Author(s):Jamal S. Rahhal
Health hazards are of great concern to cellular phone users. One important measure of the effect of electromagnetic radiation into human body is the specific absorption rate (SAR). If the human body is exposed to electromagnetic radiation, the amount of power absorbed by its tissues per mass volume should be limited and not to exceed a maximum SAR value. The cellular phone radiated through its antenna in all directions a certain amount of electromagnetic energy. This energy is concentrated in the near field region, where the user’s head is located during the call. The closest organ that is very sensitive to temperature change is the inner ear (it is just under the cellular phone antenna) where it contains a controlled viscosity liquid. In this paper we devise a two-antenna design to generate a low radiation in the direction of user’s head while using the cellular phone. By creating a null in the radiation pattern in the direction of user’s head we minimize the risk of hazards on the user. We optimize the null steering such that the device maintains a good connection to its base station and keeps the SAR level under the allowed maximum value using Lagrange method. To implement the analytical solution in real time simulated annealing (SA) algorithm is used. Results showed that we could steer the radiation pattern to optimize the radiated power in the direction of base station under the limited SAR level constraint. Simulated annealing algorithm is adopted to find the near optimal delay value to steer the antenna radiation pattern since it finds the global optimal point. It shows that a real time processing on the mobile unit can be performed to solve for the best null direction while the device is active.
The design of antennas for wireless personal communication systems is the subject of much research that is motivated by size, efficiency and health issues. In addition to maximizing the antenna radiated/accepted power of the handsets, the effects on the antenna performance from surrounding objects such as the human body must be considered. On the other hand the effect of radiation on the human body must be also considered. The closest human sensitive part to the handset in calling position is the human brain and ear in most mobile models or at least close enough to cause harmful effects. The tissues of these organs are mostly nerves plus liquid and hence, they carry electrical signals that might be affected by electromagnetic radiation from the wireless device.
The RF energy is scattered and attenuated as it propagates through the tissues of the head, and maximum energy absorption is expected in the more absorptive high water-content tissues near the surface of the head. Inner ear (that contains high water-content) is just under the mobile phone and it will be subject to the strongest
radiation from the mobile unit as shown in Figure 1.
The electromagnetic (EM) penetration into human head causes permanent damage to tissues that are exposed to high density EM energy. This could cause some organs to malfunction or to high density EM energy. This could cause some organs to malfunction or at least a disturbance in their functionality. The amount of exposed energy that can be handled by human tissues is measured by the specific absorption rate (SAR) that is given by -:
where E is the electric field intensity, σ is the tissue conductivity and ρ is the tissue density. The dependency of σ on frequency is the result of interaction between the EM waves and the tissue material, such that the existence of ions will increase the conductivity and will change the permittivity of the tissue. The complex
nature of the permittivity reflected into changing the conductivity of the tissue.
The effect of radiation in the inner ear has two folds: the effect on the neural tissues (hearing) and the effect on the filling liquid (balance). The radiation devices must be compliant to the SAR standard IEEE C95.1. The IEEE exposure criteria are based on a determination that potentially harmful biological effects can occur at an SAR level of 4 W/kg as averaged over the whole-body. Appropriate safety factors were then added to arrive at limits for both whole-body exposure (0.4 W/kg for “controlled” or “occupational” exposure and 0.08 W/kg for “uncontrolled” or “general population” exposure, respectively) and for partial-body (localized SAR), this might occur in the head of the user of a hand-held cellular telephone .
The nature of the tissues in the inner ear makes its relative dielectric permittivity in the order of (41.5 + j17.98 at 900 MHz and 40.0 + j13.98 at 1.8 GHz) and its conductivity (0.97 at 900 MHz and 1.4 at 1.8 GHz). The problem with the inner ear arises from the fact that its inner liquid heat dissipation is not suited to dissipate heat generated from high radiation near the ear. This will maximize the risk of losing balance and/or changing the physical characteristics of the inner ear and hence, hearing impairment might occur  .
This paper presents an antenna design to minimize radiation in the inner ear direction and at the same timeproduce an acceptable radiation pattern that can be used to communicate with base stations.
Journal： Int’l J. of Communications, Network and System Sciences
DOI: 10.4236/ijcns.2014.711048 (PDF)
Paper Id: 51384 (metadata)
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