基於雙頻電磁 FSS 的線性至圓形偏振變換板的設計與 分析

Abstract

在天線領域，主要的設計參數之一是所謂的電磁（EM）波的偏振。這個參數的選擇通常取決於它所要使用的應用類型。因此，在導航系統、成像和遙感或衛星通信（SATCOM）等應用中，由於圓極化（CP）相對於線性極化（LP）的優勢，通常更願意使用圓極化。其中我們可以包括對多徑衰落的更大的免疫力和減少偏振失配的影響。此外，在SATCOM中，使用CP可以減少對法拉第旋轉的敏感性，法拉第旋轉是在電磁波通過電離層傳播時發生的，對鏈路平衡有相當大的影響。這個項目是由新一代移動通信領域的衛星應用所激發的。此外，在SATCOM應用中，天線經常需要在兩個不同的和不相鄰的頻段工作。此外，經常需要這些頻段具有正交極化，以抵消SATCOM的高干擾敏感性。

為了符合上述要求，一種可能性是使用CP天線或其他類型的天線系統，這些天線具有復雜的饋電網絡，通常昂貴而笨重，或者效率不足。除了這種策略外，還有一些機制可以將LP轉化為CP，無論是在傳輸或反射模式下。使用這種機制並不特別新穎。然而，基於頻率選擇表面（FSSs）和/或元表面（MSs）的結構用於這一功能，近年來經歷了巨大的增長。

因此，這篇碩士論文致力於設計、優化、測量和分析一個基於FSS的面板，用於雙頻傳輸模式下的LP-CP極化轉換。該建議旨在X-和K-波段區域內工作，中心頻率分別定義為8.2和24.5GHz。這些中心頻率的比例為1:3，大大高於文獻中可以找到的這類設計的例子的比例。還值得一提的是，傳輸模式固有的困難，由於其在軸向比率（AR）、帶寬（BW）方面的性能較差，或者需要使用較多的層數來達到設計要求，使得它比反射式的同類產品更笨重，所以文獻中對其分析較少。然而，它的舒適性和使用的便利性使它更容易被整合到所有類型的應用中，所以值得深入研究這個領域。

該項目從根本上分為三個部分：對技術狀況和理論背景的分析，設計和優化方案，以及系統的製造和測量。第一部分涉及對該主題的技術狀況和背後的理論的廣泛分析，這為設計階段提供了信息。在這個分析中，重點放在從該主題的最基本特徵，如設計參數的操作和定義以及FSS的一般分析，到LP到CP偏振轉換的其他更具體的方面，通過FSS的類型，多波段技術，或單元格（UC）的相對尺寸對應用的工作波長的影響。其次，設計階段從一系列受分析文獻啟發的建議開始。設計過程涉及使用多種軟件進行數值模擬。在放棄了那些不太有希望的拓撲結構後，最終選定了一個。它是一種基於條狀各向異性的多層結構，具有修改過的晶格，有可能滿足最初提出的要求。該拓撲結構由兩層羅傑斯RO3006基片和其外表面的兩層銅條組成。此外，由於在設計中提供了更大的靈活性，所以選擇了使用間隔物來分離基片。為了最終完成設計階段，對提議的表面尺寸進行了優化，以測試是否可以達到所需的性能參數。為此，進行了兩方面的工作。一方面，使用商業軟件進行優化，試圖根據優化本身的進展來調整優化功能和輸入參數。同時，使用非商業軟件在多台機器上進行並行優化，目的是節省計算時間。此外，該軟件在設置函數、參數和優化過程的要求方面提供了更大的自由度，還允許將製造過程的公差納入優化方法本身。第三，設計製作了消聲室中的測量。最後，對獲得的結果進行了審查，允許檢查該設計的操作能力，重點是其主要的優點和缺點。此外，這些結果還與文獻中的一些現有設計進行了比較。

總的來說，本論文的貢獻在於提出了一種新穎的基於FSS的面板設計，它可以同時工作在兩個遙遠的頻段，並在每個頻段將LP波轉化為具有正交偏振的CP波。

In the field of antennas, one of the main design parameters is the so-called polarization of the electromagnetic (EM) wave. The choice of this parameter generally depends on the type of application for which it is intended to be used. Thus, in applications such as navigation systems, imaging and remote sensing, or satellite communications (SATCOM) it is often preferable to use circular polarization (CP) due to the advantages it provides as opposed to linear polarization (LP). Among them we may include greater immunity to multipath fading and reduction of the impact of polarization mismatch. In addition, in SATCOM, the use of CP allows reducing the sensitivity to Faraday rotation that occurs during the propagation of the EM wave through the ionosphere, which can have a considerable effect on the link balance. This project is motivated by satellite applications in the area of new generation mobile communications. Moreover, in SATCOM applications it is frequently necessary for the antenna to work in two different and non-adjacent frequency bands. In addition, it is often required that these bands have orthogonal polarizations to counteract the high interference sensitivity of SATCOMs.

In order to comply with the above requirements, one possibility is the use of CP antennas or other types of antenna systems with complex feeding networks, often expensive and bulky, or insufficiently efficient. In addition to this strategy, there are mechanisms that allow transforming LP into CP, whether in transmission or reflection mode. The use of such mechanisms is not particularly novel. However, the use of structures based on frequency selective surfaces (FSSs) and/or metasurfaces (MSs) for this function has undergone an enormous growth in recent years.

Consequently, this Master's Thesis is devoted to the design, optimization, measurement, and analysis of a FSS-based panel for LP-to-CP polarization transformation in dual-band transmission mode. The proposal aims to work within the X- and K-band regions, with center frequencies defined at 8.2 and 24.5 GHz, respectively. These center frequencies present a ratio of 1:3, considerably higher than the ratios of the examples that can be found in the literature for this type of design. It is also worth mentioning the difficulties inherent to the transmission mode, less analyzed in the literature due to its worse performance in terms of axial ratio (AR), bandwidth (BW), or the need to use a higher number of layers to achieve the design requirements, making it bulkier than their reflective counterparts. However, its comfort and convenience of use make it more easily integrable in all types of applications, so it is worth delving into this area.

The project is fundamentally divided into three parts: analysis of the state of the art and the theoretical background, design and optimization of the proposal, and manufacturing and measurement of the system. The first one involves an extensive analysis of the state of the art of the topic and the theory behind it, which informed the design phase. In this analysis, emphasis is placed on aspects ranging from the most basic features of the subject, such as the operation and definition of design parameters and general analysis of FSSs, to other more specific aspects for LP-to-CP polarization conversion, passing through FSSs types, multi-band techniques, or the impact of the relative size of the unit cell (UC) with respect to the working wavelengths of the application. Secondly, the design phase begins with a series of proposals inspired by the analyzed literature. The design process has involved the use of numerical simulations using multiple softwares. After discarding the less promising topologies, the final one is selected. It is a strip-based anisotropic multi-layer structure with a modified lattice, which has the potential to meet the initially proposed requirements. The topology consists of two layers of Rogers RO3006 substrate and their two layers of copper strips on the outer faces. In addition, the use of spacers to separate the substrates has been chosen due to the greater flexibility that it provides in design. To finalize the design stage, optimization of the proposed surface dimensions is carried out to test whether the desired performance parameters could be achieved. To this end, two lines of work are conducted. On the one hand, an optimization using commercial software, trying to adjust the optimization function and the input parameters according to the progress of the optimization itself. Simultaneously, non-commercial software is used to parallelize the optimization on multiple machines, with the aim of saving computational time. In addition, this software provides greater freedom in setting functions, parameters and requirements for the optimization process, also allowing the manufacturing process tolerances to be incorporated into the optimization approach itself. Thirdly, the design is fabricated for measurement in an anechoic chamber. Finally, the results obtained are reviewed, allowing to examine the operational capabilities of the design, with emphasis on its main advantages and disadvantages. In addition, the results are compared with some of the existing designs in the literature.

Overall, this thesis contributes by presenting a novel design of an FSS-based panel that can operate in two distant frequency bands simultaneously, and transform LP waves into CP waves with orthogonal polarizations in each band.