FIGURE 25.10 Computed normalized SAR distribution in a plane through the center of the head with d = 7.21 cm, b = 1.97 cm, and the plastic casing present at 915 MHz. The configurations are the side-mounted PIFA, the sidemounted PIFA with the spherical head, the monopole, and the back-mounted PIFA. SAR values were calculated based on an average over 1g of tissue and normalized to the power delivered to the antenna using 10 log (SAR/Pdel). (From Jensen, M.A. and Rahmat-Samii, Y., Proc. IEEE, 83(1), 7, Jan. 1995. With permission.)

SAR values and their distributions in the head. Results at 1900 MHz for the vertical monopole case are shown in Fig. 25.12. Tables 25.10 to 25.12 summarize some of the obtained results for the case of vertically polarized monopole antenna. From these results, it should be noted that taking the effect of the human body into account is certainly justifiable and, depending on the operating frequency and the distance between the head and the antenna, differences by as much as 53% may be observed [29]. Similar results for the PIFA antenna are shown in Fig. 25.13, and from Table 25.13 it may be noted that similar differences were observed and confirm the need for taking the effect of the human body into account when assessing the health effects associated with the use of handheld devices.

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

Thus far in our discussion, we focused on the safety aspects of handheld transceivers; and, to this end, issues such as safety standards and SAR distributions in the head were described. Another important aspect in studying these issues is to learn more about the radiation characteristics of the various antennas when operating near the human body. This information can be used to optimize the antenna performance but, equally important, it can be used to explore the development of new and innovative designs that can minimize sensitivity to operating conditions (head tilt and rotation, distance from the head, etc.) and also possibly minimize radiation toward the head. In this section, we summarize some of the reported results on the radiation characteristics of antennas in handheld devices.

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FIGURE 25.11 Vertical cross sections of the human body showing SARs distributions for vertical monopole antenna at 900 MHz with and without body. (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.)

Okoniewski and Stuchly [15] examined the radiation characteristics of monopole antennas and their variations as a function of distance from the human head. Figure 25.14 shows the studied geometry and some of the obtained results. As it may be seen from the θ-plane radiation patterns, there may be reduction in gain and change in the level of sidelobes as the antenna gets closer to the head. The effect of the hand placement on the receiver box was also examined and the obtained results are shown in Fig. 25.15. The effect of the hand on the input impedance of the antenna was also significant as described in Reference [14]. Figure 25.16 shows S11 of a side-mounted PIFA antenna for cases with and without hand and as a function of the distance d between the hand and the antenna [14]. From these results, it may be seen that not only the presence of the hand but also its relative location on the handset causes changes in the reflection coefficient and, more importantly, may result in a shift in the resonance frequency and hence detune the antenna. The results shown in Fig. 25.17 show a significant difference in the effect of the hand between the side-mounted and the top-mounted PIFA antennas [14]. Clearly, the effect of the hand is less severe for the case of the top-mounted PIFA antenna. As a matter of fact, placing the hand at a distance of 2.62 cm for the case of the side-mounted antenna almost detuned the antenna, whereas similar effect for the top-mounted PIFA antenna would result at distance d = 0.66 cm from the head [14]. Other studies on the radiation characteristics of antennas in the presence of a human head may also be found in the literature [43, 44].

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FIGURE 25.12 Vertical cross sections of the human head illustrating SARs distributions for vertical monopole antenna at 1900 MHz with and without body. (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 25.10 Total SARs in the Human Head: Vertical Polarization

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.

25.9 Conclusions

In this chapter, we attempt to review ongoing research in the area of EM interactions of handheld antennas with the human body. This area of research was very active in the 1970s but received less attention after the publication of safety standards, including ANSI [9], in the 1980s. In most cases, focus was placed on interaction of plane waves with the human body [1]. Various computational techniques and experimental

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TABLE 25.11 Total SARs in the Human Brain Area: Vertical

Polarization Case

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 25.12 Maximum SARs in the Human Brain: Vertical Polarization

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.

FIGURE 25.13 Vertical cross sections of the human head showing SARs distributions for vertically and horizontally oriented PIFA antenna at 900 MHz with and without the effect of the human body. (From Iskander, M.F., Yun, Z., and Quintero-Illera, R., IEEE Trans. Microwave Theory Tech., 2000. With permission.)

methods were developed to quantify the average SAR and its distribution (hot spots) in the human body. As mentioned earlier, these activities slowed down after the publication of safety standards.

The continued growth and widespread use of personal communications systems renewed interest in this area of research with focus on interaction of EM radiation from handheld devices with the human

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TABLE 25.13 SARs in the Human Head: f = 900 MHz; PIFA Antenna

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.

FIGURE 25.14 Radiation pattern for a monopole on a dielectric covered metal box at 915 MHz as a function of distance from the head model (no hand). (a) φ plane with θ = 90°, (b) θ plane with φ = 180° (away from the head), and (c) θ plane with φ = 0° (toward the head). (From Okoniewski, M. and Stuchly, M.A., IEEE Trans. Microwave Theory Tech., 44(10), 1855, Oct. 1996. With permission.)

head. Remarkably accurate models of the human head with resolution on the order of 1 mm were developed using MRI images and this, together with recent advances in computational EMs, helped in achieving accurate characterization and quantification of these interactions. SAR distributions in the head were calculated and compliance with safety standards for a wide variety of antennas was examined. Detailed effects such as distance between the antenna and the human head, placement of the hand on the transceiver, and position and rotation of the head while using these devices were examined and accurately quantified. In addition to these studies related to health effects and safety standards, antenna engineers took up the challenge and used these available computational and experimental methods to examine the impact of such interactions on the radiation and input impedance characteristics of antennas. Changes in radiation patterns, sidelobe levels, and gains were quantified; and the effect of the head and hand placement on the transceivers on the input impedance characteristics were studied. Even the diversity performance of multiple antennas in handheld devices was characterized, and this important

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FIGURE 25.15 Radiation from a monopole on a dielectric covered metal box of 915 MHz at 2 cm away from the head model, without the hand, with the hand, and with the hand and a choke. (From Okoniewski, M. and Stuchly, M.A., IEEE Trans. Microwave Theory Tech., 44(10), 1855, Oct. 1996. With permission.)

FIGURE 25.16 Computed value of S11 for the side-mounted PIFA on the handset without the hand and with the hand for three different values of d. Measured values appear for the configurations with no hand and with the hand at d = 6.56 cm. (From Jensen, M.A. and Rahmat-Samii, Y., Proc. IEEE, 83(1), 7, Jan. 1995. With permission.)

FIGURE 25.17 Computed value of S11 for the top-mounted PIFA on the handset without the plastic casing, with the casing, and with the casing and hand at four different locations. (From Jensen, M.A. and Rahmat-Samii, Y., Proc. IEEE, 83(1), 7, Jan. 1995. With permission.)

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