Thermal Effects of Mobile Phones

Transcription

1 Thermal Effects of Mobile Phones S. Kassimi 1, A. ELfadl, S. Bri 3, A. Nakheli 4, M. Habibi 5, M. Ben Ahmed 6 Systems and Telecommunications Engineering Decision Laboratory,Ibn Tofail University, Faculty des Sciences de Kénitra, B.P. 133, Kenitra - Morocco.,3,4 Materials and Instrumentations Group, ESTM, Moulay Isamil University, B. P 3103, Meknès Morocco. 6 Faculty of Sciences and Techniques, BP 416, University Abdelmalek Essaidi, Tangier- Morocco Abstract - This paper presents a study of the absorption of electromagnetic waves in the human head while using a mobile phone. This absorption is presented in the form of thermal effects of exposure to radiofrequency electromagnetic fields for long period or in the form of non-thermal effect for short period exposure. We therefore present the levels of absorption of electromagnetic waves in the human head executed by Patch antenna, using the heat equation, the specific absorption rate (SAR) and the increase in temperature. Keywords - SAR, Electromagnetic field, Heat equation, Patch antenna, Thermal effects I. INTRODUCTION The very fast evolution of wireless telecommunications systems has led to concern the public about health hazard effects of electromagnetic fields. For many years, this question has been studied and the limits of protection have been established by such international organizations as ICNIRP [1][] and IEEE [3] in order to protect the body from exposure to radiofrequency electromagnetic fields RF. While mobile phones emit electromagnetic waves, some of these waves are absorbed by the user's head, and another part is reflected. The absorbed radiation in biological tissues is evaluated and presented by the specific absorption rate SAR. If the exposure takes a considerable length of time it may be accompanied by an increase in temperature [3][4]. In this paper, we are going to present the level of the SAR and the level of temperature increase in the human head when it is exposed to the electromagnetic field near a mobile phone. We use the patch antenna operating in two frequencies 900 and 1800 separately to simulate the mobile phone. II. SPECIFIC ABSORPTION RATE The specific absorption rate (SAR) is an index that measures the level of radio frequency electromagnetic field in the human head, as emitted by the mobile phone when operating at full power, in the worst conditions. Its unit is watts per kilogram (W / kg) [5]. The specific absorption rate is calculated by the following formula: E SAR (eq.1) Where E is the electric field in V / m, ρ is the density tissue mass in kg / m³, σ is the electrical conductivity of tissue in S / m. III. HEAT EQUATION In recent years, many results have been published on mobile phone interactions - biological tissues particularly on non-thermal effects. As for thermal effects, study has become more complex than that of non-thermal effects because there are three modes of heat transfer: conduction, convection and blood perfusion. To calculate the temperature rise in biological tissues, we use the following heat equation [6-8]: C t dt dt K T ( SAR) BT (eq.) Where T is the temperature increase, K is the thermal conductivity of the tissue, Ct is the heat capacity of the tissue and B is the rate of blood flow or blood perfusion. In the table 1 we present the thermal properties of some biological tissues used for simulations: Table 1 Thermal properties of biological tissues in the human head [6][9] Tissue Ct (J/Kg. C) K (W/m. C) B (W/m 3. C) Skin Muscle Bone Brain

2 The calculation of temperature from Equation is very complex if we take into consideration the heat dissipation processes such as conduction, blood perfusion and convection. However, if we calculate the temperature increase in the worst case, where all the electromagnetic energy is used to increase the temperature and any mechanism of heat dissipation is present, this equation becomes simple: C t dt dt (SAR) (eq. 3) So from the value of the SAR, we can deduce the temperature increase "dt" for a period of time "dt" and equation 3 becomes: dt ( SAR) dt C (eq. 4) t IV. USED MODEL We use for the simulation of the mobile phone a patch antenna operating at 900 frequency at first and then at 1800 frequency: We took the dimensions of the antenna patch as follows (Fig. 1): For the 900 frequency: W=97.87 mm, L=75.9 mm, W1=.86 mm, L1=44.01 mm. For the 1800 frequency: W=48.93 mm, L=37.77 mm, W1=.86 mm, L1= mm. The antenna patch consists of a ground plane type of copper, a Bakelite type substrate permittivity ε r = 4.8 and a rectangular radiating element type copper [9]. Fig. 1: Patch Antenna We have chosen the model of the human head a sphere filled with a dielectric layer similar to the human head as shown in the figure 1. The dimensions we have chosen are as follows: for the sphere, we took a radius of 80 mm for the first layer of skin type, for the thickness, Muscle and Bone we took 5 mm, and the last layer is the Brain, we took 65 mm. The dielectric properties of different biological tissues that constitute the human head are presented in the table : Table Dielectric properties of tissues used in the simulations [10-15] Tissues 900 ε r 900 σ 1800 ε r 1800 σ Density (Kg / m 3 ) Skin Muscle Bone Brain V. RESULTS AND DISCUSSION A. Phone - head simulation of homogeneous type brain We took a homogeneous sphere of type brain and we excited it by a patch working, at first, at 900 and later at

3 We calculated the distribution of specific absorption rate (SAR) average and local level and the increase in average temperature in the head and local uniform for a period of 5 min at tree different distances from the antenna, which is 5 mm, 10 mm and 0 mm. The results are presented in the figure for a patch antenna operating at 900 and figure 3 for a patch antenna operating at 1800 : 3,0,5 Average_SAR_10mm 4 Local_SAR_10mm Average_Temperature_5mm Average_Temperature_10mm Average_Temperature_0mm 0 0,35 0,30 5 Local_Temperature_10mm 0, , Fig. : Simulation of local SAR and average, the local temperature rise and average in the homogeneous sphere composed of the brain, excited by a patch at 900 for distances 5 mm, 10 mm and 0 mm. 1

4 0 Local_SAR_10mm 14 1 Average_SAR_10mm ,6 1,4 Local_Temperature_10mm Local_Temperature_10mm Fig. 3: Simulation of local SAR and average, the local temperature rise and average in the homogeneous sphere composed of the brain, excited by a patch at 1800 for distances 5 mm, 10 mm and 0 mm. 13

5 We note that the level of the SAR has important values if the antenna is too close to the sphere (the head) and poor when it is a bit far. If we increase the frequency of 900 to 1800, the SAR value increases. From the level of the SAR, we deduce the temperature increase. We conclude that the level of the temperature can reach after 5 min of exposure, up to 0.37 C for 900 and 1.65 C for 1800, which is very important. B. Telephone - Inhomogeneous head Simulation composed of four layers: skin, muscle, bone and brain In this section, we used an inhomogeneous sphere composed of four layers: skin, muscle, bone and brain. We have excited the antenna patch used previously, and we have got the following lines: 0,16 0,14 0, ,8 1,6 1,4 14

6 Temperature (C ) Average_Temperature_5mm Average_Temperature_0mm Fig. 4: Simulation of local SAR and average, the local temperature rise and average in the inhomogeneous sphere consisting of skin, muscle, bone and brain, excited by patch antenna at 900 for distances of 5 mm and 0mm. 3,0,5 3,5 3,0,5 15

7 0,7 0,3 0,1 Average_Temperature_5mm Average_Temperature_0mm Fig. 5: Simulation of local SAR and average, the local temperature rise and average in the inhomogeneous sphere consisting of skin, muscle, bone and brain, excited by a patch at 1800 for distances of 5 mm and 0 mm. We note that the highest level of SAR is in the muscle. So, it absorbs more radiation than the other layers. As for absorption in the bone, it is almost zero. Concerning the brain, it absorbs a lot of radiation, but since it is a bit far from the radiation source and the wave is degraded through the skin, muscle and bone, it absorbs less. So these layers somewhat protect the brain. For the temperature increase, the same procedure was done and we found out an increase in temperature very important for muscle and values near zero to the bone, and the temperature degrades through the layers and s' cancels when the wave propagates deeper. VI. CONCLUSION The cell phone use becomes very important, concerns about the possible effects on health due to electromagnetic radiation to multiple scientific research goes to study the effects of RF on the human head. The objective of this work is to simulate the electromagnetic field produced in the human head through its various layers near a mobile phone. We took a phone that works with a patch antenna. This antenna has a strong influence on the human head when it is closer to her and values of local SAR and medium increases with increasing frequency. A temperature increase was noticed in the tissues of the head. We concluded that using a telephone for a period a little bit long cause heating in biological tissues that can cause some symptoms such as headaches, loss of concentration. REFERENCES [1 ] IEEE Recommended Practice for the Measurement of Potentially Hazardous Electromagnetic Fields RF and Microwave, Recognized as an American National Standard (ANSI) IEEE Std C [ ] ICNIRP Guidelines, Guidelines for limiting exposure to time varying electric, magnetic, and electromagnetic fields (up to 300 GHz), Health Phys., Vol. 74, No. 4, 494 5, [3 ] IEEE Standard for Safety Levels with Respect to Human Exposure to Radio Frequency Electromagnetic Fields, 3 khz to 300 GHz, IEEE standard C , 199. [4 ] Hsing-Yi Chen, Member, IEEE, and Hou-Hwa Wang, Current and SAR Induced in a Human Head Model by the Electromagnetic Fields Irradiated from a Cellular Phone, IEEE Transc. On Microwave and theory, Vol.4, N 1, 1994, pp [5 ] A. Ibrahiem, C. Dale, W. Tabbara and J. Wiart, «Analysis of the temperature increase linked to the power induced by RF source», Progress In Electromagnetics Research, PIER 5, 3 46, 005. [6 ] Theodoros Samaras, Andreas Christ, Anja Klingenböck, and Niels Kuster, Members, IEEE; Worst Case Temperature Rise in a One- Dimensional Tissue Model Exposed to Radiofrequency Radiation ; IEEE Transactions on biomedical engineering, vol.54, N 3;007. [7 ] Akimasa Hirata, Member, IEEE, and Toshiyuki Shiozawa, Fellow, IEEE, Correlation of Maximum Temperature Increase and Peak SAR in the Human Head Due to Handset Antennas, IEEE transactions on microwave theory and techniques, vol. 51, no. 7, july 003. [8 ] R. Otin, Numerical study of the thermal effects induced by a RFID antenna in vials of blood plasma,progress In Electromagnetics Research Letters, Vol., , 011. [9 ] B.Vedaprabhu and K. J. Vinoy, an integrated wideband multifunctional antenna using a microstrip patch with two u-slots, Progress In Electromagnetics Research B, Vol., 1-35, 010. [10 ] S. Khalatbari, D. Sardari, A. A. Mirzaee, et H. A. Sadaf, «Calculating SAR in Two Models of the Human Head Exposed to Mobile Phones Radiations at 900 and 1800», Progress In Electromagnetic Research Symposium 006, Cambridge, USA,, (1), March 6-9,pp [11 ] S.Kassimi, S. Bri, M. Habibi, A. Mamouni, Application of the finite Differences Method (FDTD) For the Modeling of SAR in biological Tissues, Phys. Chem. News 60 (011) [1 ] S.Bri, S. Kassimi, M. Habibi, A. Mamouni, Specific Absorption Rate (SAR) Distribution in the Human Head at Global System Mobil (GSM) Frequencies, European Journal of Scientific Research ISSN X Vol.49 No.4 (011), pp , EuroJournals Publishing, Inc. 011, [13 ] C. D. Moss, F. L. Teixeira, J. A. Kong, "Analysis and Compensation of Numerical Dispersion in the FDTD Method for Layered, Anisotropic Media",IEEE Transactions on Antennas and Propagation, Volume 50,Issue 9, Sept. 00, pp [14 ] Akimasa Hirata, Member, IEEE, and Toshiyuki Shiozawa,Correlation of Maximum Temperature Increase and Peak SAR in the Human Head Due to Handset Antennas, IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 51, NO. 7, JULY 003 [15 ] Rodney G. Vaughan, Neil L. Scott, Evaluation of Antenna Configurations for Reduced Power Absorption in the Head, IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, Vol. 48, N.5, September 1999, pp