseconds. A typical EMP has a duration ranging
from 400 ns to a few microseconds at half strength.
1.12.1 Induced Electromagnetic Pulse Fields in Biological Bodies
Propagation of EMP in a biological medium has been the subject of a number of investigations. The case of plane-wave EMP incidence on an air−tissue interface has been
considered for Gaussian, biphasic, and multiphase pulses (Lin 1975; Lin 1976a,b,c; Lin,
Wu, and Lam 1975). The problem is formulated analytically in the frequency domain
using frequency-dependent complex dielectric permittivity. A steady state transfer
function is obtained for the biological medium from the transmission characteristics of
a coherent plane wave incident on the medium. The temporal and spatial dependence
of the induced EMP field is derived from inverse Fourier transform. Results of these
calculations have provided insights into the behavior of transient EMPs in biological
materials.
A typical time-domain EMP with peak amplitude of 50 kV/m and half-strength time
duration of about 1 μs is shown in Figure 1.16. The points in the figure represent an average, measured EMP in the time domain that is not exceeded by a significant amount in
Coupling of Electromagnetic Fields into Biological Systems
The U.S. FCC defines an UWB device or system as any device or system in which
the fractional bandwidth is greater than 0.20% or occupies 0.5 GHz of spectrum (FCC
2002). This definition requires that UWB systems with a center frequency greater than
2.5 GHz must have a −10-dB bandwidth of at least 500 MHz, whereas UWB systems
operating with a center frequency below 2.5 GHz must have a fractional bandwidth
of at least 0.20%. The FCC based its definition of a UWB device on the −10-dB bandwidth rather than the −20-dB bandwidth used earlier, because UWB devices operate
so close to the noise floor that in many cases it is not possible to measure the −20-dB
bandwidth. The FCC also requires that the bandwidth be determined using the antenna
that is designed to be used with a UWB device. The emission limits for UWB systems
are specified in terms of effective isotropic radiated power (EIRP) measured in dBm
(dB referenced to 1 mW) with 1 MHz resolution bandwidth. The highest value is −41.3
dBm/MHz, which corresponds to 75 nW/MHz. A UWB system offers a powerful combination of low power, high throughput, wide range, and good inherent security, using
nanosecond pulses.
Earlier investigations of induced fields and power depositions and their interactions
with biological systems relied on mathematical analyses of canonical shapes of dielectric
equivalent bodies using models such as planar tissue layers and spherical objects (Lin
1975, 1976a; Lin, Wu, and Lam 1975; Lin and Lam 1976) The well-known effect of microwave hearing from pulse-induced thermoelastic pressure in the human head has been
investigated both analytically (Lin 1976b,c; 1977a,b,c; 1978; 1980; 1990) and numerically
(Lin and Wang 2010; Watanabe et al. 2000); detailed information can be found in the
works mentioned in the reference list and it is not discussed here. Section 1.12.1 presents
induced fields and power depositions for EMP and UWB fields in models of biological
bodies. Note that a significant difference between UWB and EMP is the duration of
the pulse; also, their shapes may be different. A typical UWB has a duration measured
between the half values in a few nanoseconds. A typical EMP has a duration ranging
from 400 ns to a few microseconds at half strength.
1.12.1 Induced Electromagnetic Pulse Fields in Biological Bodies
Propagation of EMP in a biological medium has been the subject of a number of investigations. The case of plane-wave EMP incidence on an air−tissue interface has been
considered for Gaussian, biphasic, and multiphase pulses (Lin 1975; Lin 1976a,b,c; Lin,
Wu, and Lam 1975). The problem is formulated analytically in the frequency domain
using frequency-dependent complex dielectric permittivity. A steady state transfer
function is obtained for the biological medium from the transmission characteristics of
a coherent plane wave incident on the medium. The temporal and spatial dependence
of the induced EMP field is derived from inverse Fourier transform. Results of these
calculations have provided insights into the behavior of transient EMPs in biological
materials.
A typical time-domain EMP with peak amplitude of 50 kV/m and half-strength time
duration of about 1 μs is shown in Figure 1.16. The points in the figure represent an average, measured EMP in the time domain that is not exceeded by a significant amount in