In many countries, over half the population uses mobile phones and the market is growing rapidly. Mobile phones emit electromagnetic radiation in the microwave range, and exposure to these electromagnetic (EM) fields emitted by the mobile phones has greater likelihood of causing adverse health effects.
Unlike X-rays and gamma rays, radiation from microwave telecommunication systems falls under non-ionising type that cannot cause ionisation in the human body. All radiations and fields of the electromagnetic spectrum that do not normally have sufficient energy to produce ionisation in matter characterised by energy per photon as less than about 12 electron volts, wavelengths greater than 100 nanometres and frequencies lower than 3×1015 Hz are termed as non-ionising.
For broadcasting purpose, high RF power is generally required to maximise the area of coverage. Electric field strengths in the vicinity of antennae can reach several hundred volts per metre. Even higher values can be found close to occupational sources. In many such applications RF safety problems arise as RF power is high.
Sources used by the general public, e.g., for wireless communication or data transmission, generate comparably much lower fields near the position of the user. But this may also depend on the behaviour of the user, especially concerning the distance to the source.
Cellular mobile communication networks, on an average, cause low levels of electromagnetic fields in areas accessible to the general public. Handsets and cell phones, however, might cause significantly higher peak levels of exposure during use.
Mobile phones are low-powered radio-frequency transceivers, operating at frequencies between 450 and 2700 MHz. Typically, a GSM (global system for mobile communication) handset emits radio waves having peak power of 2 W, and CDMA2000 (code-division multiple access) handset emits below 1W peak power. The handset transmits power only when it is turned on.
In most systems, the cellphone and the base transceiver station check the reception quality and signal strength, and the power level is increased or decreased automatically, within a certain span, to accommodate different situations such as inside or outside buildings and vehicles. The power (and hence the radio-frequency exposure to a user) falls off rapidly with increase in the distance from the handset. A person using a mobile phone 30-40 cm away from his body—for example, when text messaging, accessing the Internet or using a hands-free device—will therefore have a much lower exposure to radio-frequency fields than someone holding the handset against his head.
Thus to achieve a quantitative understanding of the biological response, dosimetric quantities such as specific absorption rate (SAR), induced electric field and current density must be quantified and correlated with the observed phenomenon. It is noteworthy that dosimetric quantities and their determinations are tissue-type dependent, and require a region of specific tissue mass for averaging and for correlation with any induced biological response. Thus a smaller averaging region is scientifically more relevant and precise.
The rate at which radiation is absorbed by the human body is measured by SAR in watts per kilogram, and is defined as the time derivative of the incremental energy absorbed by (dissipated in) an incremental mass contained in a volume element of given density.
Maximum levels of SAR for latest handsets have been set by regulating agencies like International Commission on Non-Ionizing Radiation Protection, World Health Organisation, International Non-Ionizing Radiation Committee, National Council on Radiation Protection and Measurements, and Federal Communications Commission (FCC), Standard Association of Australia. In USA, the FCC has set a SAR limit of 1.6 W/kg, averaged over a 1gm volume of tissue, for the head. In Europe, the limit is 2 W/kg, averaged over a 10gm volume of tissue.
SAR values are heavily dependent on the size of the averaging volume. Without information about the averaging volume used, comparison between different measurements cannot be done. Thus the European 10gm ratings should be compared among themselves, and the American 1gm ratings should be compared only among themselves. SAR data for specific mobile phones, along with other useful information, can be found directly on the manufacturers’ websites/manuals.
Mobile phone EMF simulations
For electromagnetic simulations, 3D EM tool CST Microwave Studio (CST MWS), based on the finite integration technique, is used. The transient solver of CST MWS with its perfect boundary approximation and thin sheet techniques is widely known for delivering accurate and fast results for various types of antennae including electrically small antennae, such as those found in mobile phones.
Once the designing of a mobile phone is completed (assuming it in free space), the performance of the phone needs to be tested in more realistic surroundings such as near the head and when placed in the hand. This is important since the field distribution and radiation pattern will be influenced by the head and hand. Finally, for certification, SAR also needs to be proven to fulfil international standards such as the IEEE (Institute of Electrical and Electronics Engineers) C95.3 standard.
Due to the fact that measurements in true biological heads cannot be carried out, SAR norms for mobile phones or electromagnetic interference problems are commonly defined in terms of standardised phantom models. In the easiest case only spherical structures are considered. To predict the SAR behaviour of a new product already in the design stage, it is suggested to include the phantom head in EM simulations.
EM simulation of the mobile phone is conducted on several system complexity levels, such as the simple antenna design and the printed circuit board (PCB), to the complete phone structure containing several hundred components and finally the entire system using the head of the standard anthropomorphic model (SAM), a homogeneous hand model and the phone (Fig. 1). The complete CAD model of the mobile phone is imported in CST MWS as a step-file. In addition, the SAM phantom head and a phantom hand are imported.
In real mobile phones, many objects in the vicinity of the antenna element have to be considered because of their effect on the performance of the system. Therefore great care must be taken when selecting and setting correct simulation parameters for relevant objects. The analysis of antenna performance in the vicinity of bodies often comprises assumptions and simplifications. In general, homogeneous bodies are used in order to measure the most conservative behaviour of the system. Because of this, homogeneous head and hand models are used for simulations as well.
In accordance with International Electrotechnical Committee (IEC) and IEEE standards, SAM is used as the head model. This head phantom model is filled with a liquid that correlates with the dielectric properties of the human head tissue. The dielectric properties of the head tissue are calculated taking into account the properties of the human brain tissue and the matching effects of the outer tissue layers of the head (e.g., skin and skull) to provide a conservative overestimate of the values. The frequency-dependent dielectric properties of the tissue-simulating liquid are also defined by IEEE standard (see the table).
Simulation of the mobile phone, with head and hand models, requires tens of millions of mesh cells and a simulation run-time of five to six hours on a PC (Intel Core i3 CPU, 64-bit operating system, 2.13GHz, 4GB RAM). The number of mesh cells can be reduced using the sub-gridding scheme. When applied, a very fine mesh is created inside the phone, a coarser one in the head and a very basic one in the vacuum (Fig. 2).
Due to high dielectric permittivity of the order of 40 in the brain tissue, the mesh needs to be much finer in the head region than the surrounding vacuum. In classical time-domain solvers, these fine mesh lines need to be extended to the boundary of the computational domain. However, the sub-gridding scheme in CST MWS allows different levels of mesh to be used for different solids in the model. Its use reduces the number of mesh cells required and also the simulation run-time.
Such a simulation can give an important insight into how the SAM phantom or the homogeneous body models affect the performance of the mobile phone. The resulting electric field distribution at 1800 MHz is shown in Fig. 3. It clearly shows how the wavelength inside the head is shortened compared to the vacuum, due to high dielectric values. The radiated field is basically guided around the head.
The far-field plot (Fig. 4) shows that the main radiation direction is located between the hand and the head.
For certification of the mobile phone, SAR is expected to be compliant to the SAR standard IEEE C95.3. Currently, the SAR value needs to be measured, but in the design stage it is very useful to check this value through simulation. In the near future, pure simulation may be sufficient for certification.
The ‘SAR 10 gm’ is the maximum SAR value averaged on 10 gm, which is obtained by averaging the SAR around each point in the volume and adding the nearest points till an average mass of 10 gm is reached with a resulting volume having the shape of a portion of sphere. The contiguous SAR 1 gm is estimated by averaging the local maximum SAR, adding the highest SAR volume in a given tissue till a mass of 1 gm is reached. SAR (point) is the local value of SAR at every point inside the head model.
Fig. 5 shows distribution of the SAR value averaged over 1gm tissue mass. It is clear that the radiated field is basically guided around the head and varies inversely with distance. This leads to an automatic inference that SAR and hence the health effects within very close proximity of head would be harmful. Thus it is clear that a person using a mobile phone 30-40 cm away from his body will have a much lower exposure to radio-frequency fields than someone holding the handset against his head. So it is recommended to keep mobile phones away from your body as much as possible.
The author is working with Bharat Sanchar Nigam Limited as a junior telecom officer and is currently posted at Ludhiana, Punjab. He is a PhD in electronics engineering from Indian Institute of Technology-BHU, Varanasi