現代通訊系統之行動通訊裝置，因為追求輕薄短小之實用性，其接地面大小往往不到一個電氣波長，而形成ㄧ個發射體，因此天線包含了地的效應就如非對稱偶極天線一般。不平衡電流也就由同軸電纜餽入點明顯地形成。不平衡電流或EMC上所討論的共模電流，本文中將從電磁相容(EMC)到現代通訊用天線觀點來討論不平衡電流之影響。
首先會由參考文獻及實驗結果中討論不平衡電流的影響，進而得到如何解決此種效應的方法。
接著一些解決之方法會由理論的分析或實驗上來得到，在EMC上常使用之Ferrite Chock 首先被介紹，但其特性通常只到1GHz，明顯可知其頻寬無法涵蓋目前的通訊產品，因此我們再提出Sleeve Balun ，傳統之單頻Sleeve Balun及新式的雙頻Sleeve Balun。

Since the size of the ground plane is decreased to be comparable to a wavelength, the ground plane should be a planar radiator. Therefore, the antenna combined with its ground plane is similar to an asymmetric dipole antenna. The unbalanced current is induced on the feeding coaxial cable consequently. The influence of unbalanced-current, or common-mode radiation, studied in this thesis is discussed in EMC firstly, but, however, it is a significant problem in modern antenna measurement because of the size of the ground plane. The topics studied in this thesis are as follow.
First of all, the influence of the unbalanced-current is studied in the summery of references and experiment results, respectively. In this part, it gives that solving of the influence is imperative.
After that, some examples of the solution are provided in theoretical and experimental analysis. Ferrite chock studied in this part firstly is a typical a choice in EMC, since it is efficient operated lower than 1GHz. However, in current portable devices, the operation frequency range is not sufficient. Consequently, two sleeve balun are induced in this thesis, a traditional single band sleeve balun and a novel dual-band sleeve balun.

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