A Study on Antenna Design for Wearable Information Terminals

随着电子技术的飞速发展，信息终端与计算机正在实现统一，并从移动终端向可穿戴终端转变。发展可穿戴信息终端的关键之一是如何将其装备到人体中，使其天线性能降低，并降低人体对电磁能量的吸收（SAR:Specific absorption Rate）。在这项研究中，我们首先开发了一个基于市售计算机模型软件的新模块，用于通过移动或可穿戴无线终端创建人类的任何姿势。这个人的体型是以日本成年男性的平均体型为基础的。在此基础上，我们根据这些终端的典型使用情况创建了一些数值人体模型，然后使用子网格时域有限差分（FDTD）算法计算了天线性能和SAR。对于位于人体头部一侧的天线，我们发现人体躯干对天线辐射方向图和峰值SAR的影响微不足道。对于此类天线的设计，一个孤立的头部模型就足够了。另一方面，对于安装在无线终端（如笔记本电脑或PDA设备）一侧的天线，我们发现身体效应主要是阻挡向身体一侧最大20 dB的辐射，而设备上操作的手主要是阻挡向头部倾斜方向最大10 dB的辐射。高SAR出现在手上，因此由于阻隔效应，导致头部SAR显著降低。作为可穿戴终端设计的指导原则，从具有稳定的辐射性能的角度来看，将天线安装在靠近手的位置并不是一个好的选择。在大多数情况下，躯干应该是更好的选择。本研究的主要结果已发表在IEEE和IEICE等参考期刊上。

With the rapid progress of electronics, information terminals and computers are being unified and changed from mobile ones to wearable ones. One of the key points in developing a wearable information terminal is how to equip it into human body with less degradation of antenna performance and low electromagnetic energy absorption (SAR:Specific Absorption Rate) inside the human body. In this study, we first developed a new module based on a commercially available computer mannequin software for creating an any posture of a human with a mobile or wearable wireless terminal. The human's bodily shape was based on averaged Japanese adult males. With this new module we created some numerical human models with typical usage of these terminals, and then calculated the antenna performance and SAR using a sub-grid finite difference time domain (FDTD) algorithm. For an antenna at one side of the human head, we have found an insignificant effect of the human torso on both the antenna radiation pattern and the peak SAR. An isolated head model is sufficient for the design of these typed antennas. On the other hand, for an antenna mounted in one side of a wireless terminal such as a laptop PC or PDA device, we have found that the body effect is mainly blocking the radiation up to 20 dB towards the body side, and the operating hands on the device are mainly blocking the radiation up to 10 dB towards the direction at an angle to the head. High SARs appear in the hands and therefore yield a significant SAR reduction in the head due to the blocking effect. As a guideline in the design of wearable terminals, equipping an antenna near the hand is not a good choice from the point of view of having a steady radiation performance. The torso should be a better candidate in most cases. The main results of this study have been published on referred journals such as IEEE and IEICE.