Modeling of Electromagnetic effects is now an increasingly important design requirement for the advancement of personal communication services(PCS).In the recent period the mobile phone industries is paying a great attention towards human health safety issues such as preventing brain tumor which is a side effect caused by Electro Magnetic Radiation. The human head model is assumed to be homogeneous and simulated with the brain equivalent material. The Antennas are assumed to be Î»/4 and (3/8)Î» monopoles mounted on a mobile handset, operated at frequencies mentioned above. With radiated power of 0.32 watts. The purpose of this simulation is for calculation of 1g average SAR on the above systems, which is recommended its limit value 1.6 W/Kg in FCC. The International Commission of Non-Ionizing Radiation Protection (ICNIRP) guidelines stipulate a maximum SAR of 2wKg or 2mW/g in any 10g of tissue in the head. The Specific Absorption Rate (SAR) is a key parameter defining the energy absorbed by the human body under electromagnetic radiation. Complex configuration of the mobile phone antenna with the human head modeling makes the evaluation of SAR more complicated, resulting in high computational resources and long simulation time in Numerical Modeling. This work is aimed at proposing a model based Dyadic Green's Function(DGF) in spherical coordinate system that can describe the field distributions inside head. Numerical results from analytical expressions are computed for the problem of the spherical head model(multilayered homogeneous lossy dielectric spherical model) in close proximity and then compared with the results of the same model using Numerical techniques(FDTD/MOM).The medium is assumed to be homogeneous, isotropic, linear, non-dispersive and stationery
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With the expansion of current use and anticipated further increases in the use of mobile phones and other personal communication services, there have been considerable research efforts devoted to interactions between antennas on the human body. These activities are important from both health and antenna performance points of view. For absorption of energy Electromagnetic fields can be divided into four frequency ranges. Significant absorption may occur in the neck and legs if the range of frequency's falls between 100KHz-20MHz. Absorption in the trunk decreases exponentially with frequency. High absorption can occur in the whole body if the frequency range falls between 20MHz-300MHz .Frequencies in the range from about 300MHz to several GHz at which significant local, non-uniform absorption occurs. Frequencies above 10GHz at which energy absorption occur primarily at the body surface.
The Specific Absorption Rate(SAR) is the most appropriate metric for determining Electromagnetic effect exposure in the near field at Radio frequency(RF) source. The SAR(W/Kg) at any point in the model can be determined from the calculated electric field (V/mt) at that point is given by:
Where 'E' is the internal electric field (RMS) value(v/mt), SAR is the Specific Absorption Rate(w/Kg), Ïƒ is the conductivity(S/m) of the tissue in which the calculation is done, 'Ï' is the mass density (kg/ cubic mt). When the microwave radiation is emitted from a cellular telephone handset, held next to a human head a portion is radiated away into the surroundings air space and other portion is scattered and absorbed by tissues in the head and neck region of the body. Typically about 50% of the radiated microwave energy is deposited into such tissues as the ear scalp, skull and the brain. The Specific Absorption Rate (SAR) is a metric to quantify and register localization of deposited microwave energy. In International standard units, SAR is expressed in units of watts per kilogram (W/Kg). As a dosemetric quantity it denotes the time rate of microwave energy absorption at a given location inside the tissue. Clearly it varies from one location to another. The accuracy and reliability of a given SAR value depends on the three key parameters namely tissue density, conductivity, and electric field, but the most significant of these is induced electric field. The induced electric field is a complex function of several physical and biological variables which include the microwave frequency , the source size and polarization and the composition of tissue and geometry as well as orientation. Microwave energy absorption in biological tissue will cause the tissue's kinetic energy to increase as a function of time. If the incident power is sufficiently high the absorbed microwave energy will produce temperature increase that rise linearly during initial period. Rate of increase is proportional to the power deposition-SAR and is measurable through a temperature gauge that's immune to microwaves.
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SAR= c ( )
Where 'c' = specific heat of a tissue phantom( J/ kg/ deg ), is the transient temperature rise( ) and is the duration in sec. of power application used for the linear portion of the temperature rise. Note that the incident power should be fairly high but not too high, in order to limit the temperature rise to or less. It's important to guard against complications due to temperature related changes in tissue properties and the associated phenomenon. All these factors mentioned above depends on Antenna design. It's not possible to test directly on human beings to predict SAR estimates, hence one should switch over to either PHANTOM materials and Numerical modeling.
3. Human body interaction with Microwave Antenna:
The energy quanta of radiation at 900MHz-1800MHz (comparable with current mobile frequencies) equals 4 and 7Î¼ev respectively. Both of these values are extremely small compared with the energy of around 1ev needed to break the weakest chemical bonds in genetic molecules( DNA) . Hence it is impossible. Therefore RF radiation could damage DNA directly which might start cells on the path to cancer.
3.1 Effect of Radiation on human body with respect to Temperature:
Temperature raised by 0.25ml of muscles by Microwave absorption is given below; 150watts generates energy of 310.6Joules per 1000ml per sec in 5 minutes energy generated by the 150watt microwave source is 310.6 x 300sec. Therefore energy required to rise:
Temperature raised by 150watt microwave source for 1000ml = 22.29; similarly temperature raised by 0.015watt for 100ml = 0.0022and temperature raise by 0.5 watt source for 0.25ml = 8.8; for digital transmission (modern mobiles) temperature raise = 4.4; but absorption of water = 2.993.
The mobile phones transmit microwave power ranging from 0.2w to 2w. Depending upon the strength of nearby tower. The minimum microwave oven heats 1000ml of water to 22 in 5min. So 0.025ml of muscle tissue of index finger will be heated ~5 in 5 minutes. Since skin and bones are transparent to microwaves. Index finger muscles and blood tissues absorb microwave power. We will not feel the heating of the tissues as sensing nerves are attached to skins. This research suggests that it is better to use mobile phones less than 5 minutes and to keep the mobile phone antenna facing outside.
3.2FDTD Formulation :
In the FDTD formulation both space and time are divided in to discrete segments. space is segmented in to box shaped cells which are small in comparison with the wavelength The electric fields (Ex(i,j,k), Ey(i,j,k) and Ez(i,j,k)) are located on the edges of the box, and the magnetic fields (Hx(i,j,k), Hy(i,j,k) and Hz(i,j,k)) are positioned on the faces as shown in Figure 1. This orientation of the fields is known as the Yee cell  and is the basis for FDTD. The time is divided into small lapses where each step represents the time required for the field to travel from one cell to the next. Given an offset in space of the magnetic fields in relation to the electric fields, the values of the field in respect to time are also offset. The electric and magnetic fields are updated using a leapfrog
scheme where the electric fields come first, then the magnetic ones are computed at each step in time. When many FDTD cells are combined together to form a three-dimensional volume, the result is an FDTD grid or mesh. Each FDTD cell will overlap the edges and faces with their neighbors. Therefore each cell will have three electric fields that begin at a common node associated with it. The electric fields at the other nine edges of the FDTD cell will belong to other adjacent cells. Each cell will also have three magnetic fields originating on the faces of the cell adjacent to the common node of the electric field as shown in fig 1.
This knowledge of field values associated with the characteristics of the tissue help to determine the SAR in the tissues without requiring an invasive measure. Now we present the Maxwell's equations in three dimensions. We suppose the absence of magnetic or electric current sources, and the existence of absorbing materials in the space.
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Where the displacement vector is related to the electric field through the complex permittivity
The various components of the fields are evaluated on the basis of neighboring components of each lapse of time and each cell in the modeling area. This method works in the time domain and allows direct visualization of Electromagnetic fields.
3.3 Modeling dipole antenna in
A simple dipole is illustrated in fig(2), consists of two metal arms. A dipole antenna functions with a a current flow through the arms, which results in radiation. FDTD simulates a dipole in the following way. The metal of the
Arms are specified by setting the Ez parameters to zero in the cells corresponding to the metal; except in place where the source is placed. This insures that the corresponding Ez field at this point remains zero as well as it would if that
point were inside the metal. The antenna length was held constant at each simulation. Perfectly Matched Layer (PML) boundary conditions were employed. The source is specified by setting the Ez field in the gap to a certain value. For the FDTD simulation, dipole is fed at the center (x = ic Î”x, y=jc Î”y, z=kc Î”z) gap of length Î”z with a Gaussian pulse . So, the electric field in the gap of the dipole is:
3.4 Electric current:
The current in the antenna at the feed point is obtained by applying Ampere's law to the surface
S with the bounding contour C on the wire at (ic, jc, kc +3/2):
3.5 The input impedance calculation :
The input impedance of an antenna is a very important parameter. After the final time domain
results are obtained, the current and voltage are transformed to those in the Fourier domain. The input impedance was calculated in the centre fed dipole over a range of frequencies. It is determined from the ratio of the Fourier transform of the voltage wave and that of the input current wave
The input impedance of the dipole antenna is shown in Figure 4. The input impedance is well matched at 75.48 + j1.12 at the resonance frequency of 1800 MHz.
3.6The input return loss (S11) and the
voltage standing wave ratio (VSWR) :
The results of input impedance are then used to obtain the return loss characteristics of the antenna. So the reflection coefficient S11 of the half-wavelength dipole antenna is:
From the calculated reflection coefficient, the voltage standing wave ratio (VSWR) can be calculated as follows:
The bandwidth of the antenna, which was determined by the impedance data, is the frequencies corresponding to a reflection coefficient of the antenna (less than or equal to
1/3) that corresponds to VSWRâ‰¤2.
In Figure 4, the resonant frequency which is around 1.8 GHz. was chosen as a frequency through the whole study.
3.7 Interaction between the handset
and the human head :
In this section, the interaction between the mobile handset and the human head has been studied. A simplified homogeneous spherical head model is used. The sphere has a radius of r = 10 cm and the tissue it contains has a relative permittivity of Îµr =43.5 and conductivity of Ïƒ
=1.15 S/m. These tissue equivalent dielectric = 10 cm and the tissue it contains has a relative permittivity of Îµr =43.5 and conductivity of Ïƒ =1.15 S/m. These tissue equivalent dielectric parameters were chosen according to  to
simulate the brain tissue at 1.8GHz. For the computation of SAR, the head tissue density is assumed to be 1030 kg/m3. The relative position of the dipole antenna relative to human
head model is illustrated in Figure 6. The interaction between the mobile handset and the human head is studied from two viewpoints: first the impact of the distance between head and phone; second the effect of head type
(homogeneous,) on the absorption and distribution of electromagnetic fields in the human head and on the radiation pattern.
The following table summarizes the dielectric constant, the conductivity Ïƒ and the mass density of the tissues used for the calculations at 900 & 1800 MHz .
As a conclusion, we can say that the important arameters affecting the energy absorbed in the human head exposed to radiation from radio is the distance between the head and the antenna. Although the power consumption in the case of the 1800 MHz frequency is lower than 900 MHz,
the maximum values of SAR are more significant for the higher frequencies.