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PART F

Electromagnetic

Radiation and

the Human Body

25Electromagnetic Interactions of Handheld Wireless Communication Antennas with the Human Body Magdy F. Iskander and Zhengqing Yun

Introduction • Exposure Standards for Radio Frequency Fields • Antenna Types for Handheld Wireless Devices • Numerical Computation of Specific Absorption Rate Distribution in the Human Head • Anatomically Based Head Model for Specific Absorption Rate Calculations • Experimental Measurement of Specific Absorption Rate in Phantoms of the Human Head • Specific Absorption Rate Distribution and Radiation Characteristics of Antenna Results • Effect of the Human Body on the Radiation Characteristics of Antennas • Conclusions

26Safety Aspects of Radio Frequency Effects in Humans from Communication Devices Alan W. Preece

Introduction • Radio Frequency Effects in Biological Tissues • Health Effects of Radio Frequency Radiation • Other Safety Issues • Exposure Guidelines for Radio Frequency Radiation • Recent Reviews • Conclusions

© 2002 by CRC Press LLC

Magdy F. Iskander

University of Utah

Zhengqing Yun

University of Utah

25

Electromagnetic Interactions of Handheld Wireless Communication Antennas with the Human Body

25.1Introduction

25.2Exposure Standards for Radio Frequency Fields

25.3Antenna Types for Handheld Wireless Devices

25.4Numerical Computation of Specific Absorption Rate Distribution in the Human Head

25.5Anatomically Based Head Model for Specific Absorption Rate Calculations

25.6Experimental Measurement of Specific Absorption Rate in Phantoms of the Human Head

25.7Specific Absorption Rate Distribution and

Radiation Characteristics of Antenna Results

Specific Absorption Rate Distribution in the Human Head Models • Effect of the Human Body on Specific Absorption Rate Values and Distributions

25.8Effect of the Human Body on the Radiation Characteristics of Antennas

25.9Conclusions

25.1 Introduction

With the explosive growth of the wireless communication technology, there have been concerns about the safety aspects of these devices and the potential hazardous effects associated with electromagnetic (EM) radiation interaction with human tissue. The general public is very much aware that people cook food and heat meat in microwave ovens and having handheld devices near the human head is certainly discomforting and a cause of serious concern. The mobility and convenience advantages that these wireless devices provide (connect to anyone, any time, anywhere), however, cannot be dismissed; hence, there is a need to carefully study this issue, accurately quantify these interactions, and determine their compliance with set safety standards.

© 2002 by CRC Press LLC

From the antenna engineering point of view, this area of human body interactions with antennas is intriguing and represents divergence from standard procedures for designing antennas for wireless communications. It represents a near-field interaction problem whereby the human head and parts or all of the body need to be taken into account in determining the antenna characteristics. These effects are expected not only to impact the gain and radiation pattern of antennas on the handheld devices but also to affect the input impedance characteristics that are essential in optimizing the low-power consumption of these devices. Antenna engineering research has actually progressed further, and more advanced antenna designs that minimize the EM radiation interaction with the human body have been developed. This provides a solution for the often unpredictable influence of the human body on the radiation and impedance characteristics of the antenna, and certainly alleviates the concerns about the potential hazardous effects of these devices.

In this chapter, we start by briefly reviewing the safety standard for EM radiation with some focus placed on the near-field aspects as well as the set standards for handheld devices. A review of the various types of antennas usually used in handheld devices are then presented, followed by sections that describe numerical techniques and experimental methods used to quantify and characterize the interactions of radiated fields with humans. Results showing the EM power deposition patterns in the human head are then presented, and more recent data illustrating the human body effects on these calculations are discussed. The chapter concludes with a section that provides illustrative examples showing the effect of these interactions on the radiation and input impedance characteristics of antennas in handheld devices.

25.2 Exposure Standards for Radio Frequency Fields

Determination of exposure standards and safety levels of radio frequency (RF) radiation has been a research area of significant interest for over three decades and, in particular, since 1966 with the publication of a safety standard by the United States of America Standards Association, currently known as the American National Standards Institute (ANSI). Initial efforts included quantification of plane wave interactions with simplified human models [1], but calculation techniques and measurement procedures continued to progress to accurately model realistic models of humans, to address additional areas of interest such as near-field interactions [2, 3], and to identify locations and levels of hot spots in the specific absorption rate (SAR) distribution in the human body [4–6]. Among the significant observations identified in these studies are the importance of accounting for the polarization effects [7] and the need for carefully considering the location and levels of hot spots [8]. The polarization effects and the resonance absorption in the frequency range from about 30 to 100 MHz resulted in reducing the ANSI standard from 10 mW/cm2 of incident plane wave power density to 1 mW/cm2 in the modified standard published in 1982 [1]. To help accommodate needs to address near-field exposure issues, safety standards are also expressed in terms of electric and magnetic field intensities instead of power density values [9].

With the growing concern about the potential health hazards associated with RF radiation from handheld devices, updated ANSI/Institute of Electrical and Electronics Engineers (IEEE) RF safety guidelines were published in 1992. These ANSI/IEEE C95.1-1992 safety guidelines were given in terms of the maximum possible exposure of electric and magnetic fields, or of RF power density [10]. These guidelines, however, were not easy to use whenever highly nonuniform fields, such as those in the near-field regions of handheld devices, were encountered. In these cases, the following alternative safety guidelines, which are based on the specific absorption rate (SAR) instead of the incident RF fields, were suggested.

An exposure condition can be considered to be acceptable if it can be shown that it produces SARs “below 0.08 W/kg as averaged over the whole body, and spatial peak SAR values not exceeding 1.6 W/kg as averaged over any 1 g of tissue.” As in the case of the originally published safety standards of the ANSI, there have been other standards published by U.S. and other international organizations [1]. These standards are often based on other considerations in addition to average SAR values and their distributions in a human body. For example, for cellular phones, a spatial peak SAR value of 8 W/kg as averaged over any 1 g of tissue has been issued by the Telecommunications Technology Council of Japan [11]. Yet another safety standard is set in Europe for the use of mobile telecommunication equipment by the

© 2002 by CRC Press LLC

public [12]. The European standard is set at 2 W/kg averaged over a volume equivalent to 10 g and a period of 6 min. Both the Japanese and European standards are clearly less restrictive than the ANSI/IEEE one enforced in the United States. These alternative safety guidelines and their reliance on absorbed power instead of incident fields generated a flurry of new studies focused on quantifying the absorption characteristics of RF radiation from handheld devices in human heads [13–18]. Existing cellular phones were examined and systems manufacturers initiated focused design efforts to help minimize these effects as well as to optimize the antenna performance in typical operating conditions. In the following sections, types of antennas used in wireless handheld devices are described, and the numerical and experimental procedures used to quantify their SAR characteristics in the human body are discussed.

25.3 Antenna Types for Handheld Wireless Devices

Antenna designers are often concerned with developing new antennas that meet gain, radiation pattern, and bandwidth requirements in a given frequency range; and incorporating advanced features such as beam steering capabilities in their designs. With the advent of wireless handheld devices, additional requirements emerged and became equally important factors in these designs. These include physical dimensions, appearance, cost, and minimum change in performance in a typical use in wireless communication environment. With the growing concerns about the biological effects and potential health hazards associated with these devices, new designs that minimize the RF radiation in the human head region are also being developed. Furthermore, more modern handheld wireless communication devices use multiple antennas to provide diversity advantages and overcome signal loss and multipath fading effects. In other words, there has been renewed interest in designing simple antenna elements with significant communication and health-related advantages in a typical terrestrial wireless communication environment.

Figure 25.1 shows four types of antennas commonly used in handheld transceiver units. These include the monopole antenna, side-mounted dual planar inverted-F antenna (PIFA), the top-mounted bent inverted-F antenna (BIFA), and the back-mounted PIFA. The variety of PIFA antenna mountings is known to be efficiently and conveniently integrated with handheld transceivers [14]. Alternative designs incorporate electrically short helical antennas operating in the normal mode condition, with maximum radiation perpendicular to the orientation of the antenna [19] and use of ferrite sheet attachments to reduce radiation toward the head from monopole antennas [20]. Examples of these ferrite attachment designs are shown in Fig. 25.2.

With the continued advances in improving quality of service and increasing capacity of modern wireless communication systems, there has been significant effort in incorporating signal processing algorithms to help develop diversity techniques to minimize signal losses and overcome fading resulting from

FIGURE 25.1 Antenna geometries and dimensions for handheld transceiver units: (a) monopole; (b) side-mounted dual PIFA; (c) top-mounted dual BIFA; (d) back-mounted PIFA. The chassis dimensions (105 cm3) shown in (a) apply to all four figures. (From Jensen, M.A. and Rahmat-Samii, Y., Proc. IEEE, 83(1), 7, Jan. 1995. With permission.)

© 2002 by CRC Press LLC

FIGURE 25.2 Portable telephones with ferrite sheets. (a) Type A. (b) Type B. (c) Type C. (From Wang, J. and Fujiwara, O., IEICE Trans. Commun., E80-B(12), 1810, Dec. 1977. With permission.)

FIGURE 25.3 Two dual-antenna diversity handset configurations. (a) Monopole/mouthpiece monopole. (b) PIFA/mouthpiece monopole. (From Green, B.M. and Jensen, M.A., IEEE Trans. Antennas Propag., 48(7), 1017, July 2000. With permission.)

multipath interference. Implementation of these techniques often requires the use of multiple antennas at the base station as well as in the handheld transceiver. An example of the use of multiple antennas in a mobile transceiver is shown in Fig. 25.3 [21]. This figure shows two possible configurations consisting of either two quarter-wavelength monopole antennas or a monopole and a PIFA antenna integrated on the mouthpiece of the transceiver.

Another type of antenna used in this application is the Yagi array shown in Fig. 25.4 [22]. In this case, a single two-element array consisting of a radiator and reflector or a pair of two-element arrays were used as shown in Fig. 25.5 [22]. In this case, the two arrays were intended for separate transmitting and receiving antennas or for multiband operation of the transceiver. Radiation characteristic results of these antennas as well as results from numerical and experimental evaluation of SAR values in a human head are presented in the results section (Section 25.7).

25.4Numerical Computation of Specific Absorption Rate Distribution in the Human Head

In this section, we briefly outline the various numerical techniques used in the calculation of the SAR and its distribution in a human head. This section also includes a description of the head model often used in these calculations.

© 2002 by CRC Press LLC

FIGURE 25.4 Geometric arrangement of the centrally placed antenna 1. The dimensions are shown in terms of the cell lengths δx, δy, and δz in the x-, y-, and z directions, respectively. For the FDTD calculations, δx = δ = 1.974 mm, δz = 1.5 mm. (From Gandhi, O.P. and Chen. J.-Y., IEEE Trans. Electromagnetic Compatibility, 7(4), 547, Nov. 1995. With permission.)

To begin with, it should be acknowledged that advances in computational EM made it possible not only to accurately and efficiently calculate SAR in realistic models of the human head, but also, through these advances, to simulate anatomically based models of the human head with resolution as small as 1 to 2 mm. In many cases, the dimensions, materials properties, and orientation of the handheld device were also taken into account in these simulations. In general, computation techniques used in these calculations include the finite difference time domain (FDTD) method [13-17, 23], transmission line matrix (TLM) method [24], modified method of moments (MOM) [25], and a hybrid MOM/FDTD technique [26]. FDTD has been by far the method of choice in many of these simulations. In a more recent effort, however, and in an attempt to examine the effect of the human body on the SAR values in the human head, it was necessary to use the multigrid FDTD code recently developed by our group at the University of Utah [27, 28].

The calculation procedure based on the FDTD method has amply been described in literature and hence needs no further discussion. In applying the TLM procedure, however, it was necessary to modify the method to account for the frequency-dependent dielectric properties of tissue [24]. Specifically, a technique that handles second-order Debye dielectric dispersion was developed, and the SAR distribution in a relatively rough model of the human head (7 × 7 × 7 mm3) was calculated. The obtained results are encouraging but additional refinements to increase the resolution are desirable. Other simulation efforts to better model the antenna structure on handsets [25] or to incorporate improved antenna modeling using the MOM while still calculating SAR in the head model using FDTD [26] were also reported.

As mentioned earlier, a multigrid FDTD code [28] was used to calculate SAR distribution in a human head, taking into account the effect of the human body [29]. In this case, two FDTD models were

© 2002 by CRC Press LLC

FIGURE 25.5 Geometric arrangement for antenna 2. Note that individual Yagi antennas T and R are used for transmitting and receiving antennas, respectively. (From Gandhi, O.P. and Chen. J.-Y., IEEE Trans. Electromagnetic Compatibility, 7(4), 547, Nov. 1995. With permission.)

FIGURE 25.6 The FDTD regions in the multigrid FDTD simulations. (From Iskander, M.F., Yun, Z., and QuinteroIllera, R., IEEE Trans. Microwave Theory Tech., vol. 48, no. 11. pp, 1979–1987, 2000. With permission.)

utilized — a fine grid for the head with a 2.45-mm cell size and a coarse grid for the human body with an 8.9-mm cell size (four times larger than the fine grid cell). Furthermore, an anatomically accurate model for the head was used, while the dielectric constant of the body was assumed homogeneous and represented by a complex permittivity value equal to 2/3 that of muscle tissue. Figure 25.6 illustrates the fine and coarse grid regions in the multigrid FDTD calculations. Further discussion of the various head models used in these and other calculations are described in the following section.

© 2002 by CRC Press LLC

FIGURE 25.7 Human head model. A fine grid computation domain was used for modeling the head. (From Iskander, M.F., Yun, Z., and Quintero-Illera, R., IEEE Trans. Microwave Theory Tech., vol. 48, no. 11, pp. 1979–1987, 2000. With permission.)

25.5Anatomically Based Head Model for Specific Absorption Rate Calculations

A variety of models was used in the SAR calculations of a human head. Simple homogeneous spherical models were used when focus was placed on evaluating and optimizing the performance of more complex antenna designs [25, 26]. A layered eccentric sphere structure was also used to help provide a concise analytic formulation and an exact solution [30]. Other simplified canonical models were also developed as part of the European COST 244 WG3 project [31]. Perhaps the most commonly used model in these calculations is the anatomically based model of the human head. These models were based on magnetic resonance imaging (MRI) [32]. The basic parts constituting the human head are shown in Fig. 25.7. The dielectric and conductivity of the tissues in the human head model are given in Tables 25.1 and 25.2 for 900 and 1900 MHz, respectively. A more detailed list of the dielectric properties and specific gravities of various tissues was also compiled in References [13, 15]. The data presented in Reference [13] is also given in Table 25.3 for completeness. Other dielectric properties for the head models were obtained from an available dielectric database [33]. In any event, images from computerized tomography (CT) scans and MRI images made it possible to model the human head to within mm-size resolution. The use of CT scan images in EM simulations, however, goes back to the late 1970s [34], but the accuracy and the resolution of the more recent images [32] have certainly played a critical role in improving SAR predictions.

TABLE 25.1 Properties of the Tissues in the FDTD Model: 900 MHz

 

Bone

Brain

Muscle

Eyeball

Fat

Skin

Lens

 

 

 

 

 

 

 

 

Dielectric constant

9.67

52.7

59.1

80.0

4.67

59.1

59.1

Conductivity [S/m]

0.0508

1.05

1.26

1.90

0.0583

1.26

1.26

 

 

 

 

 

 

 

 

From Iskander, M.F., Yun, Z., and Quintero-Illera, R., IEEE Trans. Microwave Theory

Tech., vol. 48, no. 11, pp. 1979–1987, 2000. With permission.

TABLE 25.2 Properties of the Tissues in the FDTD Model: 1900 MHz

 

Bone

Brain

Muscle

Eyeball

Fat

Skin

Lens

 

 

 

 

 

 

 

 

Dielectric constant

7.75

46.0

55.3

80.0

9.70

59.1

55.3

Conductivity [S/m]

0.105

1.65

2.0

1.90

0.270

1.26

2.0

 

 

 

 

 

 

 

 

From Iskander, M.F., Yun, Z., and Quintero-Illera, R., IEEE Trans. Microwave Theory

Tech., vol. 48, no. 11, pp. 1979–1987, 2000. With permission.

© 2002 by CRC Press LLC

TABLE 25.3 Dielectric Properties and Specific Gravities of the Various Tissues

Assumed at the Midband Mobile Telephone Frequencies of 835 and 1900 MHz

 

 

835 MHz

 

1900 MHz

 

 

 

 

 

 

 

 

Spec. Gravity

εr

σ

 

εr

σ

Tissue

103 kg/m3

S/m

 

S/m

Muscle

1.04

51.76

1.11

49.41

1.64

Fat

0.92

9.99

0.17

9.38

0.26

Bone (skull)

1.81

17.40

0.25

16.40

0.45

Cartilage

1.10

40.69

0.82

38.10

1.28

Skin

1.01

35.40

0.63

37.21

1.25

Nerve

1.04

33.40

0.60

32.05

0.90

Blood

1.06

55.50

1.86

54.20

2.27

Parotid gland

1.05

45.25

0.92

43.22

1.29

CSF

1.01

78.10

1.97

77.30

2.55

Eye humor

1.01

67.90

1.68

67.15

2.14

Sclera

1.17

54.90

1.17

52.56

1.73

Lens

1.10

36.59

0.51

42.02

1.15

Pineal gland

1.05

45.26

0.92

43.22

1.29

Pituitary gland

1.07

45.26

0.92

43.22

1.29

Brain

1.04

45.26

0.92

43.22

1.29

 

 

 

 

 

 

 

From Gandhi, O.P., Lazzi, G., and Furse, C.M., IEEE Trans. Microwave Theory Tech., 44(10), 1884, Oct. 1996. With permission.

25.6Experimental Measurement of Specific Absorption Rate in Phantoms of the Human Head

Average SAR values and SAR distribution measurements in phantoms have long existed and provided valuable information since the early 1970s [1, 6, 8]. Phantom materials for both low and high dielectric property tissues were developed and several techniques for measuring average SAR values and SAR distribution were described [1]. Procedures for preparing saline solutions with specific conductivity values are available, and techniques for measuring SAR values in solid phantoms (saline solutions with jelling agent) have long been described [8]. As may be expected, much of the effort in evaluating the potential health effects from handheld transceivers was focused on the determination of the SAR distribution in the head. To this end, more accurate phantom preparation is required to provide accurate representation of the various tissue types in this rather complex and highly sensitive part of the human body. Gandhi and Chen [22] used ingredients in available recipes for preparing phantom materials and, through systematic variations in the percentage of contribution from each ingredient and measurement of the resulting dielectric properties, developed formulas for the preparation of a wide variety of tissue properties that can be used in head models. Table 25.4 lists compositions used to simulate the dielectric properties of tissues used in an experimental model of the head [22]. The empirical formulas used to determine the percentage of water and salt (NaCl) contents in a typical phantom mixture were calculated based on the following factors [22]:

(Kσ )H2O

(Kεr )H2O

=∆σ( )

H2O %

=∆εr( )

H2O %

0.05

(25.1)

0.80

(25.2)

© 2002 by CRC Press LLC

TABLE 25.4 Compositions Used to Simulate the Dielectric Properties (εr, σ) of the Soft Tissues for the Experimental Model of the Head

 

Desired

 

 

 

 

 

 

Measured

 

 

 

 

Test Composition (Percentage)

 

 

 

 

εr

σ

 

εs

σ

Tissue

S/m

 

H2O

S.S.

P.E.P

NaCl

 

S/m

 

 

 

 

 

 

 

 

 

Brain

30.0

5.3

70.0

7.0

22.17

0.83

30.6

5.3

Muscle

40.0

4.9

80.0

13.0

7.0

0

35.5

5.1

Eyes

30.0

3.8a

92.0

3.2

3.0

1.8

46.0

6.8

Vitreous humor

63.0

7.2

 

 

 

 

 

 

 

 

Ear

23.0

2.6

70.0

7.0

23.0

0

25.0

3.5

 

 

 

 

 

 

 

 

 

 

 

a Average of the properties desired for the eyes and the vitreous humor are used to develop the tissue-simulant material for this organ.

From Gandhi, O.P. and Chen, J.-Y., IEEE Trans. Electromagnetic Compatibility,

37(4), 547, Nov. 1995. With permission.

TABLE 25.5 Compositions Used to Simulate Skull or Bone at the Experimental

Frequency of 6 GHz

 

 

 

 

 

 

Measured

 

 

Test Composition (Percentage)

 

 

 

 

 

Slab length

εr

σ

Mixture

Epoxy

Hardener

KCl or NaCl Solution*

(mm)

S/m

1

35.7

35.7

28.6

(KCl)

15.8

6.27

0.34

 

 

 

 

 

22.0

5.78

0.32

2

35.7

35.7

28.6

(NaCl)

16.2

5.76

0.31

Note: Desired εr = 6.0, σ = 0.3 S/m; the short-circuited waveguide method using different slab lengths of the material was used for the measurements.

a The composition used for KCl or NaCl solution was 130 g of salt mixed with 950 g of water.

From Gandhi, O.P. and Chen, J.-Y., IEEE Trans. Electromagnetic Compatibility, 37(4), 547, Nov. 1995. With permission.

(Kσ )

=

∆σ

= 1.00

(25.3)

NaCl(%)

 

NaCl

 

 

 

 

 

 

These formulas were obtained from the slopes of measured εr and σ vs. H2O% and NaCl% curves. As for the skull or bone tissue types, a composition of 36.4% epoxy, 36.4% hardener, and the rest a solution of potassium chloride (KCl) or sodium chloride in which 88% by weight is water (130 g of salt mixed with 950 g of water) is used [22]. Table 25.5 provides a list of the composition and the measured dielectric properties of the skull (bone) type of phantom material [22]. In the work by Gandhi and Chen [22], the NaCl-based composition was used to build a hollow human skull model of the head. This skull mold was then filled with various types of soft tissues to accurately represent a human head model.

As for the SAR distribution measurement techniques, one of the following three approaches is implemented [1]:

1.Use of nonperturbing temperature probes to measure SAR values at specified locations of interest — Nonperturbing temperature probes may include fiber optics probes [35] or the F probe available commercially [36].

2.Use of nonperturbing miniature electric-field probes [37] — Some of these implantable E-field probes are available commercially (Narda model 26089/BRH-15) and require calibration to determine conversion (scale) factor relating the actual measured E-field values to the actual ones inside

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the phantom model. The actual field values are usually obtained based on analytic expressions in phantom models of simple geometries (i.e., spheres or slabs of dielectrics).

3.Use of split phantom and thermographic camera measurements — This technique is based on developing two symmetrical parts of the head phantom model and exposing the surface at the symmetry plane to a thermographic camera for temperature distribution measurements [8]. It should also be noted that new nondegradable dry phantom materials have been developed for this application [38–40].

As in any experimental procedure, calibration procedures need to be discussed and limitations need to be pointed out. As mentioned earlier, the implantable E-field probe needs to be calibrated so as to relate the measured output voltage (electric field) from these probes to the actual electric fields at the measured locations. Although SAR values may be directly calculated from the measured and normalized E-field values using the formula SAR = σ E 2/ρ where σ is the electrical conductivity of the phantom (tissue) and ρ is the density, precautions must be exercised to minimize errors resulting from probe–phan- tom interfaces, potential airgaps at measurement locations, and interference caused by the resistive leads in the measuring probes. In the split phantom measurements, on the other hand, caution should be used concerning thermal diffusion and temperature distribution errors as a function of time after irradiating the phantom and in the process of opening the two halves to expose the symmetry surface to the thermographic camera. Calibration curves vs. time for these experiments need to be established; and extrapolation, if possible to the t = 0 time (right after terminating the phantom exposure to RF radiation), needs to be used for accurate implementation of the formula SAR = cT/t where c is the specific heat, T is the temperature rise, and t is the exposure duration time [38].

25.7Specific Absorption Rate Distribution and Radiation Characteristics of Antenna Results

In this section, we summarize the SAR distribution results either calculated numerically or measured experimentally, and also discuss the radiation characteristics of some of the used antennas when operated near the human head. Some of the more recent results illustrating the effect of the human body on the SAR distribution in the human head are also discussed.

25.7.1 Specific Absorption Rate Distribution in the Human Head Models

Several types of SAR distribution in the human head were reported in the literature. Some were intended to illustrate the effect of rotating the operator’s head while using the handheld transceiver [13, 41]; another was to illustrate differences in absorption when different types of antennas are used [14, 18, 19], whereas others were intended to examine a variety of effects such as placement of the hand [13, 18], separation distance between the handset and the head [18], and the effect of the operating frequency [13, 30, 42].

Figure 25.8 shows three configurations of the head model used to examine the effect of tilting the operator’s head when using the cellular phone [41]. The obtained peak 1-g SAR for the head, for the brain, and the average SARs in some of the selected tissues are given in Tables 25.6 and 25.7. As may be noted, results were given at both 835 MHz and 1.9 GHz. It may also be worth noting that the maximum peak 1-g SAR for both the head and the brain are maximum for the case of vertical head and minimum for the case of a 30° tilt with an additional 9° rotation in the horizontal plane [41]. A similar observation may be made even for the average SAR values in different tissue types.

Another important consideration in evaluating potential hazards and health effects is the operating frequency. Tables 25.8 and 25.9 compare results for transceivers operating at 835 and 1900 MHz, respectively [13]. As it may be noted, higher frequencies (1900 MHz) cause less than 50% of the peak 1-g SAR values for the head and brain at the lower frequency. At higher frequencies, much of the power absorption occurs at the surface and, hence, the relatively higher values in the hand at these higher frequencies. A

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FIGURE 25.8 Visualization of the various segmented head models with the telephone: Vertical, tilted 30° with a further rotation of 9° of the face toward the telephone. (From Lazzi, G. and Gandhi, O.P., IEEE Trans. Electromagnetic Compatibility, 39(1), 55, 1997. With permission.)

TABLE 25.6 Peak 1-g SARs for the Head and the Brain in W/kg at 835 and 1900 MHz for the Three Head Models Considered

Frequency

 

Vertical

Tilted 30º

Tiled 30º Head Model,

(MHz)

 

Head Model

Head Model

with Further Rotation of 9º

 

 

 

 

 

835

Peak 1-g SAR for head

2.93 (1.01 g)

2.44 (1.03 g)

2.31 (1.10 g)

 

Peak 1-g SAR for brain

1.13 (1.09 g)

0.93 (1.02 g)

0.66 (1.00 g)

1900

Peak 1-g SAR for head

1.11 (1.03 g)

1.08 (1.03 g)

1.20 (1.01 g)

 

Peak 1-g SAR for brain

0.19 (1.00 g)

0.20 (1.04 g)

0.16 (1.02 g)

 

 

 

 

 

Note: Given in parentheses are the actual weights of the subvolumes considered for the peak 1-g SARs. The telephone is assumed to be a plastic-covered metal box with a λ/4 monopole antenna mounted above it. The radiated power is assumed to be 600 mW at 835 MHz and 125 mW at 1900 MHz.

From Lazzi, G. and Gandhi, O.P., IEEE Trans. Electromagnetic Compatibility, 39(1), 55, 1997. With permission.

TABLE 25.7 Average SARs in Selected Tissues at 835 and 1900 MHz for the Three Head Models Considered

Frequency

Average SARs

Vertical

Tilted 30º

Tilted 30º Head Model,

(MHz)

(mW/kg)

Head Model

Head Model

with Further Rotation of 9º

 

 

 

 

 

835

Brain

72.3

60.6

49.1

 

CSF

72.7

66.4

53.7

 

Eye humor

31.8

20.6

20.7

1900

Brain

7.6

7.0

7.2

 

CSF

7.9

6.7

7.9

 

Eye humor

3.2

1.2

1.7

 

 

 

 

 

From Lazzi, G. and Gandhi, O.P., IEEE Trans. Electromagnetic Compatibility, 39(1), 55, 1997. With permission.

more systematic study of the effect of frequency was reported in Reference [30] for the more simplified six-layer model consisting of eccentric spheres.

There is no question that the type and location of antennas in the handset also play an important role in the amount and distribution of SAR values in the human head. Jensen and Rahmat-Samii presented results illustrating differences in SAR distributions for different antennas [14]. Figure 25.9 shows the geometric arrangement of the handset with respect to the head, and Fig. 25.10 shows results of the computed SAR distribution values [14]. As it may be seen, different antennas, even when placed at the same location from the head, would produce different SAR distributions.

© 2002 by CRC Press LLC

TABLE 25.8 Comparison of the Powers Absorbed and Peak SARs for the λ/4 and 3λ/8 Antennas at 835 MHz

Antenna

 

Peak 1-g SAR

Peak 1-g SAR

% Power Absorbed

% Power Absorbed

Length

Tilt

for Head

for Brain

by “Hand”

by Head and Neck

 

 

 

 

 

 

λ/4

2.93 (1.00 g)

1.13 (1.09 g)

9.2

45.0

λ/4

30º

2.42 (1.03 g)

0.93 (1.02 g)

12.4

39.8

3λ/8

1.60 (1.00 g)

0.65 (1.05 g)

5.6

33.7

 

 

 

 

 

 

Note: Time-averaged radiated power = 600 mW.

From Gandhi, O.P., Lazzi, G., and Furse, C.M., IEEE Trans. Microwave Theory Tech., 44(10), 1884, October 1996. With permission.

TABLE 25.9 Comparison of the Powers Absorbed and Peak SARs for the λ/4 and 3λ/8 Antennas at 1900 MHz

Antenna

 

Peak 1-g SAR

Peak 1-g SAR

% Power Absorbed

% Power Absorbed

Length

Tilt

for Head

for Brain

by “Hand”

by Head and Neck

 

 

 

 

 

 

λ/4

1.11 (1.03 g)

0.20 (1.00 g)

13.8

35.6

λ/4

30º

1.08 (1.03 g)

0.20 (1.04 g)

13.9

35.5

3λ/8

0.69 (1.06 g)

0.16 (1.00 g)

7.0

29.4

 

 

 

 

 

 

Note: Time-averaged radiated power = 125 mW.

FIGURE 25.9 Side and rear views of the FDTD head/hand/handset mode showing dimensions. (From Jensen, M.A. and Rahmat-Samii, Y., Proc. IEEE, 83(1), 7, Jan. 1995. With permission.)

25.7.2Effect of the Human Body on Specific Absorption Rate Values and Distributions

As may be noted from the bibliography, several research groups have examined SAR calculations in the human head when exposed to EM radiation from handheld devices. In all cases, however, the isolated human head was modeled and included in the simulation. It is of interest to examine the effect of the human body on these SAR values and their distribution in the human head. The multigrid FDTD code was used to address this issue [28]. In this approach, a fine grid region was placed around the head and the transceiver while a coarse grid was used to model the rest of the human body. Specifically, a fine grid with a 2.45-mm spatial resolution was used for the human head, resulting in a grid with 100 × 100 × 100 fine grid cells in this region. For the body model, on the other hand, a coarse 8.9-mm grid was used, and the dielectric constant of the body tissue was assumed to be homogeneous and ε*r the value for muscle tissue. Figure 25.11 shows the obtained SAR distribution results at 900 MHz with and without taking the human body into account [29]. From these results, it may be seen that for distances between the antenna and human head, d 4 cm, some differences that may be significant may be observed in

© 2002 by CRC Press LLC

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