應用於雙頻與MIMO天線之串接左右手傳輸線

Abstract

本篇論文中採用電氣長度60度之串接左右手傳輸線來設計應用於雙頻與MIMO之兩支天線。藉由右手傳輸線造成相位落後及左手傳輸線造成相位領先之特性，串接等電氣長度的右手及左手傳輸線，使得輸入阻抗虛部為零，再利用傳輸線的π型和T型的等效電路，透過適當的佈局來實現天線架構。在雙頻天線的設計中，第一共振頻率選擇在2.45 GHz ，再藉由選擇不同的電氣長度來設計不同的第二共振頻率。本論文中選擇電氣長度為60度，除可使天線佈局能達到更微小的尺寸以及第二共振頻率在5 GHz ，因而能適用於無線通訊網路802.11 b和802.11 g頻段。在MIMO天線設計中，延續上述60度電氣長度的左右手傳輸線的設計原理，於無線網卡上設計了三支天線，並且為了減少天線間的耦合效應，在其中兩隻天線地線加上一段金屬線，以增加天線的指向性，實際量測天線間的耦合量也都低於-10 dB。

In this thesis, a dual-band antenna and a MIMO (multi-input multi-output) antenna are proposed. The antenna design concepts are all based on using cascaded two opposite sign segments of 60 degrees electrical length transmission lines to provide zero phases at the operated frequency. The closed-form formulas for the equivalent circuit model of transmission line are used for circuit design. A π-type equivalent circuit for RH (right handed) TL (transmission line) and a T-type equivalent circuit for LH (left handed) TL are chosen and connected with an open-circuit at the unconnected port of the LH TL. These can result in a via-free layout which does not require an extra fabrication process. For the dual-band antenna, the first operated frequency is 2.45 GHz. We can design the different second resonant frequency by choosing different electrical length of transmission line. Choosing 60 degrees electrical length is convenient to design the second resonant frequency at 5GHz and reduce the antenna size. Therefore the first and the second resonant frequency of the dual-band antenna are 2.45 GHz and 5 GHz, respectively. The measured radiation pattern is approximated to omni-directional, and the largest average gain achieves 0 dBi at 2.45GHz. For the MIMO antenna, the basic design theory comes from the dual-band antenna. In order to reduce the coupling between two antennas, we add an extra ground arm, which is designed for high directivity. There are three antennas on a PCB board as a experimental prototype. The measurements of coupling between two antennas are all under -10 dB.