DRAFT Recommended Practice for Measurements and ...

1/29/98 129 C95.3-1991 Revision — 2 nd Draft
10/98 Draft
When the distance d is much smaller than λ, the term (1/d 3 ) is dominant. Here, reactive
near-field conditions produce a majority of the energy deposition in lossy dielectric
objects. The coupling in a reactive near-field situation may be analyzed as a quasi-static,
electromagnetic field problem. Capacitive (electric field) coupling provides the primary
means of inducing energy from the electric fields surrounding the radiator into the
exposed object. Magnetic fields can also induce RF energy (via internal currents),
thereby producing additional RF absorption.
D6.2 Theoretical Studies of Induced SAR: Near-Field vs Plane Wave Exposure.
For the near- field exposure situation, the induced spatially-averaged or whole-bodyaveraged
SAR is almost always much less than the whole-body average SAR induced
by a normalized plane-wave exposure. (See 5.6.2.2 for a discussion of the normalized
exposure field concept.) The only exception is when direct contact with a radiator or
reradiator induces a large RF current in the body which subsequently flows through the
body to RF ground. The degree of coupling between the object (person) and the passive
reradiator will dictate whether an enhanced or a reduced absorption situation occurs with
respect to spatially localized SAR.
The induced SAR associated with a generalized model of an active, near-field radiation
source, has been determined [D3]. Here, human exposure to a near-field source is
compared with exposure to a plane-wave field. Both exposure situations were
normalized to their respective spatial-maximum external E-fields for the reduced
absorption case. In particular, mathematical analyses were performed using a block
model of a person, and experimental verification was accomplished with human
''phantom'' models using implantable, E-field probes. The study involved a standing
person immersed in a vertically polarized, spatially non-uniform E-field. The E-field
distribution was defined as a half-cosine shape along both the vertical and horizontal
axes. The field was further defined as existing in a lateral plane located just in front of the
person's feet, simulating exposure to the emission from an RF heat-sealer. The plane
containing the field was normal to an imaginary line drawn between the center of the
radiator and the center of the person. Finally, the near field was assumed to decay
rapidly with increasing distance from the source, so that a two-dimensional analysis was
feasible.
The resulting induced, whole-body averaged SAR in the above studies was shown to be
significantly lower than the whole-body SAR induced by a plane wave. The general
relationship between the SAR induced by near-field exposures and far-field exposures is:
SARf
SARn
=
(Eq D 12 )
Av
Ah
+ ⎛ 2
⎡
⎝ ⎜ ⎞ ⎤
⎢ ⎟ ⎥ + ⎛ 2
⎡
⎣⎢
dv
⎠
⎦⎥
⎝ ⎜ ⎞ ⎤
1 ⎢1
⎟ ⎥
⎣⎢
dh
⎠
⎦⎥
where:
A v = constant
A h = constant
d v = the dimension (in wavelengths) of the vertical extent of the field in the plane
containing the object
d h = the dimension (in wavelengths) of the horizontal extent of the field in the plane
containing the object.
Copyright © 1998 IEEE. All rights reserved. This is an unapproved IEEE Standards Draft,
subject to change.