Near-fields inside a mobile phone user's head
Near-fields computed inside a phantom with belt-worn radio
Comparison of measured and simulated internal near-fields

Problem Description

The proliferation of radiating devices in modern society is forcing engineers to consider whether such radiation is harmful to humans. IEEE, Cenelec and ICNIRP standards and guidelines have been created for radiation safety levels for humans and procedures for measurement and simulation of exposure levels. Typical questions that are asked of any source of radiation is what the magnitude of field values around humans will be, what the maximum peak localised specific absorption rate (SAR) and whole body average SAR will be. Typical environments where these questions are asked are:

• Safety studies for humans in the proximity of cellular base stations.
• Safety studies for humans in the proximity of radar transmitters.
• Qualification tests for new personal communication devices, including cellular and radio telephony.
• The susceptability of pacemakers to external sources of radiation.

FEKO Solution

FEKO has a number of solution options for engineers and scientists who are involved in numerical estimation of radiation exposure in bio-electromagnetics.

The first consideration when selecting a numerical analysis technique for a bio-electromagnetics application is whether the problems requires homogeneous tissue models or whether a homogenous tissue model will suffice.

Highly Inhomogeneous Bodies

Volume discretisation techniques such as the Finite Difference Time Domain (FDTD) or Finite Element Method (FEM) are most suitable, and therefore popular, for the analysis of highly inhomogeneous dielectric bodies.

The hybrid FEM/MoM as implemented in FEKO, is particularly suitable for cases where there is a free space region of arbitrary size between the antenna and the dielectric body. The advantage offered by the hybrid MoM/FEM is that the free space between the MoM region (radiator) and the FEM region (dielectric body) does not have to be discretised leading to a reduction in memory and runtime requirements.

Partly homogeneous dielectric bodies, which can also be nested within each other (e.g. eyes and a brain inside a head volume), can be solved using the Surface Equivalence Principle (SEP). The SEP is however not efficient for highly inhomogeneous models and the FEM should then be used.

Homogeneous Bodies

For many applications it is not essential to take the inhomogeneous nature of the dielectric body into account (e.g. radiation patterns and input impedance calculations). SAR compliance measurements are normally done in phantoms filled with a homogeneous liquid simulating the human head. Simulations should therefore be done using a homogeneous body with equivalent dielectric properties in order to have proper comparison between the measurement and simulation environments.

Homogeneous dielectric bodies can be simulated in FEKO using either the MoM (SEP) or the hybrid FEM/MoM, where all tetrahedral volume elements are assigned the same dielectric properties.

SAR Computation

FEKO has been thoroughly verified through measurements as well as comparisons to other CEM codes for field calculations inside dielectric volumes. Special routines have been implemented for the extraction of maximum SAR according to the ICNIRP compliance regulations. Spatial peak SAR searching can also be done using parallel processing.

Bio-electromagnetic models that have been used with FEKO

Surface Equivalence Principle (SEP)
Articulated human phantom (SEP and FEM)	Articulated hand (SEP)	IEEE head phantom (SEP)

Finite Element Method (FEM)
Visible Human (Inhomogeneous FEM)	Visible Human Head and Shoulders (Inhomogeneous FEM)	Visible Human Head (Inhomogeneous FEM)	IEEE SAM (Homogeneous FEM)	Hugo (4 Organs FEM)