Applications of Optimization Techniques to Self-Structuring Electromagnetic Scatterers and Antenna Performance Enhancements

本論文提出三種創新的電磁應用以改善傳統架構之限制並提升工作效率。藉 由最佳化方法的智能輔助，吾人所提出之精密結構得以實現，而複雜的可重組元 件之合成問題亦得以更有效率地解決。本論文所使用之最佳化方法包含實驗計畫 法與演化式演算法，分別應用於下列創新元件中。 吾人首先提出一款適用於無線射頻標籤系統之新型雙天線標籤架構。此新型 架構使用兩支獨立工作之天線，一支專職接收來自讀取機之訊號與功率，另一支 專職將所載資料後散射回讀取機。若妥善將接收天線之輸入阻抗與後級整流電路 之晶片阻抗設計為共軛匹配，接收天線便能連續接收來自讀取機之功率，使標籤 電路之供電更為穩定；此外，若將後散射天線於開路與短路間切換，並將其輸入 阻抗設計為純實數，則後散射天線能回傳最大之散射訊號差給讀取機，可大幅提 升系統之讀取距離及讀取可靠性。由於雙天線架構須整體考慮所有設計目標，並 降低兩支天線之互耦合量，因此吾人利用實驗計畫法來掌握多目標與天線幾何參 數間的函數關係，於0.1 λ 0 × 0.1 λ 0 的面積下成功實做出此雙天線架構。此新型標籤 之效能經實驗佐證，可大幅改善傳統結構下之接收及後散射限制。 其次，吾人發展出網格化天線自動設計程式。此設計工具整合全波電磁模擬 軟體以及數種單目標及多目標演化式演算法，當天線設計情境為給定設計面積並 須考量周圍環境時，此工具僅需將設計面積切割為若干網格，就能找出工作目標 下最適合的天線幾何形狀。吾人以一多輸入多輸出天線系統來驗證多目標最佳化 方法的效能，並針對實際應用中的頻寬考量設計一更有效率的演算法，藉以改善 傳統方法的限制。此方法以手機天線設計為例，設定工作目標為同時涵蓋698–960 兆赫以及1710–2170 兆赫，其最佳化結果經模擬及量測佐證後，證實所提出之策 略確實比傳統方式更勝任多頻且寬頻的工作目標。 最後，吾人提出一款創新之自組式電磁散射體。此自組式電磁散射體為首創 之智能散射平面；它能根據下達指令自行調整其電氣形狀，完成雷達截面積最小 化或最大化等工作目標。此自組式電磁散射體利用自組式元件之原理，使用多枚 繼電器連接細長金屬片；當繼電器各自於開、關兩狀態間切換，數十億種散射組 態因應產生。藉由適當之搜尋演算法來尋找各工作目標下之最佳開關組態，雷達 截面積特性得以隨心所欲地控制。吾人提出一創新之搜尋演算法，利用部分因子 實驗設計來估計各開關之作用以及開關間的交乘影響，能極有效率地解決此合成 求解問題。吾人亦實做出此自組式電磁散射體之量測系統，以實驗佐證雷達截面 積之最小化與最大化效能，證實它能自行重組為多角度之吸收體或增強反射面。In this dissertation, three innovative electromagnetic (EM) applications are proposed to improve the efficiency and limitation of conventional employments. By using the intelligence of optimization methodologies, including design of experiments (DOE) and evolutionary algorithms, complex design processes and arduous synthesis problems are simplified and solved, and the proposed applications exhibit powerful and sophisticated capabilities which satisfy the original need. The first application is a novel dual-antenna structure for passive radio-frequency identification (RFID) tags. It is formed by two linearly tapered meander dipole antennas that are perpendicular to each other and connected to the slightly modified tag chip. One of the antennas is for receiving, while the other is for backscattering. The input impedance of the receiving antenna is designed to be conjugate matched to the highly capacitive chip impedance for the maximum power transfer. Meanwhile, the backscattering antenna is alternatively terminated by an open or a short circuit to modulate the backscattered field. By making the input impedance of the backscattering antenna real-valued, the maximum differential radar cross section (RCS) may be achieved leading to a longer read range and better reliability. With the aid of DOE, the proposed dual-antenna structure is designed to fit within a compact area of 32.8 × 32.8 mm2 while keeping relatively low mutual coupling between the two antennas. The impedance, receiving, and backscattering performances of the proposed dual-antenna structure are measured and simulated, and they agree very well. Also, it is demonstrated that the proposed dual-antenna structure outperforms the conventional single-antenna tag design in every respect. The second application is a competent antenna design tool based on the pixelized design technique. Merely with only a roughly-formed solution domain, this pixelized design tool is capable of automatically finding an antenna layout with performance satisfying the design needs. The pixelized design tool integrates a full-wave simulator and external optimization schemes, including various single-objective and multiobjective evolutionary algorithms. The capability of multiobjective operations is demonstrated by a multiple-input-multiple-output (MIMO) antenna system for handset applications, where the impedance matching of each antenna should be optimized and the mutual coupling between them should be minimized. In addition, an innovativeapproach for designing wide- and multi-band antennas within a small area is proposed and incorporated into this tool. The proposed method is verified through a handset antenna design, covering 698–960 MHz and 1710–2170 MHz. The simulated and measured results confirm that the proposed method can find an antenna configuration with satisfactorily wide bandwidth and outperforms the conventional approaches. The third application is a self-structuring electromagnetic scatterer (SSES). The SSES is the first intelligent reflective surface that can alter its electrical shape to fulfill various operational objectives, such as RCS reduction or RCS enhancement. The SSES template comprises segments of metallic thin strips interconnected via voltage-controlled switches. By opening or closing the switches, the phase of the field scattered by the strips changes, resulting in destructive or constructive interference in the total scattered field. The RCS of the SSES can thus be controlled. An efficient search algorithm based on the fractional factorial design of experiments (FFD) is adopted to find a suitable switch configuration for the SSES system. A SSES prototype was built and a series of RCS measurements were performed to demonstrate its capability to adaptively control the RCS. It is shown that the bistatic RCS can be significantly reduced in any specified direction and that the main beam maximum of the RCS pattern can be enhanced and steered within an angular range of 30 degrees