時域有限差分法在同軸型天線陣列設計和分析應用

Abstract

本文以時域--有限差分法(FDTD)來做天線的分析，基於考慮電腦資源上的限制，我們僅研究對稱性結構，我們所分析的第一個結構為monopole antenna，並探討電磁波在空氣中的傳播以及散射的產生情形，和天線罩對於天線特性的影響，所獲得結論為：（1）電磁波的散射是發生在不連續點處，由於電荷受到減/加速度作用會造成入射波能量一部分以球面波（以該不連續點為圓心）的方式輻射出去。（2）當天線的外圍覆蓋天線罩時將使得天線的共振頻率往低頻偏移，所以設計天線時必須考慮到天線罩的影響而將共振頻率設計的比實際使用的頻率稍微高一些。 我們還分析可做為行動通訊基地台的天線的Coupled Coaxial Dipole array antenna的阻抗匹配性能，並跟現有的文獻作一比較。基於結構切割上的限制，我們所分析的結構尺寸與文獻所用尺寸稍有不同。因此，分析所得 的頻譜變 化趨勢雖與文獻資料大致吻合，但的 絕對準位則有些差異。

We used finite-difference time-domain (FDTD) method to analyze linear wire antenna and antenna array. In traditional analysis, the dispersion relations of the effective dielectric constant and the characteristic impedance usually exhibit oscillatory behavior, the main cause is due to imperfect absorbing boundary condition. We used the equivalent circuit approach method to minimize oscillation. First, we analyzed monopole antenna and study about propagating and scattering of the electromagnetic wave. In addition, the effects of radome used to protect or to enhance structure strength of the antenna were examined. The following observations are found: (1) The scattering wave is produced at spatial discontinuity. When incident wave travels to discontinuity, the electron will change its velocity, and radiate part of its energy in the form of a spherical wave, this wave front is centered on the discontinuity. (2) Radome causes the resonance frequency of the antenna to be shifted toward lower frequency. Therefore, to design an antenna with radome, its resonance frequency must be made slightly higher than practically used by reason of radome’s effect. We also analyzed coupled coaxial dipole array antenna, it can be used on mobile communication’s base station, and compared simulated results with published results. Because of the limit action in structural segmentation, our antenna size is slightly different from that of Hiroaki et al. [2]. For this reason, the frequency spectrum of return loss is only roughly matched with their result.