圓極化波束掃描反射陣列

Circularly-Polarized Beam-Steering Reflectarray

洪琬婷(Wan-Ting Hung)

指導教授 ： 陳士元

國立臺灣大學/電機資訊學院/電信工程學研究所/博士(2015年)

https://doi.org/10.6342/NTU.2015.10392

摘要

本論文依序提出三款反射陣列，分別為近場聚焦反射陣列、極化可切換反射陣列與圓極化波束可掃瞄反射陣列。一般的反射陣列均為遠場輻射所設計，而在所提出之第一款反射陣列設計中，吾人修改其相位補償方式，使反射波在所設計的焦點上有相同的相位，達到聚焦效果，並藉此提高焦點位置之輻射功率密度，達到消滅H3N2流感病毒的效果，實驗結果顯示該病毒之死亡率為93%。
在圓極化反射陣列的實現方式上，有別於以往提出之圓極化入射波，吾人以線性極化天線作為饋入天線。以位於介質基板上下兩面且互相垂直之二背覆金屬帶狀偶極為基礎，藉由分別調節反射面上反射單元之水平及垂直分量之反射相位，配合極化傾角為45度之線性極化波入射，吾人便可於目標遠場方向上得到圓極化之反射波，完成圓極化反射陣列。
為了驗證所提出之設計方法，本論文分別設計極化可切換與波束可掃瞄之反射陣列。其中，極化可切換反射陣列之單位元由一對互相垂直且背覆金屬之帶狀偶極所組成，分別負責水平及垂直極化分量之相位調整。為使兩線性極化單位元之反射相位可調範圍大於360度，吾人於帶狀偶極外圍環繞一環形結構，可有效增加反射相位可調範圍至400度以上。在該反射陣列設計中，吾人使用中央饋入架構，而饋入之線性極化天線之極化傾角為45度，以此入射16×16之反射陣列，其操作頻率為10 GHz。調整水平極化的反射波相位使其領先垂直極化反射波90度，則吾人可得右手圓極化反射波。實驗量測到的右手圓極化天線增益峰值為22.3 dBic、1-dB增益頻寬為5.1%、3-dB軸比頻寬為4.7%、孔徑效率則為21.1%。
本論文提出之第三款反射陣列之單位元同樣由一對分別為水平與垂直極化之背覆金屬帶狀偶極所組成，帶狀偶極的中心均外加一變容二極體，藉由改變變容二極體之逆向偏壓可改變該偶極之極化分量反射相位。為避免兩垂直極化分量之互相耦合，吾人在此設計中並未使用環形結構，故兩垂直極化分量在5.5 GHz操作頻率下量測之可調相位範圍分別為230度與295度，略低於理想之360度。為減少金屬接地面之直接反射所造成之饋入反射波束，吾人採用六角形之單位元形狀。經適當設計其直流偏壓線路，可避免不必要之散射干擾，此具有59個單位元之反射陣列不僅可達到上述極化可切換之特性，更可實現操作在兩正交線性或與圓極化狀態下之波束掃瞄，實驗量測到的右手圓極化天線增益峰值為13.6 dBic，其於x-z平面上之波束掃瞄範圍為−20度至20度，孔徑效率則為13.6 %。

關鍵字

波束可掃瞄 ； 圓極化 ； 聚焦天線 ； 頻率選擇平面 ； 反射陣列

並列摘要

Three reflectarrays, including a near-field focusing reflectarray, a polarization-agile reflectarray, and a circularly-polarized beam-steering reflectarray, are proposed in this dissertation sequentially. In the first design, instead of designing for far-field radiation, which is common in conventional reflectarray design, we modify the methodology of phase compensation of the reflecting elements so that the reflected waves are in phase and focused at a predetermined focal point, enhancing the associated power density effectively. This design is utilized in the microwave virus sanitizer for H3N2 influenza virus. The measured death rate of the virus samples illuminated by the proposed sanitizer is up to 93%.
For the second and third reflectarrays, in order to achieve circularly-polarized (CP) radiation, a linearly-polarized (LP) antenna is first adopted as the feed instead of circularly-polarized (CP) antennas. A pair of orthogonal conductor-backed strip dipoles lying respectively on the top and bottom faces of the substrate is then used to manipulate the reflection phases of the two associated LP wave components, respectively. CP reflected waves can thus be obtained in the designed far-field direction by setting the reflection phase difference of the two orthogonal LP wave components to be 90 and making the polarization tilt angle of the LP feeding horn to be 45. As a result, a CP reflectarray can be achieved. To verify the presented method, a polarization-agile reflectarray and a CP beam-steering reflectarray are designed, fabricated, and tested. The unit cell of the polarization-agile design is composed of two orthogonally-polarized conductor-backed strip dipole elements. Each strip dipole is further encircled by a parasitic rectangular loop, resulting in a tuning phase range greater than 400. The 1616 prototype reflectarray designed at 10 GHz is center-fed and illuminated by an LP horn antenna with a 45 tile angle. By adjusting the compensation reflection phases of the two orthogonal strip dipoles to be 90, a CP main beam for the reflectarray can be realized. The measured RHCP peak gain is 22.3 dBic with the 1-dB gain bandwidth of 5.1%, and the measured 3-dB axial ratio bandwidth is 4.7%. The aperture efficiency of the polarization-agile reflectarray is 21.1%.
Likewise, the unit cell of the CP beam-steering reflectarray is composed of two orthogonal conductor-backed strip dipoles. Each dipole is loaded with a varactor diode at the center for independent control of the reflection phase of the associated LP wave component via changing the reverse bias voltage of the varactor. To prevent the coupling of the orthogonal dipoles, the parasitic loops are removed in this design; therefore, the tunable phase ranges of the two LP wave components measured at 5.5 GHz are reduced to only 235 and 290, which are lower than the optimal range of 360. In addition, the hexagonal shape of the unit cell is adopted to suppress the specular reflection due to the back conducting plane. The main beam of the CP reflectarray can be electronically steered by properly changing the bias voltages of the varactors, and thus a CP beam-steering reflectarray can be implemented. The fabricated 59-element reflectarray is capable of not only polarization agility but also beam steering for any polarization states (LP or CP). The dc bias circuits are carefully designed to avoid unwanted scattering interferences. The measured beam-scanning range of our proposed CP beam-steering reflectarray is from 20 to 20 with a RHCP peak gain of 13.6 dBic. The measured aperture efficiency is 13.6 %.

並列關鍵字

beam-steering ； circular polarization ； focusing antennas ； frequency-selective surfaces ； reflectarrays

參考文獻

[1] D. G. Berry, R. G. Malech, and W. A. Kennedy, “The reflectarray antenna,”　IEEE Trans. Antennas Propag., vol. 11, no. 6, pp. 645–651,　Nov. 1963.

[2] R. D. Javor., X.D. Wu and Kai Chang, “Design and performance of a microstrip reflectarray antenna,” IEEE Trans. Antennas Propag., Vol.43, No.9, pp.932–939, Sep. 1995

[3] J. Huang and J. A. Encinar, “Reflectarray Antenna,” in Development History, vol. 2, Wiley, Washington, 2007, pp.9–26.

[4] D. M. Pozar, S. D. Targonski, and H. D. Syrigos, “Design of millimeter wave microstrip reflectarrays,” IEEE Trans. Antennas Propag., vol. 45, no. 2, pp. 287–296, Feb. 1997.

[6] E. Carrasco, M. Barba, Jose A. Encinar, M. Arrebola, F. Rossi and A. Freni, “Design, manufacture and test of a low-cost shaped-beam reflectarray using a single layer of varying-sized printed dipoles, ” IEEE Trans. Antennas Propag., vol. 61, no. 6, pp. 3077–3085, June 2013.