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NatureInterface > No.05 > P046 [Japanese]

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Characteristics of Electromagnetic Wave Propagation on the Body Surface

KOHJI KOSHIJI (Professor, Tokyo University of Science)

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Electronic packaging technology has improved rapidly and wearable systems such as eyeglass displays and wristwatch computers will surely be developed in the foreseeable future. In order to improve the suitability of such devices and the degree of freedom they allow, wireless communication among devices is necessary. To realize wireless communication, we need to clarify the characteristics of electromagnetic wave propagation both between the inside and the outside of the human body and along the body surface. In this study, I researched the characteristics of electromagnetic wave propagation on the human body surface (using the arm as a model) by analyzing it with the Transmission-Line Modeling (TLM) Method.

The propagation features of electromagnetic waves can be divided into three categories: the TEM wave, which has no electromagnetic component in the longitudinal direction; the TE wave, which has no electric component in the longitudinal direction; and the TM wave, which has no magnetic component in the longitudinal direction. A line with two leads propagates TEM waves, while a line with only one conductor such as a rectangular waveguide or a circular waveguide propagates TE waves or TM waves according to its boundary condition. The propagation speed of energy of TE and TM waves is slower than that of TEM waves. As the frequency of TE or TM waves becomes higher, it approaches the propagation speed of light. In contrast, when the frequency of TE or TM waves gets lower, a frequency at which no signal can be transferred occurs. This is known as the cutoff frequency. The exact cutoff frequency depends on various parameters such as waveguide shape, dielectric constant, permeability, and mode number.

For this study, I designed a column as a model of a human arm and examined the state of electromagnetic wave propagation by the electromagnetic analysis using the TLM method. The analysis model is shown in Fig. 1, in which the red circle indicates the coil for excitation. The analysis of electric field strength in the direction along the arm at various frequencies of electromagnetic wave is shown in Fig. 2. The electromagnetic strength distributions at 274.2 MHz and 1.381 GHz are shown in Fig. 3. These results suggest that electromagnetic waves over a certain frequency can be propagated through the human body and that under the cutoff frequency the waves attenuate drastically. The human body can be considered to be a dielectric constant with a different boundary condition from a conductor, so the mixed TE and TM waves are thought to be propagated through the human arm.

As shown above, the human arm model has its own cutoff frequency and an electromagnetic wave whose frequency is above the cutoff frequency can be propagated along the body surface. However, there are still many questions and parameters to consider, such as modeling of the curvature of the arm, other methods of excitation, safety level, and the influence of clothes and accessories. We will examine these matters in future research.

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