CHAPTER 2 SAR ANALYSIS

Transcription

1 CHAPTER 2 23 SAR ANALYSIS 2.1 SPECIFIC ABSORPTION RATE Specific absorption rate (SAR) is a measure of the rate at which energy is absorbed by the body when exposed to a RF electromagnetic field. It is defined as the power absorbed per mass of tissue and has units of watt per kilogram. SAR is usually averaged either over the whole body, or over a small sample volume (typically 1 g or 10 g of tissue). The value cited is then the maximum level measured in the body part studied over the stated volume or mass. It can be calculated from the electric field within the tissue [36] as: where is electrical conductivity (2.1) E is electric field strength and is mass density SAR is used to measure exposure to fields between 100 khz and 10 GHz. It is commonly used to measure power absorbed from mobile phones and during MRI scans. The value will depend heavily on the geometry of the part of the body that is exposed to the RF energy and on the exact location and geometry of the RF source. Thus tests must be made with each specific source, such as a mobile phone model, and at the intended position of use. For example, when measuring the SAR due to a mobile phone the phone is placed at the head in a talk position. The SAR value is then measured at the location that has the highest absorption rate in the entire head, which in the case of a mobile phone is often as close to the phone's antenna as possible.

2 2.1.1 SAR Calculation 24 In the past few years, very rapid development in mobile cellular communication has drawn attention to possible health risks of the electromagnetic energy (EM) emitted from the transmitters of hand-held phones. The interaction between a human head and a hand-held phone under various conditions should be quantitatively evaluated in order to establish the safety in cellular mobile communication systems. These evaluated values should be within the safety limits given in ANSI/IEEE C RF Safety Guideless. Three different limits are defined for the current safety studies: 1) a whole body average SAR; 2) a local peak SAR; and 3) a Specific Absorption (SA), which limits the power of short pulses. Within these three 1) and 2) must be averaged over a defined period of time. Due to the high non-uniformity of the SAR distribution induced by a cellular mobile phone within a human head, the peak SAR is the relevant parameter to assess the risk caused by these devices. In uncontrolled environments, a peak SAR of 1.6W/kg, as averaged over any 1 g of tissue must not be exceeded. But, according to, the limit on the peak SAR is fixed at 1W/kg, as averaged over any 100g of tissue. The hand-held cellular mobile phone is stated to be safe when a cellular mobile phone's radiated power is 0.7 W (at 900MHz) and when the distance to human head is greater than 2.5 cm. Beside public biological concerns in cellular mobile communication system, there is also a great demand to know the deterioration of the antenna performance because of the existence of human head. This is an important feedback for antenna designers to develop better structures. Analyzing possible range of variations of the induced field strengths in various tissues requires an extensive effort, since local field strengths strongly depend on various parameters. Among the others operational frequency, antenna power, mutual positions of the between device and head, design of the device, size and the shape of human head, distribution of tissues within the head and electrical properties of the tissues can be listed as important parameters which strongly

3 25 affect the SAR distribution. Since some of the listed parameters [51] are different for various individuals and can even change with time, analytical formulations (even the approximate ones) in SAR distribution calculations are extremely difficult. SAR distributions in a human head exposed to EM fields from hand-held cellular phones have been estimated through experimental and numerical calculations. The models used[51] in these studies are quite different where from simple to enhanced geometries and from a few to many different tissues types are taken into account with different electrical properties. Moreover, quite different values have been used in some of these studies parallel to more accurate measurements of human tissues. For example, there are more than hundred percent differences in some of the tissue parameters[51]. Within the limits of the used models, these studies have presented the dependence of both local and global SAR distributions in a human head to the distance of the mobile cellular phone, as well as to its position. Also, it has been shown in these studies that SAR distributions within human neck and body caused by EM radiation above 300MHz are almost negligible[51]. It is, therefore, quite acceptable to use isolated head and hand-held cellular phone during SAR distribution calculations. Recent progress in computer technology enables us to use FDTD method to numerically calculate the EM interactions of a inhomogeneous, realistic human head and mobile phone models. In FDTD, spatial and time derivatives in Maxwell's equations are replaced with their central difference approximations in specially organized unit cells. FDTD has been described in several publications [51], six components of electromagnetic fields are arranged to minimize the computational storage needs. The entire computation domain is obtained by stacking these unit cells into a larger rectangular volume. FDTD is very easy to implement and to trace the wave phenomena and can handle complex geometries with

4 26 inhomogeneous materials, either conductors or lossy dielectrics. FDTD has been applied to a large amount of EM problems, including planar microstrip analysis, scattering and inverse scattering problems, antenna simulation together with nearto-far field transformations, etc FDTD Method Finite-Difference Time-Domain (FDTD) method is used to calculate the Specific Absorption Rate distribution in a human head near a hand-held cellular phone. A three dimensional FDTD algorithm is built in Cartesian coordinates. A discrete human head model, derived from a Nuclear Magnetic Resonance (NMR) image by semi-automatic algorithm, is located within FDTD volume together with a discrete hand-held receiver model. FDTD simulations are carried out for both European GSM (operating at 900MHz) and DECT (operating at 1.8GHz) systems with a quarter-wavelength antenna, mounted on top of the hand-held cellular phone. A new and an effective way of calculating input, radiated and absorbed power distributions is introduced where time-domain Poynting vector is traced over a closed virtual surface surrounding the discrete phone and head models. SAR distributions for various vertical and horizontal slices of the human head are calculated and are shown to agree with the available calculation and measurement results. The computational results are interpreted in terms of international safety guidelines for human health. The Finite Difference Time Domain (FDTD) method was originally developed by Kane S. Yee, in He proposed a three dimensional central difference approximation for Maxwells curl equations, both in space and time. FDTD is a time-domain method in that transient fields are computed as a function of time. FDTD enables the accurate characterization of complex inhomogeneous structures for which analytical methods are ill-suited. Numerical computation is

5 27 accomplished by applying some excitation to an object or group of objects in a 3 dimensional space and calculating the fields as time progresses. The FDTD method belongs in the general class of grid-based differential time-domain numerical modeling methods. The resulting finite-difference equations are solved in either software or hardware in a leapfrog manner: the electric field vector components in a volume of space are solved at a given instant in time; then the magnetic field vector components in the same spatial volume are solved at the next instant in time; and the process is repeated over and over again until the desired transient or steady-state electromagnetic field behavior is fully evolved Simulation region The simulation region must be divided into Yee Cells as shown in Figure 2.1 in order for the FDTD method to be applied. Illustration of a standard Cartesian Yee cell used for FDTD, about which electric and magnetic field vector components are distributed (Yee 1966), visualized as a cubic voxel, the electric field components form the edges of the cube, and the magnetic field components form the normal to the faces of the cube. A three-dimensional space lattice is comprised of a multiplicity of such Yee cells.

6 28 Figure 2.1 Standard Yee Cell In the Cartesian coordinate system, each Yee cell contains six field components, Ex, Ey, Ez, Hx, Hy and Hz. The fields are all offset by half a space step as shown in the above diagram Operation of FDTD When Maxwell's differential equations are examined, it can be seen that the change in the E-field in time (the time derivative) is dependent on the change in the H-field across space (the curl). This results in the basic FDTD time-stepping relation that, at any point in space, the updated value of the E-field in time is dependent on the stored value of the E-field and the numerical curl of the local distribution of the H-field in space (Yee 1966). The H-field is time-stepped in a similar manner. At any point in space, the updated value of the H-field in time is dependent on the stored value of the H-field and the numerical curl of the local distribution of the E-field in space. Iterating the

7 29 E-field and H-field updates results in a marching-in-time process wherein sampled-data analogs of the continuous electromagnetic waves under consideration propagate in a numerical grid stored in the computer memory Strengths of FDTD FDTD is a versatile modeling technique used to solve Maxwell's equations. It is a time-domain technique, and when a broadband pulse (such as a Gaussian pulse) is used as the source, the response of the system over a wide range of frequencies can be obtained with a single simulation. FDTD technique allows the user to specify the material at all points within the computational domain. A wide variety of linear and nonlinear dielectric and magnetic materials can be naturally and easily modeled. FDTD allows the effects of apertures to be determined directly. Shielding effects can be found, and the fields both inside and outside a structure can be found directly or indirectly. FDTD uses the E and H fields directly. Since most EMI/EMC modeling applications are interested in the E and H fields, it is convenient that no conversions must be made after the simulation has run to get these values Weakness of FDTD FDTD requires that the entire computational domain be gridded, and the grid spatial discretization must be sufficiently fine to resolve both the smallest electromagnetic wavelength and the smallest geometrical feature in the model, very large computational domains can be developed, which results in very long solution times.

8 30 FDTD finds the E/H fields directly everywhere in the computational domain. If the field values at some distance are desired, it is likely that this distance will force the computational domain to be excessively large. FDTD simulations calculate the E and H fields at all points within the computational domain, the computational domain must be finite to permit its residence in the computer memory. Because FDTD is solved by propagating the fields forward in the time domain, the electromagnetic time response of the medium must be modeled explicitly. For an arbitrary response, this involves a computationally expensive time convolution, although in most cases the time response of the medium can be adequately and simply modeled using either the recursive convolution (RC) technique, the auxiliary differential equation (ADE) technique, or the Z-transform technique. 2.2 SAR DISTRIBUTION Head Model The head model was created by taking the superimposition of different tissues like skin, skull, bone, gray matter, white matter, fat, humor, CSF. Above all tissues have different mass densities and dielectric properties at different frequencies. And also they have different thermal properties. The head model had 500X341 pixels. According to range of pixel values the different tissues of head are created. Sixteen different types of tissues and organs have been evidenced, and particular attention has been devoted to the modeling of the ear, skin, skull, air and brain simulate its morphology during a phone call. The tissue parameters used in the FDTD simulations [6] are given in Table 2.1 at frequency of 900 MHz. In some simulations, the hand grasping the phone has been considered. The hand tissue has been assigned electric properties(conductivity and dielectric constant)

9 31 equal to two-thirds of those used for the muscle since the depth of penetration for muscle is high. The hand is wrapped around the case on three sides, leaving free the side facing the head. The hand tissue has been assigned electric properties equal to two-thirds of those used for the muscle. The hand is wrapped around the case on three sides, leaving free the side facing the head. Table 2.1 Mass density and dielectric properties of the head tissues at 900 MHz Tissue Skin Muscle Bone Blood Fat Cartilage Gray Matter White Matter Humor Lens Sclera/ Cornea Cerebellum Hypophysis CSF Parotid Tongue r

10 2.2.2 SAR Distribution at 900MHz and 1800MHz 32 Figure 2.2 and Figure 2.3 show the SAR distribution in the head at 900 MHz and 1800 MHz. As power level increases (frequency) effect is more in near tissues to the source near the source SAR distribution is high. As distance increases from the source SAR decreases. The SAR distribution is high at 1800 MHz compared to 900 MHz. Figure 2.2 SAR Distribution in the human head model at 900MHz

11 33 Figure 2.3 SAR Distribution in the human head model at 1800MHz 2.3 HANDSET ANTENNA AND SAR The development of modern wireless communications devices has put significant pressure on the antenna performance, as the size of the device has decreased in parallel to the increase in the number of communications systems supported by a single terminal. This challenge has been the inspiration for scores of studies. In practice the synthesis of an antenna is done through analysis, since there are no universal mathematical models to create antenna structures based on the target properties. The desired antenna performance is pursued by trying different antenna types and changing their parameters. This emphasizes the role of experience and source literature, not forgetting the importance of the knowledge of the fundamental theory. Due to the reciprocal nature of antennas, and assuming an isotropic medium, the transmitting and receiving characteristics of an antenna are identical. This is a helpful aspect in the design and measurement process of

12 34 antennas. However, the requirements of a transmitting antenna are often different from those of a receiving antenna. In addition to the electrical requirements, the design of a handset antenna has to take into account the resulting exposure of the user. Compliancy with the SAR limits is one challenge more to the antenna designer, but it also bears a reward. Namely, the reduction of the power absorbed by the user is desirable in terms of increasing the total radiation efficiency of the handset. Research on the matter has produced known principles that can be utilized to reduce the SAR. However, it must be said that these principles only generate trends to the SAR levels and do not guarantee any absolute SAR values. Table 2.2 presents the frequency ranges of some of the most widely used wireless communications systems[69]. Of these systems E-GSM 900 and GSM 1800 are used in most of the world, including Europe and Asia, and GSM 850 and GSM 1900 in the Americas. UMTS 2100, which is a newer system deployed in Europe and Asia, belongs to the third generation (3G) of wireless systems. Table 2.2 Wireless communications system bands System Frequency [MHz] Relative bandwidth (Br) [%] GSM E-GSM GSM GSM UMTS In addition to the systems of Table 2.2 there can be a number of other supplemental systems integrated in a single modern handset, e.g. Bluetooth (BT)

13 35 and Wireless Local Area Network (WLAN). These systems, however, normally operate at a lower power level and do not require the handset to be held near the head of the user. A variable greatly affecting the resulting SAR values of handsets is naturally the applied transmitting power level. Table 2.3 summarizes the transmit frequencies[69] and maximum powers of the mobile communication systems already presented in Table 2.2. GSM systems divide transmissions into eight time slots. Table 2.3 Maximum transmit powers in mobile communication systems System Uplink frequency[mhz] Peak power [W] Mean (rms) power [W] GSM E-GSM GSM GSM UMTS < 1, typically During a call each phone is allocated one of the slots, during which it may transmit. Therefore the average power is one eighth of the peak burst power. The more recent UMTS system has a continuous, albeit varying, signal. The transmit power of mobile terminals in GSM and UMTS systems is adjusted automatically depending on the quality of the connection to the base station. The exposure is then highest when the base station is far away or the connection is weak due to some other reason. The radiation in newer systems is not more powerful, although their data transfer rate is far superior.

14 2.3.1 Handset Antennas Patch Antenna 36 A patch antenna (also known as a Rectangular Microstrip Antenna) with folded slit is a popular antenna type [12]. Its name is attributed to the fact that it consists of a single metal patch suspended over a ground plane. The assembly is usually contained inside a plastic radome, which protects the antenna structure from damage (as well as concealing its essential simplicity). Patch antennas are simple to fabricate and easy to modify and customize. The radiation mechanism arises from discontinuities at each truncated edge of the microstrip transmission line. The radiation at the edges causes the antenna to be slightly larger than its physical dimension electrically. In order to obtain a resonant condition at the antenna driving point, a shorter than a one-half wavelength section of microstrip transmission line is used. A patch antenna is generally constructed on a dielectric substrate, usually employing the same sort of lithographic patterning used to fabricate printed circuit boards. An integrated triple band antenna for the GSM800, DCS and PCS is designed. The frequency range of operation is MHz and MHz. The front view and side view of patch antenna are shown in Figure 2.4 and Figure 2.5 respectively. The mobile consists of a PCB where the antenna is mounted on. The mobile is covered by a plastic casing of 1 mm thickness with a relative permittivity of r = 3. In the simulation all remaining parts of the mobile are treated as perfect conductors. In the following investigation the size of the mobile phone is 111 x 42 x 10 mm without changing any dimension in the upper part of the phone near the antenna. The radiating part is made of aluminum has the conductivity of 3.8 x 10 7 S/m and its dimension is 40 x 25mm.

15 37 The substrate used here is FR4 which has the r =4.6. The shape of the antenna has been modified in order to fit closer to the shape of a realistic mobile. The relative permittivity of the plastic casing is r and its thickness is 1 mm. The size of the antenna parameters are given in micrometer scale. Figure 2.4 Patch antenna Front View Figure 2.5 Patch antenna side View Monopole Antenna The internal antennas have several advantages over conventional monopole-like antenna for mobile phones [1]. They are less prone to damage, compact in total size and aesthetic from the appearance point of view. Hence, small and low profile structures such as the planar antenna that can be mounted on the portable equipment are becoming very attractive for the mobile communications. But, they must consider about EMI/EMC and SAR problem. In particular, if the handset antenna radiates the energy in the direction of the head then it will, as a consequence, increase the SAR. In this project a novel compact internal planar antenna for multi-band operation covering the PCS, IMT-2000 and WLAN(2450 GHz) bands have been designed. The frequency range of operation

16 38 is GHz and GHz. The front view and side view of monopole antenna are shown in Figure 2.6 and Figure 2.7 respectively. Antenna structure size is 60mm 24mm. Ground plane and radiating part thickness is 0.1 mm. The main radiating part of the substrate is used of meander line with 10 mm 24 mm size. A 50 ohm microstrip line is used to feed the monopole planar antenna, and is printed on the same substrate. On the other side of the substrate, there is a ground plane below the microstrip feed line. Ground plane and radiating conductor is copper which has the conductivity of 5.8 x 10 7 S/m. Substrate dielectric used is FR4, its thickness is 1.6mm and relative permittivity is Adjust parameters to reduce the SAR are antenna size, substrate thickness and relative permittivity. Figure 2.6 Monopole Front View Figure 2.7 Monopole Side View

17 Inverted F Antenna 39 Inverted F Antenna (IFA) typically consists of a rectangular planar element located above a ground plane, a short circuiting plate or pin, and a feeding mechanism for the planar element. The Inverted F antenna is a variant of the monopole where the top section has been folded down so as to be parallel with the ground plane. This is done to reduce the height of the antenna, while maintaining a resonant trace length. This parallel section introduces capacitance to the input impedance of the antenna, which is compensated by implementing a short-circuit stub. The ground plane of the antenna plays a significant role in its operation. Excitation of currents in the printed IFA causes excitation of currents in the ground plane. The resulting electromagnetic field is formed by the interaction of the IFA and an image of itself below the ground plane. Its behavior as a perfect energy reflector is consistent only when the ground plane is infinite or very much larger in its dimensions than the monopole itself. In practice the metallic layers are of comparable dimensions to the monopole and act as the other part of the dipole. The antenna/ground combination will behave as an asymmetric dipole, the differences in current distribution on the two-dipole arms being responsible for some distortion of the radiation pattern. In general, the required PCB ground plane multi lobed. On the other hand, if the ground plane is significantly smaller than degrades. The optimum location of the IFA in order to achieve an omni-directional far- to be close to the edge of the Printed Circuit Board. The miter is used to avoid a right angle microstrip bend, which results in a poor current flow on the stub. The taper is needed in order to compensate the abrupt step transition encountered between the microstrip line feed

18 40 and the antenna. The omni-directional behaviour of the IFA with gain values that ensure adequate performance for typical indoor environments taking into account the standard values of the output power and receiver sensitivity of short range radio devices. Although, currently, many wireless systems are vertically polarized, it has been predicted that using horizontal antennas at both the receiver and the transmitter results in 10dB more power in the median as compared to the power received using vertical antennas at both ends of the link. The IFA bandwidth increases with its thickness. The front view and side view of IFA are shown in Figure 2.8 and Figure 2.9 respectively. IFA is designed for WLAN band. The frequency range of operation is GHz. Antenna structure size is 85mm 54mm and radiating part thickness is 0.2 mm. The radiating conductor is copper has conductivity of 5.8 x 10 7 S/m. Substrate dielectric used is Rogers RT/Duroid5880. Substrate thickness is 0.5mm and relative permittivity 2.20 and tangent delta of 9 x Figure 2.8 IFA Front View Figure 2.9 IFA Side View

19 Planar Inverted F Antenna 41 PIFA can be considered as a kind of linear Inverted F antenna (IFA) with the wire radiator element replaced by a plate to expand the bandwidth [9]. One advantage of PIFA is that it can be hidden into the housing of the mobile when comparable to whip/rod/helix antennas. Second advantage of PIFA is having reduced backward rad electromagnetic wave power absorption (SAR) and enhance antenna performance. Third advantage is that PIFA it exhibits moderate to high gain in both vertical and horizontal states of polarization. This feature is very useful in certain wireless communications where the antenna orientation is not fixed and the reflections are present from the different corners of the environment. In those cases, the important parameter to be considered is the total field that is the vector sum of horizontal and vertical states of polarization. Narrow bandwidth characteristic of PIFA is one of the limitations for its commercial application for wireless mobile. The shorting post near the feed probe point of usual PIFA types is good method for reducing the antenna size, but this results into the narrow impedance bandwidth. Bandwidth is affected very much by the size of the ground plane. By varying the size of the ground plane, the bandwidth of a PIFA can be adjusted. Reducing the ground plane can effectively broadened the bandwidth of the antenna system. To reduce the quality factor of the structure (and to increase the bandwidth), can be inserted several slits at the ground plane edges. Using stacked elements it will increase the bandwidth. One method of reducing PIFA size is simply by shortening the antenna. However, this approach affects the impedance at the antenna terminals such that the radiation resistance becomes reactive as well. This can be compensated with capacitive top loading. In practice, the missing antenna height is replaced with an

20 42 equivalent circuit, which improves the impedance match and the efficiency. The front view and side view of PIFA are shown in Figure 2.10 and Figure 2.11 respectively. PIFA is designed to work in UMTS-2100 band. The frequency range of operation is MHz. Antenna structure size is 80mm 46mm. Ground plane and radiating part thickness = 0.2 mm. Substrate Thickness is 0.5mm and relative permittivity 2.20 and tangent delta of 9 x Ground plane and radiating conductor is copper which has the conductivity of 5.8 x 10 7 S/m. Figure 2.10 PIFA Front View Figure 2.11 PIFA Side View SAR Calculation The inhomogeneous head model is chosen based on data from the Visible Human Project. The model is segmented into 1 mm voxels with more than twenty different tissue parameters. The dielectric parameters of the different tissues are automatically calculated for the selected frequency.

21 43 A voxel editor, available with EMPIRE allows to apply modification (rotations, change of material parameters or any voxel manipulation) to the model which is needed to adjust the orientation of the head with respect to the mobile in order to reach the so called cheek position. This model, developed from magnetic resonance imaging (MRI) scans of a human head, had a spatial resolution of 1.1x 1.1 x 1.4 mm from the top of the head to the ear-lobe level and a resolution of 4x 4 x4 mm in the remaining part of the head. Figure 2.12 shows human head model. For obtaining a good accuracy of the EM solution with the FDTD method at the frequencies of interest, a cell dimension of 1 mm is sufficient. Therefore, the original model has been resampled to obtain a final resolution of 2 x 2 x 2 mm. Twenty different types of tissues and organs have been evidenced, and particular attention has been devoted to the modeling of the head in order to simulate its morphology during a phone call. Figure 2.12 Human Head Model

22 44 Cell phones have low-power transmitters and most car phones have a transmitter power of 3 watts. A handheld cell phone operates on about 0.75 to 1 watt of power. The position of a transmitter inside a phone varies depending on the manufacturer, but it is usually in close proximity to the phone's antenna. The radio waves that send the encoded signal are made up of electromagnetic radiation propagated by the antenna. The function of an antenna in any radio transmitter is to launch the radio waves into space; in the case of cell phones, these waves are picked up by a receiver in the cell-phone tower. Figure 2.13 shows how the radiation enters in to the head. As with most controversial topics, different studies have different results. Some say that cell phones are linked to higher occurrences of cancer and other ailments, while other studies report that cell-phone users have no higher rate of cancer than the population as a whole. No study to date has provided conclusive evidence that cell phones can cause any of these illnesses. However, there are ongoing studies that are examining the issue more closely. Figure 2.13 Human Head with cell phone

23 45 At high levels, radio-frequency energy can rapidly heat biological tissue and cause damage such as burns. The mobile phones operate at power levels well below the point at which such heating effects would take place. The amount of radiation emitted from the devices is actually minute and there is a limits on how much radiation a cell phone can emit. The designed antennas are placed near the inhomogeneous head model and the SAR values are noted for each antenna at different frequency bands. SAR values for Patch Antenna, Monopole Antenna, Inverted F-Antenna and Planar IFA are tabulated in Tables 2.4, 2.5, 2.6 and 2.7 respectively. Table 2.4 SAR Values for Patch Antenna FREQUENCY SAR1g (W/Kg) SAR10g(W/Kg) 835 MHz MHz Table 2.5 SAR Values for Monopole Antenna FREQUENCY SAR1g (W/Kg) SAR10g(W/Kg) 1800 MHz MHz

24 46 Table 2.6 SAR Values for IFA Antenna FREQUENCY SAR1g (W/Kg) SAR10g(W/Kg) 2.46 GHz Table 2.7 SAR Values for PIFA Antenna FREQUENCY SAR1g (W/Kg) SAR10g(W/Kg) 1950 MHz The maximum SAR values for all the antennas at one gram and ten gram average shows that at higher frequencies the depth of penetration is higher than the lower frequency for a particular antenna. The average SAR may be well below the standard values, but the measured maximum SAR is high in some cases. The maximum SAR should also be maintained well below the safe level. So SAR reduction is needed SAR Reduction There are different approaches to SAR reduction. First approach is to investigate if the angle of phone in relation to human head have significant impact, second approach is to investigate if the size and type of the material covering the antenna have significant impact on the SAR radiation and as third approach is to investigate if it is possible to change the composition of the material which would ultimately lead to SAR reduction. The distance and antenna variations will not be

25 47 investigated. The investigations will be performed in a simulation program called EMPIRE XCcel and the results will be showed in text and figurative form. To reduce the SAR several tries of simulations were performed. By changing the angle the SAR values do not decrease but rather they move to another place in the head. So by changing the angle you lower SAR values at one place while increasing at another. The other two methods were tried and the SAR is significantly reduced. In the case of patch antenna the radiating aluminum is changed to copper having the conductivity of 5.8 x 10 7 S/m, which causes increase in conductivity and results in reduction of SAR. SAR Values are compared for Patch Antenna after reduction as shown in Table 2.8. Table 2.8 Comparison of SAR Values for Patch Antenna after Reduction FREQUENCY SAR1g (W/Kg) SAR1g (W/Kg) After Reduction SAR10g (W/Kg) SAR10g (W/Kg) After Reduction 835 MHz MHz In the case of monopole antenna the material used and the size of the antenna were changed. Changing substrate thickness from 1.6 mm to 1.7 mm and changing the relative permittivity from 4.62 to 4.9 gives reduction in SAR which is shown in Table 2.9.

26 Table 2.9 Comparison of SAR Values for Monopole Antenna after Reduction 48 FREQUENCY SAR1g (W/Kg) SAR1g (W/Kg) After Reduction SAR10g (W/Kg) SAR10g (W/Kg) After Reduction 1800 MHz MHz In the case of IFA and PIFA the substrate dielectric used is Rogers RT/Duroid5880. Substrate Thickness is 0.5mm and relative permittivity 2.20 and tangent delta of 9.000e-04. When we gradually change the relative permittivity of the dielectric from 2.20 to 4.56, there is a change in SAR for both the antenna types and it is shown in Table 2.10 and Table Table 2.10 Comparison of SAR Values for IFA Antenna after Reduction FREQUENCY SAR1g (W/Kg) SAR1g (W/Kg) After Reduction SAR10g (W/Kg) SAR10g (W/Kg) After Reduction 2.46 GHz Table 2.11 Comparison of SAR Values for PIFA Antenna after Reduction FREQUENCY SAR1g (W/Kg) SAR1g (W/Kg) After Reduction SAR10g (W/Kg) SAR10g (W/Kg) After Reduction 1950 MHz The change in size and type of the material covering the antenna and the composition of the material significantly reduces the SAR values. So these two

27 49 methods play a vital role in the reduction of SAR without affecting the antenna performance. 2.4 THREE LAYER HEAD MODEL AND SAR The human head model is designed with three layers such as skin, brain, bone. The outer skin has 8.25cm radius, middle bone has 7.75cm radius and the inner brain with 7.2cm radius [9]. The head model is filled with suitable materials having dielectric properties equal to human head tissues. The mass density and dielectric properties of corresponding head tissues are shown in Table Table 2.12 Mass density and dielectric properties of head tissues Tissues 3 ) 900MHz 1800MHz 2.4GHz 5GHz r r r r Skin 1020 Bone 1130 Brain Head model is subjected to RF sources like mobile phone, Bluetooth and Wi-Fi antennas and the absorbed electromagnetic power (SAR) inside the head tissues like skin, bone and brain is calculated. First specific absorption rate due to mobile antenna on human head is calculated by placing antenna at 0.5mm from head. Likewise SAR due to Bluetooth and Wi-Fi is also calculated. The three layer head model for SAR calculation is shown in Figure 2.14.

28 50 Figure 2.14 Three layer head model Mobile communication technology is booming and number of peoples using this increases tremendously. Radio frequency emitted from mobile phones affects a person who uses it. Mobile phone is an important RF source used worldwide. Use of mobile phone causes temperature rise in human head. So it is necessary to measure the how much it affects human being. To prevent harmful effects of localized tissue heating during exposure to radiofrequency radiation, national and international bodies have recommended restrictions on the specific absorption rate (SAR) permissible in various parts of the body. According to FCC the accepted limit is 1.6W/kg for SAR averaged over 1g volume of tissue. CENELEC specifies the limit as 2W/kg over 10g volume of tissue, for the head [29]. To predict the temperature increase in the human head, a head model is subjected to a mobile phone antenna. Specific Absorption Rate (SAR) is measured by subjecting head model to patch antenna and PIFA antenna which are widely used in mobile phone. SAR value is measured for skin, bone and

29 51 brain at GSM frequencies like 900MHz and 1800MHz. Temperature increase in head tissues is calculated by using Bio heat equation Patch antenna Patch antenna is designed with infinite ground plane at base and a substrate of height h=32mm is placed above the infinite ground plane. The computing time for a patch antenna with infinite ground plane using image theory is much shorter than that for a finite ground plane. Length and width of the substrate is 100mm and 90mm respectively. Substrate and infinite ground plane and patch are made up of Rogers RT/duroid Patch is designed with length of 40mm and width of in Figure Figure 2.15 Patch antenna Patch Antenna is placed at 0.5mm from human head model and antenna is operated at GSM frequencies at 900MHz and 1800MHz. Specific Absorption Rate

30 52 (SAR) is measured for head tissues like skin, bone and brain. Patch antenna with head model is shown in Figure SAR at skin is higher than bone and brain. This is due to wave attenuation (skin depth) when travelling over the distance. Figure 2.16 Head model with patch antenna At 900MHz over 1g volume of tissue skin has the SAR value of 3.25W/kg while bone and brain have the SAR value of 2.70W/kg and 2.10W/kg respectively. SAR value is measured at 900MHz over 1g volume of tissue is shown in Figure SAR is measured from outer skin. In Figure 2.17 first 5mm distance to skin, next 5mm to bone and 10 to 15mm for brain. Patch antenna at the same frequency of operation shows the SAR value of 2.82W/kg over 10g volume of tissue while bone and brain has 2.35W/kg and 1.86W/kg respectively. SAR at 900MHz over 10g volume of tissue is shown in Figure 2.18.

31 53 Figure 2.17 SAR at 900MHz over 1g volume for patch antenna Figure 2.18 SAR at 900MHz over 10g volume for patch antenna Patch antenna excited with 1800MHz and the SAR value measured over 1g volume. Skin has the SAR value of 2.62W/kg whereas bone and brain shows 2.15W/kg and 1.70W/kg respectively. SAR at 1800MHz over 1g volume of tissue is shown in Figure SAR value measured over 10g volume of tissue. Skin and bone has the SAR value of 2.20W/kg and 1.80W/kg while brain has 1.40W/kg. SAR at 1800MHz over 10g volume of tissue is shown in Figure Figure 2.19 SAR at 1800MHz over 1g volume for patch antenna Figure 2.20 SAR at 1800MHz over 10g volume for patch antenna

32 2.4.2 Planar Inverted F Antenna 54 A radiator with width W = 18mm and length L = 40mm is placed above a ground plane with dimensions 100mmx40mm is shown in Figure The feed pin is fixed to one side of the radiator and the short pin is fixed at other side of Figure 2.21 PIFA Antenna The electromagnetic energy absorbed on human head from PIFA antenna is measured. At 900MHz operating frequency the SAR value measured in skin over 1g volume of tissue is 3.50W/kg. Bone and brain has the SAR value of 2.80W/kg and 2.30W/kg. SAR value at 900MHz over 1g volume for patch antenna is shown in Figure Similarly SAR measured at 10g volume for skin, bone and brain is 3.15W/kg, 2.60W/kg and 2.10W/kg respectively. SAR at 900MHz over 10g volume is shown in Figure 2.23.

33 55 Figure 2.22 SAR at 900MHz over 1g volume for PIFA antenna Figure 2.23 SAR at 900MHz over 10g volume for PIFA antenna When the frequency of operation is 1800MHz, SAR averaged over 1g volume of tissue at skin, bone and brain is 2.90W/kg, 2.40W/kg and 1.80W/kg respectively. SAR at 1800MHz over 1g volume of tissue for PIFA antenna is shown in Figure Figure 2.24 SAR at 1800MHz over 1g volume for PIFA antenna Figure 2.25 SAR at 1800MHz over 10g volume for PIFA antenna

34 At the same frequency, SAR measured over 10g volume for skin and bone is 2.55W/kg and 2.10W/kg respectively. Brain has the SAR value of 1.60W/kg. SAR at 1800MHz over 10g volume of tissue for PIFA antenna is shown in Figure SAR results of mobile antennas are shown in Table RF Source Table 2.13 SAR results of mobile antennas Tissues SAR 1g (W/kg) 900MHz SAR 10g (W/kg) SAR 1g (W/kg) 1800MHz 56 SAR 10g (W/kg) Patch antenna Head Bone Brain PIFA antenna Head Bone Brain Specific Absorption Rate observed for mobile antennas higher when compared to international safety limit. At 900MHz mobile phone shows higher SAR value compared to 1800MHz. this is due to output power radiated at 900MHz is higher when compared to 1800MHz. when frequency of operation increases the SAR value decreases. So SAR is inversely proportional to frequency Bluetooth Antenna Bluetooth (IEEE ) is used for voice and data transfer over short distances. Its frequency of operation is 2.45GHz. It is designed for short range communications with a range of about 10m. A high performance monopole antenna fabricated using a folded planar line as radiator is used as Bluetooth antenna. The antenna is designed of compact size of 20mm x 9 mm [7]. This long

35 folded wire radiator has a total length of about 29 mm. The short metal patch radiator has a total length of about 12 mm is shown in Figure Figure 2.26 Bluetooth antenna SAR value is measured by keeping 0.1m distance between Head and Bluetooth antenna. Specific Absorption Rate over 1g volume of tissue at skin, bone and brain is 1.22W/kg, 0.99W/kg and 0.74W/kg respectively. SAR of Bluetooth antenna at 0.1mm distance for 1g volume of tissue is shown in Figure Similarly for 10g volume of tissue skin, bone and brain has 1.07W/kg, 0.84W/kg and 0.64W/kg respectively. SAR of Bluetooth antenna at 0.1mm distance for 10g volume of tissue is shown in Figure Figure 2.27 SAR over 1g volume at 0.1m for Bluetooth Figure 2.28 SAR over 10g volume at 0.1m for Bluetooth

36 58 Similarly keeping 0.5m distance between Head and Bluetooth antenna SAR measured over 1g volume of tissue at skin, bone and brain is 0.74W/kg, 0.69W/kg and 0.59W/kg respectively. SAR of Bluetooth antenna at 0.5mm distance for 1g volume of tissue is shown in Figure Similarly for 10g volume of tissue skin, bone and brain has 0.62W/kg, 0.58W/kg and 0.49W/kg respectively. SAR of Bluetooth antenna at 0.5mm distance for 10g volume of tissue is shown in Figure SAR obtained for Bluetooth antenna is lesser than mobile antenna. So mobile phone affects human highly compared to Bluetooth and other RF sources. Bluetooth devices emit low output power compared to mobile phone and hence effect due to it on human head is negligible. Figure 2.29 SAR over 1g volume at 0.5m for Bluetooth Figure 2.30 SAR over 10g volume at 0.5m for Bluetooth Wi-Fi Antenna Wi-Fi is the wireless technologies which allow devices to inter-connect and communicate with each other. Radio waves are electromagnetic waves and have different frequencies. These technologies are radio frequencies. Wi-Fi works in two frequency bands 2.4GHz and 5GHz. Wi-Fi or Wireless Fidelity, has a range of

37 59 about 100m and allows for faster data transfer rate between 10-54Mbps is the wireless standard set by The Institute of Electrical and Electronic Engineers. There are three different wireless standards under Wi-Fi, a, b and g. Wi-Fi is used to create wireless Local Area Networks (WLAN). The most widely used standard is b and g is expected to grow rapidly. These two standards are relatively inexpensive and can be found providing wireless connectivity in airports, railway stations, cafes, bars, restaurants and other public areas. The main difference between the two is the speed b has data transfer rate of upto 11Mbps and g has a rate of upto 54Mbps g is a relatively new and has yet to be adopted widely a is more expensive and as a result it is not available for public access. Equivalent Isotropically Radiated Power (EIRP) in the EU is limited to 20dBm (100mW). Wi-Fi antenna designed is WLAN dual band F shaped monopole antenna [11]. Ground plane and substrate designed with length L=80mm and width W=45mm. Substrate constructed with height H=32mm and coaxial feed is used is shown in Figure Figure 2.31 Wi-Fi Antenna SAR value is measured after keeping 0.1m distance between Wi-Fi antenna and head model. At 2.4GHz over 1g volume of tissue skin, bone and brain shows 1.16W/kg, 0.95W/kg and 0.70W/kg respectively. SAR of Wi-Fi antenna at 0.1mm distance for 1g volume of tissue is shown in Figure 2.32.

38 60 Figure 2.32 SAR at 2.4GHz over 1g volume 0.1m for Wi-Fi Figure 2.33 SAR at 2.4GHz over 10g volume at 0.1m for Wi-Fi Similarly for 10g volume of tissue skin, bone and brain shows 1.04W/kg, 0.80W/kg and 0.62W/kg respectively. SAR of Wi-Fi antenna at 0.1mm distance for 10g volume of tissue is shown in Figure At 0.5m distance SAR measured for 2.4GHz over 1g volume of tissue skin, bone and brain shows 0.68W/kg, 0.64W/kg and 0.52W/kg respectively. SAR of Wi-Fi antenna at 0.5mm distance for 1g volume of tissue is shown in Figure Similarly for 10g volume of tissue skin, bone and brain shows 0.52W/kg, 0.47W/kg and 0.40W/kg respectively. SAR of Wi-Fi antenna at 0.5mm distance for 10g volume of tissue is shown in Figure Figure 2.34 SAR at 2.4GHz over 1g volume 0.5m for Wi-Fi Figure 2.35 SAR at 2.4GHz over 10g volume at 0.5m for Wi-Fi

39 61 After keeping 0.1m distance between Wi-Fi antenna and head model the SAR value is measured. At 5GHz over 1g volume of tissue skin, bone and brain shows 0.94W/kg, 0.74W/kg and 0.54W/kg respectively. SAR of Wi-Fi antenna at 0.1mm distance for 1g volume of tissue is shown in Figure Similarly for 10g volume of tissue skin, bone and brain shows 0.80W/kg, 0.60W/kg and 0.44W/kg respectively. SAR of Wi-Fi antenna at 0.1mm distance for 10g volume of tissue is shown in Figure Figure 2.36 SAR at 5GHz over 1g volume 0.1m for Wi-Fi Figure 2.37 SAR at 5GHz over 10g volume at 0.1m for Wi-Fi At 0.5m distance SAR measured for 5GHz over 1g volume of tissue skin, bone and brain shows 0.52W/kg, 0.47W/kg and 0.38W/kg respectively. SAR of Wi-Fi antenna at 0.5mm distance for 1g volume of tissue is shown in Figure Similarly for 10g volume of tissue skin, bone and brain shows 0.42W/kg, 0.37W/kg and 0.28W/kg respectively. SAR of Wi-Fi antenna at 0.5mm distance for 10g volume of tissue is shown in Figure 2.39.

40 62 Figure 2.38 SAR at 5GHz over 1g volume 0.5m for Wi-Fi Figure 2.39 SAR at 5GHz over 10g volume at 0.5m for Wi-Fi Comparison of SAR results of Bluetooth and Wi-Fi antennas is shown in Table Specific Absorption Rate observed for Bluetooth and Wi-Fi are low and within the safety limit of ICNIRP and its effect to human is lesser. Table 2.14 SAR results of Bluetooth and Wi-Fi 0.1m 0.5m RF Source Frequency (GHz) Bluetooth 2.45 Wi-Fi Tissues SAR 1g (W/kg) SAR 10g (W/kg) SAR 1g (W/kg) SAR 10g (W/kg) Skin Bone Brain Skin Bone Brain Skin Bone Brain

41 2.4.5 SAR Reduction 63 Temperature rise due to mobile phone in human head is high compared to Bluetooth and Wi-Fi devices. To reduce temperate effects attach (RF) shields on mobile phone to reduce Specific Absorption Rate (SAR) in the spherical head model. Shields made of ferromagnetic materials are used to suppress surface currents on the mobile phone. Many kinds of simulation are performed to investigate the effect of thickness of the shield on the SAR and also on the antenna performance. The most important group of ferri magnetic materials is the ferrites. In ferrites the conductivity is low which results in much smaller induced currents in the material when electromagnetic waves are applied. Also when an electromagnetic wave hits ferrite particles, the magnetic field part of the wave is cancelled [58]. RF shield of size 4.0x4.0 cm 2 is attached on to the front top surface of mobile phone. Shield thickness is varied from 0.5mm to 3mm. SAR obtained was highest for a thickness of 3mm.Mobile phone with ferrite sheet is shown in Figure Figure 2.40 Mobile phone with Ferrite sheet

42 64 Mobile antenna is placed inside a conducting box, the conducting box acts as a part of the antenna. The current flowing onto the conducting box spreads on all box surfaces. The local SAR on the head surface is roughly proportional to the squared surface-current-density on the box. Current density on the conducting box can be suppressed by attaching RF shield on to it which in turn reduces the corresponding surface SAR. Results have shown that attachment of RF shield on mobile phone not only reduces SAR in head model but also the radiated power. The point of concern here is whether suppression of surface current affects the radiation pattern. No degradations in the radiation pattern with RF shield were observed as compared to without shield. SAR reduction is done with varying Ferrite sheet thickness from 0.1mm to 0.3mm [35]. Figure 2.41 shows reduction of SAR did for 0.1mm ferrite thickness. The obtained SAR value is higher than the international standard value. For 0.2mm ferrite thickness SAR value reduced compared to 0.1mm thickness but this value also higher compared to standard value. SAR value for 0.2mm thickness is shown in Figure 2.42.

43 Skin Brain Bone 0 Patch_900MHz Patch_1800MHz PIFA_900MHz PIFA_1800MHz Mobile antenna Figure 2.41 SAR Reduction for 0.1mm ferrite thickness Skin Brain Bone Patch_900MHz Patch_1800MHz PIFA_900MHz PIFA_1800MHz Mobile antenna Figure 2.42 SAR Reduction for 0.2mm ferrite thickness

44 66 SAR reduction for 0.3mm thickness is high and within the acceptable limit is shown in Figure So 0.3mm is the best thickness of ferrite sheet for the SAR reduction. Figure 2.43 SAR Reduction for 0.3mm ferrite thickness After attaching ferrite sheet of 0.3mm thickness to Patch antenna with frequency of operation 900MHz and the SAR value measured over 1g volume. Skin has the SAR value of 1.38W/kg whereas bone and brain shows 1.12W/kg and 0.86W/kg respectively. SAR at 900MHz over 1g volume of tissue is shown in Figure SAR value measured over 10g volume of tissue. Skin and bone has the SAR value of 1.18W/kg and 0.96/kg while brain has 0.72W/kg. SAR at 1800MHz over 10g volume of tissue is shown in Figure 2.45.

45 67 Figure 2.44 SAR 1g at 900MHz for 0.3mm Ferrite of patch antenna Figure 2.45 SAR 10g at 900MHz for 0.3mm Ferrite of patch antenna Patch antenna, at 1800MHz over 1g volume of tissue skin has the SAR value of 1.11W/kg while bone and brain have the SAR value of 0.91W/kg and 0.68W/kg respectively. SAR value is measured at 900MHz over 1g volume of tissue is shown in Figure Figure 2.46 SAR 1g at 1800MHz for 0.3mm Ferrite of patch antenna Figure 2.47 SAR 10g at 1800MHz for 0.3mm Ferrite of patch antenna At the same frequency of operation shows the SAR value of 0.94W/kg over 10g volume of tissue while bone and brain has 0.72W/kg and 0.56W/kg respectively. SAR at 1800MHz over 10g volume of tissue is shown in Figure PIFA antenna with frequency of operation 900MHz SAR 1g for skin, bone and brain is 1.48W/kg, 1.19W/kg and 0.90W/kg respectively. SAR for 1g volume of tissue at 900MHz for 0.3mm ferrite of PIFA antenna is shown in Figure Skin and bone has the SAR value of 1.32W/kg and 1.08W/kg while brain has 0.84W/kg over 10g volume. SAR 10g at 900MHz for 0.3mm ferrite of PIFA antenna is shown in Figure 2.49.

46 68 Figure 2.48 SAR 1g at 900MHz for 0.3mm Ferrite of PIFA antenna Figure 2.49 SAR 10g at 900MHz for 0.3mm Ferrite of PIFA antenna For 1800MHz frequency of operation SAR 1g for skin, bone and brain is 1.22W/kg, 0.96W/kg and 0.72W/kg respectively.sar for 1g of tissue at 1800MHz for 0.3mm ferrite of PIFA antenna is shown in Figure Figure 2.50 SAR 1g at 1800MHz for 0.3mm Ferrite of PIFA antenna Figure 2.51 SAR 10g at 1800MHz for 0.3mm Ferrite of PIFA antenna Skin, bone and brain has the SAR value of 1.08W/kg, 0.88W/kg and 0.65W/kg respectively over 10g volume. SAR for 10g of tissue at 1800MHz for 0.3mm ferrite of PIFA antenna is shown in Figure 2.51.