適用於極小型無線網路收發器之雙緊鄰天線設計

Abstract

本論文提出適用於2.4 GHz極小型無線網路收發器的雙天線架構。由於設計面積僅為6.3 × 12.9 mm2 (0.0514 λ0 × 0.1053 λ0)，此面積相對於天線操作頻率是相當小的，在這麼狹小的空間裡，天線間的耦合效應是非常嚴重的。由於一般天線設計隔離度僅約為2 dB左右，因此天線要達到良好的阻抗匹配與隔離度相當具有挑戰性。本篇論文在考慮實際內部電路屏蔽盒環境下，提出簡潔的雙天線設計概念，以增加此天線設計的實用性，使得天線在達到良好的阻抗匹配的同時，也能獲得較好的隔離度。首先，透過饋入點與天線開路點擺放位置的探討，可得知電感性耦合較電容性耦合影響隔離度較小，所以本論文採用饋入點靠近方式來設計天線。而天線繞線設計採用螺旋狀的走線，藉由螺旋狀走線提供的電感效應，幫助天線縮小化。接著在兩支天線中間加入寄生元件，也可以進一步改善隔離度。因為天線設計空間尺寸極小，為了提升天線輻射特性，可利用雙層疊板架構來增加天線設計空間。本論文提出的天線架構，饋入點間距離只有1.9 mm (0.0155 λ0)，其實際量測的最小隔離度可達到8.54 dB，在全向性平面上的平均輻射增益為 -1.43 dBi，模擬與量測結果一致皆符合無線網路應用需求。

In this thesis, a MIMO (Multi-input Multi-output) antenna system for small 2.4GHz WiFi dongle applications is investigated. The overall size of the MIMO antenna system is only 6.3 × 12.9 mm2 (0.0514 λ0 × 0.1053 λ0), which is a quite small area as respect to the antenna operating frequency. There is an extremely strong mutual coupling between antennas. The isolation is only about 2 dB when normal antenna design is adopted. It is challenging to achieve good impedance matching and isolation. In this thesis, the simple dual-antenna design concepts is proposed in detail to achieve the goal with considering shielding box to fit the practical situation. First, through the discussions of positions of ports and open ends, it is clear to know that the conductive coupling is better than capacitive in isolation. Thus, the antenna design of proposed structure with close ports. This structure consists of two antennas, which are PIFAs(Planar Inverted-F Antenna) with spiral tails. The spiral tails provides inductive loading, which is helpful for miniaturalization. Furthermore, it can also improve isolation by inserting a parasitic element in the middle of antennas. Since the size of antenna design space is very limited, a double layer structure is used to enhance radiation properties. The proposed antennas with 1.9 mm (0.0155λ0) separation between the antenna ports have measured isolation of 8.54 dB. The measured average gain is -1.43 dBi on the omnidirectional radiation plane. The simulation and measurement results come to a great agreement.