| Abstract
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Wireless devices, such as mobile phones, personal digital assistants, and Bluetooth headsets emit electromagnetic (EM) radiation. Such devices when used near a user result in EM energy absorption in the head or body of the user. The effect of this absorption is thermal and non-ionizing and the metric that is used to characterize the extent of this absorption is called the specific absorption rate (SAR), which is a function of the electric field intensity emitted by the antenna. There are national and international standards organizations that have developed safe limits of SAR based on animal studies, which ensure that the temperature rise in the user's head or body is small enough not to cause any harmful effects. Currently, in order to determine the SAR induced by a particular wireless device measurements are performed in a head or body simulating dielectric liquid phantom using a robot and near-field probes. The process is very complicated and tedious. Reports on SAR computation and modeling techniques are abound but they mostly focus on algorithm refinement or the development of different types of head or body models. At present there is no simple straightforward method to even distinguish the SAR induced by an extremely small RFID (radio frequency identification device) antenna and that by a mobile phone primarily because of our lack of understanding of the relationship between antenna performance characteristics (eg. bandwidth, directivity) and SAR. Yet such relationship can enable us to predict the SAR of an antenna more easily and quickly and for some antennas possibly eliminate the need for testing altogether if they emit a power below a certain threshold. To achieve this goal, in this work, we start from the definition of the fundamental limits of antenna quality factor, design antennas of different types, shapes, and sizes and investigate their performance characteristics and SAR over a wide frequency range both numerically and experimentally. Our results on dipole antennas elucidate that a focusing factor can be defined for antennas near a lossy dielectric object, such as the human head or body and that the smaller antennas focus more energy in the SAR averaging mass than the longer. We also conclude that among all classes, shapes, and sizes of antennas investigated in this study, the dipole antennas generally induce higher SAR compared to all other antennas and that the antenna free-space bandwidth is strongly related to the SAR. Based on these findings we develop a simple easy to use formula which can be used to estimate the threshold power that directly corresponds to the SAR induced by an antenna once the antenna free-space bandwidth is known. We also design, fabricate, and test two completely new antennas with unique performance characteristics and test our SAR estimation formula on them. The first is a new planar microstrip-fed dual-band Hilbert slot antenna which has two distinct patterns, end-fire and broad-side, at the low and high resonant frequencies, respectively and the second is a miniature spiral diversity antenna with high overall gain and low mutual coupling characteristics.
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