DRAFT Recommended Practice for Measurements and ...

1/29/98 92 C95.3-1991 Revision — 2 nd Draft
10/98 Draft
probe has a response time of the order of a few milliseconds, continuous line scans of
the internal E-fields may be dynamically recorded by means of a robot that generates
position data as the probe is moved along a path. Extensive data in an object can, thus,
be plotted in a relatively short period of time, and the possibility of missing a local peak is
reduced [Bassen, et al., 1977].
There are several inherent sources of error associated with the use of implantable E-field
probes for SAR measurements. Regardless of the quality of the specific probe used,
calibration (in terms of absolute field strength in high water-content biological tissue or
tissue simulant) is difficult (see below). Large gradients in the internal E-fields and
imprecise knowledge of the conductivity and mass density of the biological tissue or
tissue simulant add additional degrees of uncertainty. Although commercially available
implantable probes are available, custom-designed probes are frequently developed,
evaluated and calibrated by the developer and/or user. Therefore, instrument
performance limitations, as well as errors introduced during the measurement
procedure, should be understood by the user and steps taken to minimize errors
introduced by these factors. Detailed discussions on the evaluation, calibration and use
of implantable probes have been published [Bassen, et al., 1977, Stuchly, 1987]. In
summary, SAR measurement uncertainties of at least ±2 dB are to be expected, even
under optimum measurement conditions.
The high sensitivity of the E-field probes make them ideally suited for the measurement
of SARs associated with low-power (of the order of one watt or less) localized sources
such as hand-held radio transceivers, e.g., cellular and personal communications
equipment. The low power output and small size of the antenna make thermal
measurements extremely difficult [Balzano, et al., 1978a; Balzano, et al., 1979; Balzano,
et al., 1978]. Increasing the power of such sources by a factor of ten times so that
thermal techniques could be used would entail a substantial modification of the device to
the point of being non-representative of an actual transceiver. Since the exposure of
concern from such low power sources localized sources is within about 5 cm of the
antenna, accurate positioning of the sensing probe is crucial for performing repeatable
measurements. The probe positioning should be performed by machine, e.g., a robot,
rather than by hand.Simulated human tissue used for such measurements may vary
from case to case depending on the specific device being evaluated. For cellular
phones, for example, the head and upper part of the torso would usually be sufficient
[Balzano, 1995], while a
150 MHz two-way radio strapped to the belt with the antenna operated by remote
switching would require a full-size human phantom [Fujimoto, 1994].
The conductivity of the tissue simulant must be correct for the frequency being tested.
The mixing of such materials and the measurement of their corresponding electrical
properties present substantial challenges to achieve accuracy and repeatability. It is not
possible to use a single formulation over a wide frequency range, e.g., more than an
octave, without running into relatively large departures (±5 %) from published
conductivities for biological tissues. To obtain repeatable results (±3 %) it is advisable to
restrict the frequency band and to purchase the primary materials from the same
supplier. Well documented mixing procedures should also be adhered to, e.g., accurate
weights of the components, temperature of the liquids during mixing, length of the mixing
time, RPM of the stirring device. Measurement of the dielectric properties of tissue
simulants is also difficult to perform accurately. Acceptable results can be obtained
Copyright © 1998 IEEE. All rights reserved. This is an unapproved IEEE Standards Draft,
subject to change.