可抑制後波瓣的單一導體帶狀天線之設計

Abstract

在本論文中，首先介紹了漏波的條件，接著在理論上採用全波分析法-頻域法(spectral domain approach)得到單一導體結構的第一高階洩漏模的傳播常數，其中在空間波輻射頻段，正規化的相位常數非常接近1，透過洩漏波天線洩漏角度之關係式可得知此天線之主波束(main beam)幾乎固定在端射(end-fire)方向。由於單一導體結構的第一高階洩漏模的基本物理特性為完美電牆(perfect electric wall)中心對稱，表示縱向電流為奇對稱分布而橫向電流為偶對稱分布，所以我們使用平衡式微帶線和反向平衡式微帶線來饋入以激發此模態，並成功地設計出具有高增益與寬頻的單一導體帶狀洩漏波天線。

從此天線的輻射波瓣圖(radiation pattern)之模擬結果，我們觀察到後波瓣(back lobe)的輻射相當大。在分析了單一導體帶狀洩漏波天線的結構之後，我們推斷出饋入電路中的訊號回流機制會造成向天線左右兩端以及後端的輻射。為了抑制這種現象，我們利用兩個寬頻巴倫(balun)以上下顛倒的方式分別接在饋入電路的左右兩側，而實驗的結果也證實天線的前後輻射比(front-to-back ratio)確實有了明顯的改善(此一構想是將回流路徑中的訊號當作巴倫的輸入訊號，而在兩輸出端可產生大小相同、相位差卻為 的反相訊號，最後將其匯合後來達到抵消的效果，因此向後端福射的效應也隨之減小)。

This thesis presents the condition of leakage and mode distinction at first; then the well-known full-wave method, spectral domain approach, is applied to investigate the propagation characteristics of the first higher-order mode of the single-conductor strip structure. In this case, the normalized phase constant is very close to 1 in the space-wave leaky region. By the leakage angle equation of leaky-wave antenna, we find that the main beam of this antenna is fixed in the end-fire direction over a broadband region. For the first higher-order leaky mode in this structure, a virtual perfect electric wall is assumed at the center of the strip, this means that the longitudinal currents are odd-symmetric and the transverse currents are even-symmetric with respect to the center. A broadband planar feeding structure based on the balanced microstrip lines and the inverted balanced microstrip lines is developed to feed this single-conductor strip structure and thus excite the first higher-order leaky mode. Then, a high gain and wideband leaky-wave antenna is implemented.

From the radiation patterns of our single-conductor strip leaky-wave antenna, we observe that the back lobe is quite large. After analyzing the single-conductor strip leaky-wave antenna, we conclude that the feeding structure of this antenna causes this undesired effect. In order to alleviate this undesired radiation from the feeding structure, we turn two broadband planar baluns upside down and attach themselves to the left and right sides of the feeding structure, respectively. Experimental results show significant improvement of the front-to-back ratio of this antenna.