SAR of a Human Head for Mobile and WiFi
Users of mobile electronic devices such as cell phones (and other wireless devices) are exposed to radio frequency (RF) radiation. The amount of RF exposure absorbed by a body is measured by the Specific Absorption Rate (SAR), which represents the rate of RF energy. To help design safer devices, engineers can use COMSOL Multiphysics® software to calculate the local SAR values of the human head model while absorbing the wave from an antenna.
Using SAR to Measure RF Absorption Rates
When we use wireless devices, our body is exposed to RF energy from an antenna connected to that device. Therefore, it is important to understand the health consequences of electromagnetic (EM) exposure. RF energy from the antenna passes through the tissues and is dissipated as heat that can be absorbed by our body. The amount of RF energy absorbed varies according to the frequency of the RF signal.
When engineers prototype devices that emit RF energy, they design them with features that comply with safety rules. This is to ensure that they do not exceed their maximum exposure level. We can measure the amount of RF exposure emitted by SAR tests to see if these devices are safe to use. SAR testing provides suitable features for determining maximum exposure to EM from a variety of devices.
Using the COMSOL Multiphysics and RF Module, we can calculate and analyze local SAR values on a simplified human head and brain alongside a microband patch antenna operating in the Wi-Fi frequency range. This model shows how a human head absorbs the wave from an antenna under RF emission.
Simulation of a human head ghost under RF emission
This training model uses imported human head geometry that is the same as the Specific Anthropomorphic Manikin (SAM) provided by IEEE, IEC, and CENELEC from the SAR value measurement standard specifications. After importing the geometry into COMSOL Multiphysics, minor adjustments are made and the original geometry is reduced by 60% to reduce the size of the problem. Using an ellipsoid geometry, we can create a simplified shape of the brain and also characterize parts of the human head using the properties of cortical bone tissue.
The antenna next to the human head consists of a thin metal sheet, a rectangular FR4 dielectric plate, and a ground plane. In the electromagnetic wave interface, in the frequency domain, we can represent the metallic parts of the antenna (microstrip feed line, antenna radiator, and ground plane) as perfect electric conductive (PEC) surface when the loss is negligible. A pooled port boundary condition (a simplified port boundary condition) is added to represent a power supply. In this case, the antenna gets power from a packaged 50 Ω port.
To simulate antenna testing in infinite free space, we turned the phantom of the human head and the antenna into a spherical airfield using a perfectly matched layer (PML). A PML works like an anechoic chamber by absorbing all the energy from outgoing waves and avoiding unwanted reflections.
Analysis of Human Head RF and SAR Effects
The highest SAR value in the model is the surface area facing the incident electric field. Normally, the SAR effect depends on the antenna’s location and dielectric characteristics. The human body contains different dielectric property values (permeability and conductivity), which are functions of variables such as frequency and geometry. The conductivity and permeability of human tissues are factors for RF communication and absorbed radiation. When there is an increase in resistive loss, the SAR value increases.
We usually think of an electrical signal as a unidirectional phenomenon going from one source to another. But with RF, electrical signals can travel in both directions due to reflection (similar to the reflection of light from a mirror). The following results show the area emanating from the microband patch antenna distorted by reflection from the human head, and the far-field results include these effects.
The SAR value is of special interest to the designers and can be easily obtained using COMSOL Multiphysics. After simulation, the SAR can also be arbitrarily evaluated in/from the user-defined location (s).