DRAFT Recommended Practice for Measurements and ...

1/29/98 96 C95.3-1991 Revision — 2 nd Draft
10/98 Draft
MPE at one or more points in that region, a violation of the MPE is implied. However,
exposure to nonuniform fields produces a nonuniform SAR distribution throughout the
object occupying the region and the maximum induced SAR may or may not exceed the
peak or the whole-body averaged SAR limits of the MPE depending on whether an
‘’enhanced’’ or ‘’reduced’’ absorption situation exists.
Although the degree of induced SAR in a person exposed to a near-field source is difficult
to relate to the maximum value of the local field strength (E or H) it can be normalized
with respect to a standard plane-wave-exposure situation. The term normalized planewave
absorption is defined as the SAR induced by a plane wave whose field strength is
equal to the maximum field strength (E or H) measured in the near-field exposure
situation in question at the site that a person would occupy. The maximum spatial SAR
induced by this normalized plane wave can then serve as a reference level for evaluation
of the near-field exposure. If the corresponding maximum spatial SAR induced in a
person’s body by the near-field source exceeds the normalized plane-wave SAR, an
enhanced absorption case exists; otherwise, a reduced absorption case exists.
The interpretation of field strength measurements near a passive reradiator or active
radiator should be performed carefully to determine if an enhanced absorption case
actually exists. When a typical, modern isotropic hazard survey meter is placed near a
passive reradiator or an active radiator, the meter should accurately measure the field
strengths if the guidelines in 5.3.6 are followed during the measurement procedure and,
in many cases, these values may exceed the plane-wave limits of the MPE. However,
the maximum RF energy that is actually absorbed in any part of a person’s body and,
hence, the maximum peak SAR, might not exceed the local SAR provisions of the same
MPE. Conversely, some situations exist where the local SAR in a part of the body may
indeed exceed the limits of the MPE [B46]. These two extreme cases (enhanced or
reduced absorption) and their physical basis are examined in 5.6.2.1 and 5.6.2.2.
5.6.2.1 The Reduced Absorption Case. In any situations involving the exposure of
a person to near fields from a passive reradiator or active radiator, the spatial maximum
(local) SAR is relatively ‘’low’’ with respect to the maximum local SAR that would be
induced by a normalized plane-wave exposure, as discussed above. In virtually all
cases, the whole-body-averaged SAR induced in a person under near-field exposure
conditions is much less than the whole-body- averaged SAR associated with plane-wave
exposure [B115]. Very high field strengths can be measured (with respect to the MPE)
when a survey meter is placed close to a resonant (at the frequency of irradiation)
passive reradiator. Also, high field strengths may occur in the near field of active
radiators. The field strengths that are indicated by a survey meter, in close proximity to a
passive reradiator or active radiator, may be much greater than the ambient field
strengths existing only a fraction of a meter away (i.e., large field strength gradients
exist). Thorough experimental and theoretical studies of near-field exposure from active
radiators have been performed (see 5.6.2.3). Many cases result in reduced absorption.
An example of the reduced hazard situation for a passive reradiator easily can be
envisioned, but little experimental or theoretical SAR data are available to confirm this
situation.
For example, high field strengths (relative to the ambient field strength) are generally
measured in the immediate vicinity of small, resonant, passive reradiators. These
passive reradiators include automobile steering wheels [B4, B85], and/or short linear
metal objects. If a person is near, but not touching the resonant object, little energy
Copyright © 1998 IEEE. All rights reserved. This is an unapproved IEEE Standards Draft,
subject to change.