2013 IEEE International RF and Microwave Conference (RFM2013), December 09-11, 2013 - Penang, Malaysia
Effects of Electromagnetic Radiations Generated by Mobile Antennas on the Human Brain Moumen Cherif, and Benslama Malek Laboratoire d’Electromagnétisme et de Télécommunication, Université de Constantine, Algeria e-mail :
[email protected] ;
[email protected]
is quantified by the power absorptive per unit of exposed biomaterial mass. It is defined by specific absorption rate (SAR).
Abstract—Mobile telephony became an essential instrument in our everyday life, and with dimensions of more than 36 million mobile phones, one counts some 35000 basic antennas or stations, where the surrounding inhabitants worry about the consequences that they could have on health [1]-[2]. In this article, the dielectric human tissue properties and their evolution will be approached according to the variations of the frequency, as well as the resolution of the equations of Maxwell's in order to determine the expressions of the electromagnetic fields in the human body, thus broadcasting capacity of energy (RF) in the layers of the human head most exposed to these radiations, this by the observation of the variation of the power absorptive by tissue of the human head.
II.
The electromagnetic wave penetrated in the tissue depends on the following electric properties [4]: the electric permittivity ε, the magnetic permeability μ and the electric conductivity σ of various tissues, as well as the thickness of tissue, incidental power and the frequency of operation. The electric properties of tissue vary with their water content; the water content in fatty tissue is much larger than in non-fatty tissue, the human biomass can be regarded as dielectric:
Key words: dielectric parameters of tissue; specific flow of absorption SAR; incidental power on the head; power absorptive by the head.
I.
ε = ε 0 (ε ' − jε '' ) = ε 0 (ε ' − j (σ / ωε 0 )) ε 0 : Electric permittivity in the space, ε ' and σ
INTRODUCTION
(1) are
dependent on the frequency, and this frequency has two significant values [0.9 GHz and 1.8 GHz] at a temperature of 37°, as illustrated in the equations (1), (2) & (3) [5] :
Since its creation, networks of mobile telephony and communication networks have so much developed that they have become an essential tool of communication [3]; however this communication puts more and more in sets of problems of effects on health. Field RF is the association of an electric field and a magnetic field which vary in time and are propagated in space. These fields are likely to move electric charges, and are characterized by several physical properties whose principal ones are : the frequency, the wavelength, intensity and power. Therefore any living matter contains electric charges (ions, molecules...) and insulating materials; it is thus a medium slightly conducting (called dielectric). According to the position of the mobile telephony compared to the human head figure (4), the tissue (layers of the head) is subjected to a field RF, part of the field is reflected, and the other penetrates in the organism. The radiation produced by this interaction must be quantified, because it can be at the origin of biological effects. The field which penetrates inside the tissue can be calculated using electromagnetic models, and the energy absorptive by transformation into heat proportions
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PROPRIETES DIELECTRIC OF TISSUE
ε ' = 1.71 f −1.13 + (ε sm − 4) /[1 + ( f / 25) 2 ] + 4 σ = 1.35 f
σ 0.1 + [0.00222 (ε
0 .13
m s
f is the frequency in GHz, and conductivity in 0.1 GHz,
ε sm =
2
σ 0.1 =
0.05 is the
8.5 is the extrapolated
permittivity of the electromagnetic wave [6].
368
(2)
− 4) f ] /[1 + ( f / 25) ] (3) 2
B. Penetration depth
Figure 1. Relative permittivity in function to the frequency.
The penetration depth is defined as being the distance so that the density of power is reduced to 13% of its initial value, the distance z = 1/α is called penetration depth (SAR) represented on the figure (3).
SAR =
1
α
=2
ε0 ε ' μ0 σ
(11)
Figure 2. Conductivity in function to the frequency. Figures (1) and (2) are identical by contribution to practical measurements, and small variation between f = 1 GHz and f = 10 GHz is due to the water content in the biomass. III.
EXPRESSIONS OF THE FIELDS E/M IN THE HUMAN BODY
Figure 3. Penetration depth in function to the frequency.
The expressions of the electromagnetic fields in the human body can be obtained by the resolution of the equation of Maxwell's, on the assumption of a wave with uniform incidence and being propagated along direction Z.
Ε y = Ε 0 exp(−αz ) exp( j (ωt − βz ))
(4)
Η z = −(Κ z / ωμ 0 )Ε y
(5)
With : Κ z =
ω
εr − j
c
σ = β + jα ωε 0
As the figure indicates it (3) above, the SAR of the man depends on the frequency of operation, the SAR decreases quickly when the frequency increases, that means that quantity of incidental energy is absorbed by the human body, although the evaluated SAR is lower than the limits of health protection recommended in official regulations [7]. IV.
(6)
Properties of reflection and transmission in various tissues are in function dielectric properties of these various tissues: frequency of operation, angle of incidence, type of polarization and the thickness of each layer.
Kz is the complex constant of propagation, α represent the coefficient attenuation and β is the constant of phase. A. Coefficients of the attenuations The solution ( Ε y
/ Η z ) of the equations of
A. Model of the human head
Maxwell's led to calculate the constant of attenuation ( α ), the constant of phase ( β ), intrinsic impedance of the wave ( η ) and the propagation speed V, represented in the relations (7), (8), (9) & (10).
α
=
μ
σ ε
2
0
0 ε
'
2 ⎛ ⎛ ε '' ⎞ ⎞⎟ ⎜ β = ω μ 0ε 0ε 1 + 0.125⎜⎜ ' ⎟⎟ ⎜ ⎝ ε ⎠ ⎟⎠ ⎝ '
η=
μ0 ⎛ σ ⎞ ⎜1 + j ⎟ ' ⎜ ε 0ε ⎝ 2ωε 0 ε ' ⎟⎠
V =
⎛ 1 ⎛ σ ⎞2 ⎞ ⎜1 − ⎜ ⎟ ⎜ ωε ε ' ⎟⎟ ⎟ ' ⎜ 8 μ 0ε 0ε ⎝ ⎝ 0 ⎠ ⎠ 1
RF ENERGY PROPAGATION THROUGH THE HUMAN HEAD
It is more practical to take the human head like a model simple to study in the form of four layers : Skin, Fat, Bone and Brain (figure 5), as well as their density and electric properties for the two significant frequencies 0.9 GHz and 1.8 GHz, table 1 [8].
(7)
(8)
(9)
(10)
Figure 4. Position of the mobile on the human head
369
Figure 6. Incidental power in function to the fréq with z=0. The incidental power reaches a maximum level, then decreases quickly according to the increase in the frequency.
Figure 5. Simple model of the human head.
Bone
0.9 1.8 0.9 1.8 all all
43.74 41.36 0.855 1.21 1 1100
5.46 5.35 0.051 0.078 1 920
12.45 11.78 0.143 0.275 1 1850
Pr = Pi Rc2
Brain
Fat
σ [Sm-1] μr [ - ] ρ [Kgm-3]
Skin
εr [ - ]
Fréquence [GHz]
C. Reflected power
D. Transmitted power
52.73 50.08 0.942 1.391 1 1050
Pt = Pi Tc2
Rc and Tc
(15) are the coefficients of reflexion and
transmission in tissues [9]-[10], for a wave with uniform incidence (θ = 0).
R=
Table 1. Characteristics and electric properties of the layers of the human head.
ε1 − ε 2 ε1 + ε 2
,T
= 1+
ε1 − ε 2 ε2 + ε2
(16)
ε 1 and ε 2 are the permittivity of medium 1 and medium2.
Thus εr the relative permittivity, σ the electric conductivity [Sm-1], μr the relative permeability, ρ density [kgm-3].
1)
Absorptive power with Z = 0
By the application of the principle of conservation of energy :
B. Incidental power
Pa (0) = Pi − Pr − Pt = 2 R (1 + R )Pi
The flow of the incidental power can be calculated by G using the theorem of Poynting, the vector of Poynting p is equal to the average power which crosses the unit of G area of the plan of wave, therefore the flow of p represents a power, in our case a wave which is G propagated in direction OZ, the vector p only one component has
(14)
(17)
p z who is written :
1 P( z ) = − Re[ E y H x* ] 2
(12) Figure 7. Power absorptive in function to the fréq with z=0.
Therefore the incidental power is written:
⎛ ε '' ⎞ 1 ε 0ε ' ⎛⎜ Pi ( z ) = 1 + 0.125⎜⎜ ' ⎟⎟ 2 μ0 ⎜ ⎝ε ⎠ ⎝
2
⎞ ⎟ E 2 exp(− 2αz ) ⎟ 0 ⎠
2) (13)
Absorptive power with Z ≠ 0
Pa (Z ) = Pa (0 ) exp (− 2α h ) h represent the thickness of the layer in question.
370
(18)
induce physiological changes on the brain which results in unpleasant moods which on the long term [11] can influence the metabolism at the level of the brain. Recent work starts to clarify the problems of the electromagnetic environment. The aspect of pollution E/M starts to be posed with acuity by [12]. Theoretical and experimental works were born [13], they pose the problem of the human behaviour subjected to an electromagnetic influence in a clear way [11,14]. Figure 8. Power absorptive in function to depth z for f=0.9GHz.
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Figure 9. Power absorptive in function to depth z for f=1.8GHz.
Figure 10. Power absorptive in function to depth z for f=3GHz.
E. CONCLUSION Analysis of the relative power absorptive in function to the penetration depth (z) leads to the following results: 1- With Z = 0 (surface head): two figures (6) and (7) are identical on the form and not on the amplitude (absorptive energy < incidental energy), this justifies that the human head absorbs a some quantity of energy. 2- with Z ≠ 0 (inside the head): in various tissues appears (8), figure (9) and figure (10), when the frequency increases (f = 0.9GHz, f = 1.8 GHz and f = 3 GHz): the amplitude of the absorptive power undergoes a light reduction, and the penetration depth (z) undergoes a very fast reduction. When the frequency increases, the wave with a less penetration is due to the obstacles (living matter) on the course of the wave, what is called the interaction (wave / living matter), which produces an incidental energy absorption along the wave propagation until the complete weakening of the wave. The analysis carried out shows that the absorptive energy by the human head is weak; however, the interaction wave / living matter is not negligible. This can
[14] E. Keller, and al, “Theoretical evaluations of therapeutic systemic and local cerebral hypothermia”, Journal of Neuroscience Methods 178 (2009), pp. 345–349.
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