對於個人通信 (PCS)的系統服務來說，由於越來越多樣服務的提供以及受限於可攜式行動裝置上的有限空間，因此對雙頻帶/多頻帶的平面天線之需求與日俱增；近年來，不少研究人員使用多分枝的天線架構以獲得想要的多頻帶的操作。在本論文中，以CI及EI形態的新穎的多分枝單極天線架構被引進以實現包括 GSM 900，GSM 1800/1900及2.4 GHz ISM的多頻帶的操作，其返回損失皆小於-10dB。 本論文探討的天線以50Ω的共平面波導（CPW）饋入，使用共平面波導的好處包括只使用單一的金屬層，其他有吸引力的特徵包括傳輸線特性的穩定帶寬較大、架構簡單、不用焊點以及容易與主動元件結合等。 本論文詳細探討此一多分枝天線的多頻帶特性並經由實驗的驗證，吾人發現較長的 C分支（或 E 分枝）得負責對最低頻帶（0.9 GHz）的輻射，較短的I 分支對負責中間頻帶（1.8/1.9GHz），此外，較高的 2.4 GHz頻帶則由 C分支的高階（higher order）模態的被激發所貢獻。吾人進一步發現，使用非勻稱的接地架構（延伸右邊的接地平面使其有較大的尺寸）可以取得一相當寬帶的操作，在1.4 GHz～3 GHz的範圍內，其返回損失皆小於-9dB。 For systems of personal communication services (PCS), there are continuous needs for dual-band/multi-band planar antennas because of the various services available and the limited space on the portable devices. In recent years, there had been researchers employing the multiple branch structures to obtain the multi-band operations desired. In this paper, a novel CI-shaped multiple-branch monopole antenna is introduced in order to achieve multi-band operations that cover the GSM 900, GSM 1800/1900, and the 2.4GHz ISM bands with return-loss less than -10dB. The proposed antenna is fed by a 50Ω coplanar waveguide (CPW) transmission line to benefit from the simple structure of single metallic layer and other attractive features such as wide bandwidth, no soldering point needed, and easy integration with active devices, while the multi-band characteristics of the proposed antenna are carefully studied and experimentally verified. It is found that the longer C branch (or E branch) is responsible for the radiation of the lowest band (0.9GHz), while the shorter I branch is responsible for the higher band (1.8/1.9GHz). In additions, the 2.4GHz band is contributed from the excitation of higher order mode along the C branch (or E branch). Moreover, the un-symmetric structure with extended right-hand side ground plane with extra height is employed to achieve a wide band operation covering the range from 1.4GHz to 3GHz with return-loss less than -9dB.