寬頻微小化多頻帶毫米波天線

Abstract

傳統的毫米波單頻天線已無法滿足未來5G行動通訊的多頻帶需求，因此多頻帶且寬頻的毫米波天線將逐漸受到重視。天線必須具備高增益的特性，但提高增益往往需要較大的天線面積，又會與手持裝置的微小化需求相違背。因此，要如何在有限尺寸下設計一個寬頻且多頻帶的高增益毫米波天線設計是一大挑戰。 由於5G毫米波多頻帶天線相關議題非常新穎，就吾人所知，目前文獻多致力於毫米波雙頻帶天線設計，有些文獻設計兩個共振結構達到雙頻帶操作，結構相對簡單但卻因為其窄頻共振特性而使其頻寬較窄，不易滿足美國FCC規定的頻寬標準。另外，相關研究也有在寬頻天線裡使用帶拒結構來達到雙頻帶特性，這種方式雖然可以得到較寬的頻寬，但需要加入帶拒結構會讓設計較為複雜，且為了增強帶拒效果往往需要較大的面積。也有文獻使用多層板結構縮小天線的面積，並且透過耦合饋入的方式改善頻寬，但天線的多層結構會使整體架構變複雜。 為了滿足28/38/60 GHz三頻帶需求，本論文以quasi-Yagi天線為基礎設計一個具有三個Arm與圍繞式彎曲型結構的三頻帶天線，各個Arm具有多功能，例如: Arm 2可以等效作為28 GHz頻段的director、38 GHz頻段的driver與60 GHz頻段的reflector，而Arm 3可以等效作為28 GHz頻段的director、38 GHz頻段的director與60 GHz頻段的driver。由於不需要每個頻段都額外具備相對應的director與reflector，可在面積減少的情況下維持高指向性。然而該機制使得三個Arm彼此經由傳輸線相連，造成相互間的強烈耦合而影響各頻段天線的整體增益與頻寬，所以提出漸進線之設計以及三個Arm的寬度設計來減少耦合效應之影響，使各頻段電流可以較集中在各頻段的driver上。 另外，圍繞式彎曲型結構可以提供另外70 GHz頻段的共振點，以解決頻寬不足之問題，且同時可作為director以達到較好的輻射特性。最後透過接地面尾端三角形與共用路徑上的開路殘段之設計，能夠在不增加天線面積下改善天線頻寬。 本天線使用Rogers RO4003基板，整體大小僅5×8×0.203 mm3，為單層介質板結構，低成本且製作十分容易。經量測後的天線頻寬範圍為25.53-30.26 GHz、34.66-40.4 GHz與50.22-67 GHz，除了67 GHz以上目前受限於量測儀器而無法量測外，其它都有滿足三個寬頻帶的設計目標。場型為end-fire的輻射，在28/38/60 GHz的模擬增益分別是6.2/7.56/9.42 dBi，具有良好的輻射特性。本天線的設計細節及實驗結果在論文中皆有詳細討論。

Traditional single-band Millimeter-Wave antennas are already unable to satisfy the need of future fifth-generation (5G) mobile communication, therefore the wideband and multi-band millimeter-wave antennas have gained substantial attention gradually. The antennas must have high gain performance. However, increasing antenna gain often needs bigger antenna size, which violates the need of miniaturization of handheld device. Consequently, how to design a wideband, multi-band and high gain millimeter-wave antenna is a big challenge under the limit of small-size. Because the issue of 5G multi-band millimeter-wave antenna design is very novel. As far as I know, literatures have reported the millimeter-wave dual-band antenna design presently. Several literatures designed two resonated structures to obtain dual-band characteristic, and the structures were simple relatively. However the resonated characteristic let the bandwidth become narrower, and the frequency response was not easy to meet the bandwidth standard of 5G mobile communication. In addition, several literatures used notched structures to get dual-band characteristic, which had wider bandwidth. But the method needed addition notched structures which let the design more complex and often needed more size in order to produce higher gain suppression. Furthermore, some literatures used multi-layer structure to reduce the antenna size and improve the bandwidth through the method of coupled feeding, but the multi-layer structure of antenna let the whole structure become complex. In order to meet the need of 28/38/60 GHz band, the thesis proposed a tri-band antenna based on quasi-Yagi antenna with three arms and round and bended structures. Each arm has multiple functions. For example, Arm 2 functions as a director for 28 GHz band, functions as a driver for 38 GHz band and functions as a reflector for 60 GHz band. Arm 3 functions as a director for 28 GHz band, functions as a director for 38 GHz band and functions as a driver for 60 GHz band. The design maintains high gain performance in the situration of reducing antenna size, and doesn’t need the whole and corresponding director and reflector in each band. However, the three arms connects each other through transmission lines and causes strong coupled effect between them, which influences the antenna gain and bandwidth of each band. So the design of tapered line and the width design of the three arms have been introduced to reduce the influence of coupled effect, which the current of each band can concentrate on the driver of each band. Furthermore, the round and bended structures provide another resonance to satisfy the 57-71 GHz frequency range, and also gain better radiation performance as directors simultaneously. Finally, we use the triangles in the end of ground and the open stub on the common path to increase antenna bandwidth without increasing the overall antenna size. The proposed antenna has been fabricated on a Rogers RO4003 substrate with single-layer structure, and the overall size of the antenna is 5×8×0.203 mm3. The measured impedance bandwidth is 25.53-30.26 GHz, 34.66-40.4 GHz and 50.22-67 GHz expect above 67 GHz which can not measure data under the limit of measuring instruments presently. In addition, the radiation pattern is end-fire direction and the simulated gain at 28/38/60 GHz is 6.2/7.56/9.42 dBi respectively, which has good radiation performance.