DRAFT Recommended Practice for Measurements and ...

1/29/98 117 C95.3-1991 Revision — 2 nd Draft
10/98 Draft
both E and H coupling are weak. For K-polarization (where the wave propagation vector,
k, is parallel to the long axis of the object), E coupling is weak, but H coupling is strong.
Thus, for frequencies below resonance, the SAR is greatest for E-polarization, least for
H-polarization, and intermediate for K-polarization.
Fig D2 Prolate Spheroid Placed in a Quasi-static Magnetic Field, H inc
Another important characteristic is related to interdependence of the fields in different
parts of an absorber. At low frequencies (long wavelength with respect to the irradiated
object), the fields in one part of a body are strongly affected by the other parts of the
body, and field concentrations can be produced by combinations of size, shape, and
orientation of the body in the incident field. At intermediate frequencies, the fields inside
can be thought of as propagating waves that are partially transmitted and partially
reflected at the boundaries, with constructive and destructive interference causing
significant variations in field strength inside the body. Also, at these intermediate
frequencies, E and H cannot exist separately and are strongly coupled together. At very
high frequencies, however, the internal fields in different parts of an absorber can be
essentially uncoupled, as explained below.
D4. High-Frequency Behavior
At very high frequencies, where the wavelength of the incident wave is very small
compared with the size of the object, geometrical optics concepts apply. That is, the EM
waves can be thought of as rays. In this case, for lossy objects, the effects are mostly
surface effects because the depth of penetration decreases rapidly as frequency
increases. This is the so-called skin effect. The penetration depth depends on both
conductivity and frequency. At 10 GHz, for example, the skin depth in tissue is less than
5 mm. At these very high frequencies, the fields in one part of the body are affected only
very slightly by other parts of the body.
D5. Methods of Calculation
In principle, the internal fields in any object irradiated by EM-fields can be calculated by
solving Maxwell's equations. In practice, this is very difficult and, until recently, could be
done only for a few very special cases (idealized models, such as spheres or infinitely
long cylinders. Because of the mathematical complexities involved in calculating SAR, a
combination of techniques has been used to obtain SAR for various models as functions
of frequency. Each of these techniques provides information over a limited range of
parameters. Combining the information thus obtained, gives a reasonably good
description of SAR as a function of frequency over a wide range of frequencies for a
number of useful models.
Copyright © 1998 IEEE. All rights reserved. This is an unapproved IEEE Standards Draft,
subject to change.