蕈狀結構於複合左右手洩漏波天線的應用

Abstract

傳統洩漏波天線，需要較複雜的饋入電路以激發高階模，只能前向掃描。而複合左右手(composite right/left handed, CRLH)洩漏波天線具有從後向到前向掃描能力，而且只需簡單和有效的饋入結構。 蕈狀結構有很多獨特的電磁特性，也可被用作一種二維的複合式左右手結構超材料(metamaterials)。在本論文中，利用特徵模態(eigenmode)分析蕈狀(mushroom)結構的色散圖，並觀察各模態的電場和磁場，確定結構的特性。由色散圖得到可輻射的頻率範圍，並建立等效電路。然而使用特徵模態解出的第一個模態，是不會存在背向的洩漏波，和等效電路預測的不同，我們會透過量測，驗證哪一個才是正確。 先利用單一微帶線去饋入蕈狀結構之複合左右手洩漏波天線，但天線在左手洩漏波區時，增益比較低，所以利用一分二的功率分波器，去饋入天線。頻寬從5.6GHz到9.35GHz，增益在頻段內都有達到10dBi以上，掃描角度為-51度順時針轉到69度。而由遠場量測得到的主波束輻射角，來推得相位常數。

Typically, the leaky wave antenna is fed by a complex feeding structure to excite the higher order mode and it exhibits forward angle scanning characteristic. The composite right/left handed (CRLH) leaky wave antenna exhibits continuous backward-to-forward angle scanning characteristic and the broadside radiation capability. Moreover, it can be fed by a very simple and efficient (small and broadband) feeding structure. Mushroom structure exhibits unique electromagnetic properties, and it can be used for one kind of the 2-dimentional metamaterials. In the thesis, we use the eigenmode to analysis the dispersion diagram of the mushroom structure and we observe the electric field and magnetic field of each mode to understand characteristics of the mushroom structure. We acquire the radiation region from the dispersion diagram, and the mushroom structure can be represented by an equivalent circuit model. One of the mode solved by eigenmode method shows that this structure is unable to exhibit backward leaky wave; the circuit model constructed, however, predicts that the structure should be able to support backward leaky wave. We verified this contradiction by measuring the actual structure. First, we use single microstrip line to feed the composite right/left handed leaky wave antenna composed by the mushroom structure. However, the antenna gain is lower in the left-handed leaky wave region than in the right-handed leaky wave region. Thus, we use a 2-way power divider to feed the antenna. The bandwidth of the antenna fed by 2-way divider is from 5.6 GHz to 9.35 GHz, and the angle of radiation rotates clockwise from -51 to 69 degree. The antenna gain is over 10 dBi within the bandwidth. In addition, we acquire the angle of radiation of the main beam by far-field measurement, so we can calculate the phase constant.