SAR of a Human Head Next to a Wi-Fi Antenna

Users of consumer electronics with radiating devices are exposed to radio frequency (RF) emission. The amount of exposure is defined as the specific absorption rate (SAR). That is, the SAR value represents the radio frequency (RF) energy rate absorbed by a body. This model computes local SAR values over a simplified human head and brain mock-up when a microstrip patch antenna operating in the Wi-Fi frequency range is placed close to the head.

Figure 1: Human head phantom next to a microstrip patch antenna resonant around the Wi-Fi frequency range. The surrounding air domain and perfectly matched layer are removed from the figure for visualization purposes.

Model Definition

The human head geometry is the same as the specific anthropomorphic mannequin (SAM) phantom provided by IEEE, IEC, and CENELEC from their standard specification of SAR value measurements. The geometry is imported into COMSOL Multiphysics after minor adjustments and scaling down to 60 % of the original geometry to reduce the problem size. The shape of brain is simplified using an ellipsoid geometry. This model takes material properties for the human brain from a presentation by Ref. 1. The following table shows some important properties from this publication at 2.45 GHz.


Parameter	Value	description
σ	2.09 S/m	Conductivity
εr	54.7	Relative permittivity

Other parts of the human head is characterized using the properties of cortical bone tissue (Ref. 2), as displayed below.


Parameter	Value	description
σ	0.4 S/m	Conductivity
εr	11.35	Relative permittivity

This example is an introductory model showing how to analyze SAR. It is assumed here that all materials are homogeneous, which is an oversimplification. For more realistic brain material characterization, see another application library example, Specific Absorption Rate (SAR) in the Human Brain in which material parameters based on the imported MRI image data with a volumetric interpolation function characterize the variation of tissue type inside the head.

The microstrip patch antenna next to the human head is composed of a thin layer of metal, a rectangular FR4 dielectric block, and a ground plane. The microstrip feed line, antenna radiator and ground plane are modeled as perfect electric conductor (PEC) surfaces. The antenna is fed by a 50 Ω lumped port, representing a feed from the power source.

The human head phantom and antenna are enclosed by a spherical air domain which is truncated by perfectly matched layers. This mimics the antenna testing in infinite free space. The perfectly matched layers work like an anechoic chamber in reality preventing unwanted reflection from the outer walls.

Results and Discussion

The model simulates the local SAR values in the head under effect of the RF emission caused by a microstrip patch antenna resonant in the Wi-Fi frequency range. The highest SAR value is observed in the mock-up brain area close to the surface of the head facing the incident electric field.

Figure 2: The computed local SAR values are visualized for a part of the head using a filter subfeature. The brain part closest to the radiating antenna has the highest SAR values.

References

1. G. Schmid, G. Neubauer, and P.R. Mazal, “Dielectric properties of human brain tissue measured less than 10 h postmortem at frequencies from 800 to 2450 MHz,” Bioelectromagnetics, vol. 24, pp 423–430, 2003.

2. M. Vallejo and others, “Accurate Human Tissue Characterization for Energy-Efficient Wireless On-Body Communications,” Sensors, vol. 13. pp. 7546–7569, 2013.

RF_Module/EMI_EMC_Applications/sar_wifi_antenna

Modeling Instructions

From the menu, choose .

New

In the window, click

Model Wizard

In the window, click

In the tree, select .

Click .

Click

In the tree, select .

Click

Study 1

Step 1: Frequency Domain

In the window, under click .

In the window for , locate the section.

In the text field, type 2.45[GHz].

Geometry 1

The head geometry has been created outside COMSOL Multiphysics, so you import it from an MPHBIN-file. Then create the PML, air, simplified brain, and antenna domains manually.

Import 1 (imp1)

In the toolbar, click

In the window for , locate the section.

Click

Browse to the model’s Application Libraries folder and double-click the file sar_wifi_antenna.mphbin.

Click

Click the

button in the toolbar.

Add a simplified brain domain using an ellipsoid.

Ellipsoid 1 (elp1)

In the toolbar, click

and choose .

In the window for , locate the section.

In the text field, type 0.035.

In the text field, type 0.045.

In the text field, type 0.025.

Locate the section. In the text field, type -0.005.

In the text field, type 0.04.

Build a geometry for a microstrip patch antenna operating in the Wi-Fi frequency range.

Block 1 (blk1)

In the toolbar, click

In the window for , locate the section.

In the text field, type 0.004.

In the text field, type 0.05.

Locate the section. From the list, choose .

In the text field, type 0.10.

Work Plane 1 (wp1)

In the toolbar, click

In the window for , locate the section.

From the list, choose .

On the object , select Boundary 2 only.

Work Plane 1 (wp1)>Plane Geometry

In the window, click .

Work Plane 1 (wp1)>Square 1 (sq1)

In the toolbar, click

In the window for , locate the section.

In the text field, type 0.0275.

Locate the section. From the list, choose .

Work Plane 1 (wp1)>Rectangle 1 (r1)

In the toolbar, click

In the window for , locate the section.

In the text field, type 0.0039.

In the text field, type 0.01125.

Locate the section. From the list, choose .

In the text field, type 0.019375.

Work Plane 1 (wp1)>Union 1 (uni1)

In the toolbar, click

and choose .

Click in the window and then press Ctrl+A to select both objects.

In the window for , locate the section.

Clear the check box.

Work Plane 1 (wp1)>Rectangle 2 (r2)

In the toolbar, click

In the window for , locate the section.

In the text field, type 0.006.

In the text field, type 0.00925.

Locate the section. In the text field, type 0.00195.

In the text field, type 0.0045.

Work Plane 1 (wp1)>Mirror 1 (mir1)

In the toolbar, click

and choose .

Select the object only.

In the window for , locate the section.

Select the check box.

Work Plane 1 (wp1)>Difference 1 (dif1)

In the toolbar, click

and choose .

Select the object only.

In the window for , locate the section.

Find the subsection. Click to select the

toggle button.

Select the objects and only.

Click

Work Plane 2 (wp2)

In the window, right-click and choose .

In the window for , locate the section.

From the list, choose .

On the object , select Boundary 1 only.

Work Plane 2 (wp2)>Plane Geometry

In the window, click .

Work Plane 2 (wp2)>Rectangle 1 (r1)

In the toolbar, click

In the window for , locate the section.

In the text field, type 0.004.

In the text field, type 0.0039.

Locate the section. From the list, choose .

Click

A lumped port will be set to this boundary.

Click the

button in the toolbar.

Sphere 1 (sph1)

In the toolbar, click

In the window for , locate the section.

In the text field, type 0.18.

Click to expand the section. In the table, enter the following settings:


Layer name	Thickness (m)
Layer 1	0.05

In this layer, a perfectly matched layer will be assigned.

In the toolbar, click

Definitions

Perfectly Matched Layer 1 (pml1)

In the toolbar, click

Select Domains 1–4 and 8–11 only.

In the window for , locate the section.

From the list, choose .

Electromagnetic Waves, Frequency Domain (emw)

Perfect Electric Conductor 2

In the window, under right-click and choose the boundary condition .

Select Boundaries 62 and 66 only.

The metal parts of the antenna substrate are defined as perfect electric conductors, assuming that the conductivity of copper is high enough to have negligible loss.

Lumped Port 1

In the toolbar, click

and choose .

Select Boundary 63 only.

Far-Field Domain 1

In the toolbar, click

and choose .

Select Domain 5 only.

The feature defines a wave number for use by its subfeature - the feature.

Specific Absorption Rate 1

In the toolbar, click

and choose .

Select Domains 6 and 7 only.

The specific absorption rate feature defines the SAR postprocessing variable. It is calculated from the electromagnetic dissipation density and the specified density for the human head.

Add Material

In the toolbar, click

to open the window.

Go to the window.

In the tree, select .

Click in the window toolbar.

In the tree, select .

Click in the window toolbar.

In the toolbar, click

to close the window.

Materials

FR4 (Circuit Board) (mat2)

Select Domain 12 only.

Brain

In the window, right-click and choose .

In the window for , type Brain in the text field.

Select Domain 7 only.

Locate the section. In the table, enter the following settings:


Property	Variable	Value	Unit	Property group
Relative permittivity	epsilonr_iso ; epsilonrii = epsilonr_iso, epsilonrij = 0	54.7	1	Basic
Relative permeability	mur_iso ; murii = mur_iso, murij = 0	1	1	Basic
Electrical conductivity	sigma_iso ; sigmaii = sigma_iso, sigmaij = 0	2.09	S/m	Basic
Density	rho	1000	kg/m³	Basic

Head

Right-click and choose .

In the window for , type Head in the text field.

Select Domain 6 only.

Locate the section. In the table, enter the following settings:


Property	Variable	Value	Unit	Property group
Relative permittivity	epsilonr_iso ; epsilonrii = epsilonr_iso, epsilonrij = 0	11.35	1	Basic
Relative permeability	mur_iso ; murii = mur_iso, murij = 0	1	1	Basic
Electrical conductivity	sigma_iso ; sigmaii = sigma_iso, sigmaij = 0	0.4	S/m	Basic
Density	rho	2000	kg/m³	Basic