A Study to Determine Safe Levels of Radiation from Mobile Antennas using Feko
One of the key factors that determine if a mobile phone can be sold in a country’s market is its compliance with the federal regulations on the RF radiation from the device. The regulations provide two different limits that needs to be satisfied in order to consider a device as compliant, the Specific Absorbance Rate (SAR) and the Maximum Permissible Exposure (MPE). These two limits define the partial or localized and full body exposure compliance from a device. In this work, the focus is on simulating a scenario that showcases the highly localized closed-in exposure of human body or in other words computing SAR over a small sample volume of tissue (1g or 10g). SAR is a parameter that defines the rate of absorption of RF energy into tissues of a human body when exposed to electromagnetic fields emitted from a transceiver device such as a mobile phone. Since SAR is power absorbed per mass of tissue, it has the units of Watts per kilogram (Wkg-1). FCC defines the limits of exposure of humans from mobile phones to be 1.6 Watts per Kg. The SAR value for any point in the human head is defined by (1)
where,
The value of the SAR depends directly on the electrical properties of the exposed parts of the body. Specifically for the cell phone exposure compliance testing, the mobile phone with an active radiating RF source (Antenna) is placed against the representation of a human head phantom near the ears and the SAR value is measured across the human head by computing the induced electric fields.
For example, the SAR simulation can be accomplished in Feko by requesting SAR computations either for 1g or 10 g tissue volume or the average volume exposure. For the purpose of representation of a practical scenario, a humanoid phantom with different dielectric layers that closely represent a real human face tissue with their corresponding dielectric constant and loss tangent is used. A dipole antenna operating at 1 GHz is placed at a distance from the head as shown in Fig 1 (see attached Example_SAR.cfx) and a parametric simulation is performed to compute the SAR values of the head due to its exposure to the dipoles radiating fields at different distances. The computed SAR values at different distances from the head is shown in Fig 2.
Fig 1. Simulation set up for computing SAR on a humanoid phantom from a representative mobile phone antenna.
Fig 2. Variation in 1g SAR vs representative antenna’s distance from head
Such studies help to determine whether the designed antenna is safety to be used in close proximity to the human body. The key to successfully conducting such safety simulations lies in the ability to create a humanoid that could accurately represent the human body at the frequency of operation.