近場通訊系統之天線耦合分析

Analysis of Antenna Coupling in Near-Field Communication Systems

陳晏笙(Yen-Sheng Chen)

指導教授 ： 李學智

國立臺灣大學/電機資訊學院/電信工程學研究所/碩士(2009年)

https://doi.org/10.6342/NTU.2009.00080

摘要

近年來，近場通訊系統的應用日趨廣泛，諸如超高頻近場射頻辨識系統、近距離無線通訊技術(NFC)。在近場通訊系統中，我們希望成功的設計天線，並優化系統的效能，因此收發兩端的天線耦合分析就變得很重要。本論文提出一個簡單的公式，可以計算近場通訊系統中傳送天線和接收天線之間的功率耦合係數(power coupling coefficient)。此公式的適用性不拘天線的種類、形式、尺寸、工作頻率；所需的資訊為收發兩端天線在該工作頻率下的三維遠場場形、天線之間的相對傾斜角度及距離。換言之，我們所提出的公式可視為遠場福利斯傳輸方程式(Friis transmission equation)在近場的類比。

為了驗證公式的準確性，我們先考慮幾種常用的天線，利用他們的場形標準式(close-form pattern)，將計算的近場耦合係數和全波分析模擬(HFSS)作比較。此外，我們也以超高頻近場射頻辨識系統為例，設計符合此應用的天線，並且比較量測、模擬、理論計算的結果。透過這些驗證，我們確認所提出的公式可以精準的算出近場功率耦合係數。在實驗中我們也發現了一些影響耦合程度的參數，諸如接收天線的阻抗匹配、發送天線的指向性。藉由提出的公式，我們還可以計算超高頻近場射頻辨識系統的讀取距離以及可靠度。因此，這篇論文提出的成果相信有助於應用在近場通訊系統。

關鍵字

電磁耦合 ； 近場 ； 能量傳輸 ； 射頻辨識 ； 超高頻天線

並列摘要

Recently, near-field communication systems have been widely used in many applications such as the near-field UHF RFID item-level tagging, the Near Field Communication (NFC) device, and the mCoupons. To successfully design and optimize the near-field communication systems, it is important to investigate the near-field coupling between the transmitting and receiving antennas. In this thesis, a simple formula has been presented for computing the coupling coefficient between two antennas that are placed in the near field of each other. The choices of the two antennas are arbitrary, and all the information needed includes the corresponding normalized vector far-field patterns along with their relative orientations and the antenna spacing.
To verify the proposed formulation, the coupling coefficients in several near-field scenarios, including a practical near-field UHF RFID system, are computed and compared to those measured and full-wave simulated using Ansoft HFSS. They are all in good agreement. Additionally, it is shown that several factors may influence the coupling coefficient, such as the impedance matching of the receiving antenna and the directivity of the transmitting antenna. With the aid of the proposed formulation, the near-field read range and read reliability can be determined and the near-field coupling phenomena can be investigated. The results thus obtained may be useful in the near-field communication systems.

並列關鍵字

Electromagnetic coupling ； near field ； power transmission ； RFID ； UHF antennas

參考文獻

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