【作者】 易敏；

【导师】 李书芳；

【作者基本信息】 北京邮电大学 ， 电磁场与微波技术， 2009， 博士

【摘要】 目前,诸如手机和无绳电话等无线电手持设备已经成为人们生活中必不可少的一部分,由此引起的电磁暴露问题也日益受到公众的广泛关注。无线电手持设备作为低功率的发射机,其天线紧贴人体头部,产生的感应场和辐射场可能会过量照射人体头部而对人体造成伤害。如何评估和避免其电磁辐射对人体的伤害,一直是学术界研究的热点,也是本文的主要研究内容。通过本文的研究工作,力争为开展生物电磁学实验和制订合理的非电离辐射卫生防护标准提供参考,为绿色环保型无线电手持设备的开发提供有益的指导。本文在综合国内外相关领域研究成果的基础上,应用电磁场与天线相关理论以及计算电磁学的数值方法,从射频电磁暴露评估关键技术、无线电手持设备天线设计与优化、儿童与成人头部吸收无线手持设备辐射电磁能量差异等方面进行了深入研究。研究内容包括无线电手持设备与人体相互作用的数值建模和仿真,比吸收率测量系统关键技术的研究、以及RFID读写器天线的仿真和优化等方面的工作。研究工作的主要创新点如下:(1)采用蒙特卡罗方法得到电场探头校准的高置信水平(置信概率)对应的置信区间。在整个测量范围内,肖特基检波二极管的压缩特性导致电场探头测量结果的非线性。因此,在电场探头用于电磁剂量学测量前,必须对电场探头进行校准并分析其不确定度。蒙特卡罗方法使用概率密度函数来分析电场探头校准的不确定度,避免了采用“测量不确定度表示指南”中的规则评估不确定度引入的近似,从而可以精确求解电场探头校准的高置信水平对应的置信区间。(2)通过数值仿真和实验测试研究工作在UHF频段的双频平面倒F天线的电磁辐射在SAM头部模型中产生的比吸收率(SAR)分布情况。由于双频平面倒F天线(PIFA)天线能够同时工作在900MHz和1800 MHz的移动通信频段,因此在手机中得到了广泛的应用。论文利用时域有限差分法仿真分析了人体头部在PIFA天线近场的能量吸收机制,得到人体头部内的SAR分布。测试结果与仿真结果进行了比对分析。(3)为了克服平面倒F天线的两个谐振点频率相差大、带宽较窄的缺点,论文提出了一种新型射频识别读写器天线。该天线的两个工作频段的中心频点分别位于我国分配给射频识别技术(RFID)使用的两个超高频频段内。该天线很好地缓解了天线带宽增加和天线效率下降的矛盾,在我国为RFID分配的频段内具有较高的天线效率。在ISO/IEC推荐RFID使用频段内的反射损失小于-10dB,天线的相对带宽达到17%以上,能够工作在ISO/IEC推荐使用的频段内,达到实际应用的要求。论文采用最大暴露允许值(MPE)评估该手持RFID读写器天线电磁辐射特性。(4)利用数值仿真技术分析儿童头部与成人头部吸收无线电手持设备辐射电磁能量的差异。通过数值仿真分析了以往研究存在差异的主要原因,重点研究了天线和人体头部模型间的有效距离、手机的不同放置位置及仿真结果归一化处理对研究结果的影响。为进一步研究手机的绿色设计进行了有益地探索。更多还原

【Abstract】 With rapidly increasing demand for wireless communication, wireless handsets such as cellular and cordless telephones have been widely used. Electromagnetic exposure of a wireless handset near human head is a major public concern. Wireless handset as a low power transmitter can make human head super-irradiated due to the induction and radiation field of the antennas of the handset. The microwave energy which is absorbed by human body may induce thermal and non-thermal effects that may lead to damages to human body. Specific Absorption Rate (SAR), which is based on the electric field within the exposed body, is a key evaluation index for such exposure accepted by most of the international standard organizations.The dissertation, articulates my researches on evaluation for electromagnetic exposure of wireless handsets, the effects of head size on the electromagnetic energy absorption, the design of antennas for RFID handsets and its key technology, are based on electromagnetism, antenna theory, computational electromagnetism and Exposure Assessment of RF Technologies. These researches results not only provide effective inquiry data for RF bioeffects study, but also make a guiding role for the green design of wireless handsets.The main efforts and contribution of this dissertation are as following:(1) The confidence interval of E-field probes calibration for a high level of confidence is derived from Monte Carlo method. The compression characteristic of Schottky detector diode causes E-field probes highly non-linear over signal strength. Therefore it is important to accurately calibrate E-field probes in simulation liquid at mobile communications frequencies before probes are used in dosimetric measurements. The Monte Carlo method uses the probability density function rather than the mean, standard deviation and degree of freedom. So it avoids the approximation in the evaluation of uncertainty of probe calibration in Guide to the Expression of Uncertainty in Measurement.(2) The SAR distribution in a SAM head model which induced by a planar inverted-F antenna are studied by simulation and measurement. The PIFA antenna is widely applied to mobile service in which the 900MHz frequency band and 1800MHz frequency band are the current carrier frequency bands in China. The impacts to human health are investigated using the finite-difference time-domain technique. Assessments of specific absorption rate in the head were conducted through simulating the energy absorption mechanism of head in the near field. The experimental results are in good agreement with the simulative results.(3) A novel antenna and feed networks are proposed for a dual frequency of 840MHz～845MHz and 920MHz～925MHz application which is allocated for radio-frequency identification (RFID) use in China. Due to enhanced bandwidth, the antenna can also be applied to ultra high frequency of 860MHz-960MHz that is proposed for RFID use by ISO/IEC. The feed networks which are used to attain circular polarization characteristics of the antenna were introduced. Simulation and measurement were performed and the frequency band with a return loss less than -10dB was achieved. The evaluation for electromagnetic Exposure of the antenna is discussed in details.(4) Electromagnetic exposure of a mobile phone near children’s head which is compared with adult’s head is investigated by using finite-difference time-domain method. The factors which cause variation of the results in previous research as are evaluated by Numerical simulation. We focus on the effects from different relative positions of mobile phones, the effective distance between the antennas and the human head, normalization of numerical simulation. The thesis results are important and can work as a foundation to conduct further research in green design for wireless handset.更多还原