自諧振結構之微型化寬頻標籤天線

Abstract

本論文研究著重於使用在頻率860~960MHz的寬頻標籤天線，以及微型化仍保有較高的輻射效率。標籤上的微晶片通常是低電阻高電容的阻抗，而且高電容產生的抗值會隨著頻率而變化，因此天線與微晶片達到寬頻阻抗共軛匹配具有相當的困難度，一般寬頻標籤天線採用單閉路迴圈耦合雙電流路徑輻射體，單閉路迴圈提供大電感來抵銷微晶片的高電容，進行阻抗匹配，並靠著雙電流路徑形成兩個共振頻率達成寬頻效果，但使用單閉路迴圈需要大面積才能提供高電感，且雙電流路徑的分流降低了輻射效率。我們提出一個寬頻標籤天線使用單閉路迴圈加入雙殘段形成的自振腔，再耦合至單一電流路徑輻射體，自振腔在小面積下就能形成大電感，而雙殘段的設計，讓自振腔中的電流路徑保持於單閉路迴圈上，使得與輻射體之間的耦合量保持不變，同時自振腔提供一個自諧振頻率，因此只需耦合至單一電流路徑輻射體形成另一個共振頻率，即可達到寬頻效果，而單一電流路徑的輻射體比雙電流路徑大幅提高了輻射效率。此寬頻RFID標籤天線的大小為80 × 10.5 × 0.4 mm^3，透過數值模擬軟體及實作量測得到在頻段860~960 MHz具有相當好的匹配程度，且模擬結果天線增益皆為3dBi以上，測得實際的讀取距離約為8公尺。本天線的設計細節及實驗結果在論文中有詳細討論。

This thesis focuses on the 860~960 MHz broadband RFID tag antenna design and realization that has smaller antenna size than common broadband tag antennas while retaining high radiation efficiency. The impedance of the microchip of a RFID tag usually has low resistance and high capacitance, and its reactance caused by high capacitance varies with frequency. Therefore, it is quite difficult to achieve broadband impedance conjugate matching between the antenna and the microchip of a broadband RFID tag. A common broadband tag antenna adopts a single closed loop coupling to a dual current path radiator. The high inductance induces by the closed loop offsets the high capacitance of the microchip to match impedance, and the dual current path forms two resonant frequencies to yield broadband effect. However, a closed loop requires a large area in order to generate a high inductance, and the diversion of dual current path reduces the radiation efficiency. We propose a broadband tag antenna utilizes a closed loop with double stubs to form a self-resonance which then couples to a single current path radiator. The self-resonance is able to produce large inductance with a small area. Due to the design of double stubs, the current path of the self-resonance can be kept on the closed loop so that the amounts of the coupling between the radiators remain unchanged. Besides, since the self-resonance provides a self-resonant frequency, broadband effect can be achieved by simply coupling to a single current path radiator to form another resonant frequency. In addition, the radiator with single current path significantly increases radiation efficiency comparing to the dual current path radiator. This antenna is realized by a 80 × 10.5 × 0.4 mm^3 FR4 substrate. Considering the read range of more than eight meters, we show that our design have pretty good impedance matching between the bandwidth of 860~960 MHz either by numerical simulations or our experiments. Our simulation results also show that the gain of the antenna is 3 dBi above. The design of the antenna and the experiment results will have further detailed discussion in the thesis.