使用複合式左右手傳輸線製作雙臂螺旋零階共振天線與指向耦合器實現的雙頻分工器

Abstract

本論文主要探討複合式左右手傳輸線於天線與方向耦合器的應用。本論文分成兩部份：第一部分為利用複合式左右手傳輸線的獨特零階共振特性達到天線的縮小化，第二部分則利用複合式左右手傳輸線做成的指向耦合器實現完全向前耦合(Complete forward coupling)與向後耦合(Complete backward coupling)的雙頻分工器(Diplexer)。 隨著無線通訊的廣泛使用，人們對於微小的可攜電子產品的需求逐日增加。因此，天線的縮小化與整合性成為天線設計的一大挑戰。複合式左右手傳輸線所實現的零階共振天線具有無限大的波導波長使天線的等效面積大幅增加，進而改善了天線的輻射效率。此外，由於其操作的頻率與複合式左右手傳輸線的等效電路元件值有關，不同於一般共振型天線需製造出至少四分之波長的共振路徑，而只需調整電路元件所相對應的天線結構即可大幅節省所使用的面積，達到天線的縮小化。因此於第一部份，本論文利用一端貫孔的雙臂螺旋結構來實現複合式左右手傳輸線最重要的兩個元件：串聯電容以及並聯電感。此結構因有效地增加邊緣耦合及縮小所佔的覆蓋區域，使該天線尺寸面積成為目前最小之零階共振型天線，且具有偶極天線的輻射場型，可接收四面八方而來的訊號，符合一般攜帶式電子產品對於天線的要求。量測出來的天線增益及輻射效率分別為 -0.53 dBi及53%。 除了前述的天線外，本論文也利用左右手傳輸線做成的對稱方向耦合器來實現雙頻分工器。此雙頻分工器的最大特色在於使用向前耦合埠與向後耦合埠做為分工器的分工路徑，此外，對稱的結構也使分析過程更為簡單，以奇偶模態的分析方法，得到只有一組特定的色散圖與布洛赫阻抗才可在向前耦合區域內得到最佳的指向性。除此之外，兩個頻帶的距離可藉由調整結構中兩個貫孔彼此之間的距離來控制。在指向性、頻寬、電路大小和金屬盒的模態等的考量下，本論文選擇了具有七個單元的複合式左右手傳輸線作為雙頻分工器所需的耦合線，兩個頻帶之量測結果分別是從4.19 GHz 到 4.9 GHz 以及 5.6 GHz 到 6.23 GHz，其部份頻寬分別為15.6% 和 10.7%， 而兩個頻帶最大的饋入損耗為 -1.3 dB 和 -0.9 dB。

This thesis dedicates to the applications of composite right/left-handed (CRLH) transmission line, and it consists of two parts. A miniaturized and zeroth-order-resonant (ZOR) antenna based on dual-arm spiral structure is presented in the first part. The second part introduces a novel bandwidth-engineered diplexer realized by a symmetric CRLH coupled-line directional coupler with complete forward and backward couplings. Widespread use of the wireless communications in our daily life demands the development of compact and portable electronic products. As an essential element in the communication systems, small antennas are required to fit the limited space available. Unlike the conventional resonant antenna needs at least half-wave resonant length, the ZOR antenna created by the CRLH transmission line has zero phase constant and is independent of the resonant length. Therefore, ZOR antennas provide a good alternative to miniaturize antenna size without considering the resonant lengths. In addition, the ZOR antenna supports infinite guided wavelength, this substantially increases the effective area of antenna and thus improves the radiation efficiency. This thesis presents a compact ZOR antenna by using two spiral arms with a shorted via. The proposed dual-arm spiral unit cell is able to implement and tailor the CRLH lumped elements easily in a compact fashion. Particularly, the gap coupling between the spiral arms contributes to the series capacitance C_L and the metallic via at the end of one arm results in the shunt inductance L_L. The resonant frequency and thus the miniaturization factor can be engineered easily by adjusting the spiral parameters that determine the corresponding circuit elements. These arms not only realize the most essential components of CRLH transmission line mentioned above, but also effectively increase the edge coupling and shrink the antenna size, occupying the smallest footprint among the ZOR antennas in terms of electrical wavelength. Moreover, this compact antenna exhibits dipole-like radiation patterns with a peak gain of -0.53 dBi and a radiation efficiency of 53%, and meeting the omi-directional requirement of antenna in the portable device. The second part proposes a novel bandwidth engineering diplexer realized by the symmetric CRLH coupled-line directional coupler with complete forward and backward couplings. The operating principle is investigated by the even/odd mode analysis, and it deduces that only a set of dispersion diagrams and Bloch impedances rendering the best isolation in the forward region. In addition, the bandwidth engineering is demonstrated by controlling the distance between the vias. The 7-unit-cell case is implemented under the consideration of the isolation, bandwidth, coupling length and the cavity modes of metal housing. The measured 3-dB bandwidth of forward and backward coupling bands are in the range from 4.19 GHz to 4.9 GHz and 5.6 GHz to 6.23 GHz with the maximum insertion loss of -1.3 dB and -0.9 dB, the corresponding fractional bandwidths are respectively 15.6% and 10.7%.