DRAFT Recommended Practice for Measurements and ...

1/29/98 27 C95.3-1991 Revision — 2 nd Draft
10/98 Draft
reradiated field. An electromagnetic field resulting from currents induced in a
secondary, predominantly conducting object by electromagnetic waves incident on that
object from one or more primary radiating structures or antennas. Reradiated fields are
sometimes called ''reflected'' or, more correctly ''scattered fields.'' The scattering object
is sometimes called a ''reradiator'' or ''secondary radiator.'' See: reradiator.
reradiator, passive or parasitic. Electrically conducting structures that, when
illuminated by a primary RF source or ambient electromagnetic fields, act as a
secondary radiating source because of currents induced in the structure. In some
cases, reradiators can produce localized EM fields significantly more intense than the
fields at the location of the reradiator that are associated with the prime source.
response time. The time required for a field-measuring instrument to reach some
specified percentage of the final value after being placed in the field to be measured. In
this document, 90% of the final value is assumed.
scattering. The process that causes waves incident on discontinuities or boundaries of
media to be changed in direction, frequency, phase or polarization.
specific absorption rate (SAR). The time derivative (rate) of the incremental energy
(dU ) absorbed by (dissipated in) an incremental mass (dm) contained in a volume
element (dV ) of a given density (ρ).
SAR =
d
dt
⎛
⎜
⎝
dU⎞
⎟ =
dm⎠
d
dt
⎛ d ⎞
⎜ ⎟
⎝ ρdV⎠
SAR is expressed in units of watts per kilogram (W/kg).
NOTES: (1) SAR can be related to the electric field at a point by
E
SAR = σ 2
| |
ρ
where:
σ = conductivity of the tissue (S/m)
ρ = mass density of the tissue (kg/m 3 )
E = Total RMS electric field strength (V/m)
(2) SAR can be related to the increase in temperature at a point by
SAR = c∆T ∆t
where:
∆T = change in temperature ( o C)
∆t = duration of exposure (seconds)
c = specific heat capacity (J kg -1 degree C -1 )
This assumes that measurements are made under ''ideal'' non-thermodynamic
circumstances, i.e. no heat loss by thermal diffusion, heat radiation, or
thermoregulation (blood flow, sweating, etc.).
standing wave. A spatially periodic or repeating field pattern of amplitude maxima and
minima that is generated by two equal-wavelength propagating waves traveling in
different directions. For any component of the field, the ratio of the amplitude at one point
to that at any other point does not vary with time.
standing wave ratio. The ratio of maximum field strength to minimum field strength
along the direction of propagation of two waves traveling in opposite directions on a
transmission line.
Copyright © 1998 IEEE. All rights reserved. This is an unapproved IEEE Standards Draft,
subject to change.