人體區域網路之超寬頻天線設計

Abstract

人體區域網路，由布置於人體上之無線感知節點所組成的區域網路，感知節點整合感測電路及無線通訊系統，利用超寬頻無線通訊系統將感測訊號（心跳、血壓、等）傳達至個人資訊中心，來達成遠端監測的目的。由於無線感測裝置是配戴在人體上，因此其對天線的需求有1.天線小且高度低，不干擾人體的日常活動;2.當感知節點放在人體上時，天線必須避免頻率偏移及輻射場型等特性改變;3.天線場型要減少背向輻射（朝人體方向），以增加輻射效率及降低人體吸收率（Specific Absorption Rate, SAR）。本篇論文設計一支可應用在人體區域網路的新型超寬頻天線，其高度為一公分。天線操作頻寬涵蓋3.1 GHz ∼12 GHz 並且符合上述所有需求。在天線設計中，我們利用斜角的饋入結構使輸入阻抗緩慢變化來達到寬頻效果。為減低背向輻射，我們設計了一個類似patch天線的輻射體，使天線場型具有指向性且其增益約在3 dBi∼5 dBi。輻射體包含一個巧妙設計的彎折結構，使操作頻寬符合10dB反射損失的需求。本論文也實際量測天線放在人體上之後的特性，其反射損失幾乎不會受到人體影響。此外我們將肌肉模型放入模擬軟體中計算人體吸收率（SAR），並與其他平面超寬頻單極天線做比較。 為了避免干擾現存頻譜無線網路（5∼6 GHz）系統，我們將彎曲槽線設計在饋入結構上，使天線反射損失產生帶拒效果，並維持天線原有之特性。我們可以藉由調整槽線的長度與距離來控制帶拒頻率及頻寬，因此我們將帶拒頻率設計在5GHz∼6GHz之間，達到阻絕無線網路干擾的問題。

Wireless Body Area Network (WBAN) is wireless sensor network which is applicable on the human body. The sensors, which are applied on the human body, transmit the wanted/desired signal to the center devices for remote monitoring. The sensor devices are placed on the human body so there are three requirements for the antenna (which is integrated with the sensor device): 1. Compact form and low height: The antenna should be unobtrusive when it is placed on the human body in order to avoid interfering in the human’s daily activities. 2. Low mutual effect between the antennas and the human body: The antenna should keep the characteristics such as operating bandwidth when applying to the body. 3. Low backward (direction to the human body) radiation: In order to decrease the Specific Absorption Rate and increase the radiation efficiency. Owing to these three restrictions on antenna characteristic, designing an UWB antenna for WBAN is a challenge. In this thesis, we propose a novel patch-like UWB antenna used in WBAN application. The proposed antenna consists of a bevel edge fed structure and a truncated radiator. The bevel edge of the fed structure is designed for slow impedance variation and achieves very wide bandwidth. The patch-like radiator is designed for reducing the backward radiation and results in the directional radiation patterns. Moreover, the folded strip of the radiator is a key factor in antenna design, it affect the impedance match of the desired bandwidth. By suitable designing the folded strip, the return loss is over 10dB condition within the operating bandwidth. In this thesis, we also compare the measured return losses of the proposed antenna in free space and on the human body. In order to obtain the SAR values, the muscle models are employed in simulation. In order to avoid interfering the existent wireless application such as WLAN, a meander slit, a band-stop resonator, is designed at the fed structure of the proposed UWB antenna. The additional slit creates the band-rejected property while keeping the other characteristics almost the same. We can control the notch property including frequency and bandwidth by adjusting the length and distance of the slit. In final design, the band-rejected property is designed in 5GHz ~ 6GHz to prevent the interference from the WLAN.