使用超穎材質以降低特定吸收比及交互方向內隱式時域有限差分法之穩定度改善

Abstract

目前行動通訊裝置使用人數日與俱增，而人們也開始關心手機電磁輻射對人體的影響。其中，藉由特定吸收效率可以瞭解人頭電磁能量吸收的多寡。目前，時域有限差分法已應用於人頭特定吸收效率的計算。最近，超穎材質由於具有特殊物理特性而引起研究人員對其興趣。超穎材質為人造材質，由於其介電係數與導磁係數為負值，因此超穎材質電磁傳輸特性不同於一般材質。 在本研究中，我們將使用超穎材質以減低人頭與天線間交互電磁現象。首先，我們利用時域有限差分法結合Drude模型來模擬超穎材質。在模擬中，超穎材質置於天線與人頭間。從模擬結果可知，藉由擺放超穎材質可以有效減低人頭的特定吸收效率。我們也探討擺放超穎材質對於天線影響。藉由適當擺放超穎材質，超穎材質對於天線輻射能量和天線場型影響不大。而我們進一步探討擺放位置，尺寸大小，超穎材質介質係數對於減低特定吸收效率的影響。超穎材質可以藉由設計分離式環形共振器來實現。在本研究中，我們設計分離式環形共振器，使其工作頻率為900 MHz和1800 MHz。設計流程也將詳細描述。我們將設計的分離式環形共振器置於天線與介質體中間，從結果可以發現介質體的特定吸收效率將會減低。此研究可以提供減低特定吸收效率的方法。 在研究中，我們也發展無條件穩定ADI-FDTD模擬方法。我們發現，當使用吸收邊界於ADI-FDTD時，可能會造成演算法不穩定問題。首先，我們探討Mur吸收邊界於ADI-FDTD的數值穩定分析。在此演算法中，電磁波傳播方向將會影響該演算法穩定度。從模擬結果可知，該演算法只有在波行進方向為0度，45度，90度時會穩定。我們也推導出該演算法數值色散關係式。發現此演算法數值不穩定是無法改善。接著，我們探討應用分離場完美匹配層於ADI-FDTD的穩定度分析。穩定度理論分析可以藉由推導穩定矩陣實現。我們發現應用分離場完美匹配層於ADI-FDTD會造成數值不穩定結果。完美匹配層中電導會影響該演算法穩定度。因此，我們提出改良完美匹配層電導，以改善該演算法穩定度。最後藉由數值模擬可以驗證分離場完美匹配層和Mur吸收邊界於ADI-FDTD的數值不穩定效應。CN-FDTD為另一種無條件穩定演算法。ADI-FDTD與CN-FDTD差別只是二階近似項。在本研究中，我們將分析分離場完美匹配層，非分離場完美匹配層，複數頻率轉換完美匹配層對於ADI-FDTD與CN-FDTD的影響。我們發現ADI-FDTD不穩定是二階近似項所造成。藉由此研究可以提供未來發展應用於ADI-FDTD簡潔穩定的完美匹配層。由於在ADI-FDTD中，其時間步階不受穩定法則限制，故該演算法非常適合模擬超大型積體電路。藉由改良完美匹配層，我們發現所提出的演算法可以有效率以及準確模擬超大型積體電路時域與頻域電磁特性。 在多天線系統中，增加天線隔絕度為重要參數。在本研究中，藉由在兩天線間增加一耦合元件，可以有效增加隔絕度。此耦合元件優點為不增加額外天線面積。在研究中，耦合元件尺寸對於天線隔絕度效率與共振頻率也將做進一步探討，經由天線量測結果，所提出的耦合元件可以增加天線15dB的隔絕度，增加耦合元件對於天線場型亦只有減少1dB增益以內的影響

The use of the mobile devices has been growing rapidly in the global communities. The influence of electromagnetic (EM) waves from cellular phones on the human head has been widely discussed recently. The specific absorbing rate (SAR) is a defined parameter for evaluating power deposition in human tissue. The finite-difference time-domain (FDTD) is widely used to study the peak SAR in the human head. Recently, metamaterials have inspired great interests in their unique physical properties and novel application. Metamaterials denote artificially constructed materials having electromagnetic properties not general found in nature. Two important parameters, electric permittivity and magnetic permeability determine the response of the materials to the electromagnetic propagation. In this work, we use the metamaterials to reduce the EM interaction between the antenna and human head. Preliminary simulation of metamaterials is performed by FDTD method with lossy Drude model. The metamaterials are placed between the antenna and human head. From the simulation result, it is found that the peak SAR in the human head can be reduced with the placement of metamaterials. We also study the antenna performance with metamaterials. The antenna radiated power and antenna pattern can be less affected with placement of metamaterials properly. The effects of placement position of metamaterials, metamaterials size, and the medium parameters of metamaterials on the SAR reduction effectiveness are investigated. The metamaterials can be constructed from split ring resonators (SRRs). In this work, we also design the SRRs operated at 900 MHz and 1800 MHz. The design procedure of the SRRs is described. The designed SRRs are placed between the antenna and a dielectric cube. It is found that the peak SAR in the dielectric cube is reduced significantly. This study can provide useful methodology for SAR reduction. In this work, we develop the alternating direction implicit (ADI) finite-difference time-domain (FDTD) method. However, when employing the absorbing boundary conditions (ABCs) for ADI-FDTD method, this scheme can lead to instability. First, the stability analysis of the Mur’s ABC for ADI-FDTD method is also studied. The effect of the wave propagation direction on the stability of this scheme is investigated. It is found that this scheme can be stable only when the incident wave directions are 0 degree, 45 degree, and 90 degree. We also derive the dispersion relation of this scheme. The instability of this scheme can not be avoided. Then, the stability analysis of split-field perfectly matched layer (PML) for ADI-FDTD is studied. The theoretical stability analysis of this scheme is performed by deriving the amplification matrix. It is found that the split-field PML scheme for ADI-FDTD method will be unstable. The effect of the PML conductivity profile on the stability of this scheme is studied. We propose the modified PML conductivity profile to improve the stability of this scheme. Finally, numerical simulations are performed to validate the instability of the split-field PML and Mur’s ABC for ADI-FDTD method. The Crank-Nicolson FDTD (CN-FDTD) is also an unconditionally stable scheme. The difference between the ADI-FDTD and CN-FDTD is the second order perturbation term. In this work, the stability analysis of split-field PML and unsplit-field PML for ADI-FDTD and CN-FDTD are studied. It is found that the instability of PML schemes for ADI-FDTD is due to the perturbation term. This study can provide information to develop a simple and stable PML scheme for ADI-FDTD in future work. The ADI-FDTD can simulate the VLSI circuits effectively since the time step is not restricted by the Courant stability condition. The modified PML scheme for ADI-FDTD method is employed to simulate the VLSI circuits. It is found that the proposed scheme can model the time domain and frequency domain electromagnetic characteristics of VLSI circuits accurately and effectively. A coupling element to enhance the isolation between two closely packed antennas for 2.4 GHz wireless local area network (WLAN) application is introduced. The proposed structure occupies two antenna elements and a coupling element in between. By putting a coupling element which artificially creates an additional coupling path between the antenna elements, the antenna isolation can be enhanced. The advantage of this design is that no extra space is needed for antenna elements. With the proposed design, more than 15 dB isolation can be achieved for two parallel individual planar inverted F antennas (PIFAs) with 5 mm spacing. Parametric studies for the design are also included to show how to increase isolation bandwidth and control the isolation frequency.