Specific Absorption Rate testing

Specific Absorption Rate (SAR) is a measure of the amount of RF energy absorbed in the human body due to a radiofrequency (RF) device such as a cell phone or medical implant with wireless power or telemetry. For safety, the FCC, along with other international organizations, limits the amount of exposure from such devices.  In the case of the FCC, exposure is limited to a level of 1.6 watts per kilogram (1.6 W/kg), which is a measure of the local RF power (in watts) deposited to a standardized mass of tissue (in kilograms).  In order to certify that a device is operating safely within the regulatory limits, both numerical and experimental studies should be performed.

Here at ITEMS, we use the D-H Finite Difference Time Domain (D-H FDTD) numerical method to compute the electromagnetic fields induced in the human body due to external sources of electromagnetic radiation [1], and from those fields and the tissue properties we can compute the local SAR. The human body model used in our simulations is extracted from ‘the visible human project’ [2], and the tissue properties are obtained from Gabriel [3]. The FDTD method is based on a numerical solution of Maxwell’s equations. This method, introduced by Yee, is a well-established method where solutions to Maxwell’s equations are derived through central difference approximations of the time and space derivatives, with fields staggered in space and time that progress through a “leapfrog” scheme. The explicit equations are iterated in space and time until steady state is achieved (in the case of sinusoidal excitation). Our computational models are terminated using Perfectly Matched Layer (PML) absorbing boundary conditions to approximate a simulation space of infinite size and, therefore, terminate the computational mesh with a reflectionless material (non-physical). Compared to the conventional E-H formulation of the FDTD method, the D-H formulation has the benefit that the PML absorbing layers are independent of the background material. The field solutions from this numerical method are then coupled with the local tissue properties to compute the Specific Absorption Rate, and validate the safe operation of devices.

[1] G. Lazzi, “Unconditionally stable D-H FDTD formulation with anisotropic PML boundary conditions,” in IEEE Microwave and Wireless Components Letters, vol. 11, no. 4, pp. 149-151, April 2001.

[2] The National Library of Medicine, “The Visible Human Project,” 2000. [Online] Available: http://www.nlm.nih.gov/research/visible/

[3] C. Gabriel: Compilation of the dielectric properties of body tissues at RF and microwave frequencies, Report N.AL/OE-TR- 1996-0037, Occupational and environmental health directorate, Radiofrequency Radiation Division, Brooks Air Force Base, Texas (USA), June 1996.