电离层背景大纵横比VLF天线特性数值方法研究

The ground-based VLF antenna has disadvantages of huge equipment, high energy consumption, and being vulnerable to attack. Characterized by small volume, strong mobility, and high efficiency, the study on the spaceborne VLF antennas has significant strategic importance. Considering the difficulty of experimental researches, the numerical simulation of the spaceborne VLF antenna in the ionosphere has been a hotspot in this field. In the ionosphere (anisotropic dispersive medium), the huge difference between the wavelength and the longitudinal size of the antenna make it difficult to analyze the related problems by numerical methods, and traditional numerical methods generally cannot meet the requirements. This project intends to develop the discontinuous Galerkin time-domain (DGTD) method that has gained increasing popularity in more than a decade. The DGTD method will be extended to handle the anisotropic dispersive medium and analyze VLF antenna in this backgound. Furthermore, the parallel DGTD algorithm based on high-order hierachical basis functions will be studied to improve the efficiency and expand its application. Finally, combined with the transmission line theory, the scheme will be presented for calculation of the impedance characteristic, current distribution and propagation characteristic of the VLF antenna in the ionosphere. The results of this project will act as foundation for the design of spaceborne VLF antennas, as well as boosting the research of related numerical methods. It lays the foundations for further simulation of complex antennas.

地基VLF天线体积庞大、能耗惊人，易受攻击。星载VLF天线尺寸小、机动性强，辐射范围广。因而对星载VLF天线研究具有重要的战略意义。由于难以进行实验研究，电离层背景VLF天线的仿真分析成为该领域的研究热点。复杂电离层背景（各向异性色散介质）下波长与天线纵向几何尺寸的巨大差异使得采用数值算法对相关问题分析面临许多困难，传统算法往往难以满足计算需求。本项目拟发展十多年来得到广泛关注的时域非连续伽略金（DGTD）法，使之适用于各向异性色散介质背景天线电磁特性的分析需要。进一步研究基于高阶叠层型基函数的DGTD大规模并行计算方法，提升算法效率，拓展其应用范围。最后，结合传输线理论给出电离层中的线状VLF天线的阻抗特性、电流分布及传播特性的计算方案。本项目研究成果将为星载VLF天线的设计分析提供依据，推进相关数值算法的研究，并为进一步的复杂天线仿真分析打下基础。

本项目旨在基于星载VLF设计分析需求，研究各向异性色散介质背景中大纵横比天线设计相关的数值算法。针对所面临问题深入研究DGTD算法，突破因多尺度网格、吸收边界、天线激励等问题带来的技术难题，使DGTD具备分析大尺度范围内的长VLF天线的能力。以推进相关热点领域的研究。.研究内容包括：.1）各向异性色散介质背景中大纵横比天线设计相关的DGTD算法研究。.2）面向长天线的DGTD与传输线的混合计算研究。.3）电离层VLF天线在大尺度范围内的传播特性研究。.研究中突破了复杂介质背景、细天线快速算法、并行计算等研究难点，完成了研究内容。其中采用电流方程描述细天线，无需建模可进行快速分析，是本项目的关键成果。此外对复杂介质背景（电离层）及并行计算的研究用于VLF天线对地耦合分析，有助于VLF导航通信的进一步研究。.计算成果表明：.1）对特定工作频率，等离子体背景中的天线其长度可以短于自由空间中的天线，符合预期。此外带绝缘层线会稍微降低了其阻抗谐振点。.2）地球磁场对VLF天线工作有较大影响，随着磁场与天线的夹角增大，天线工作电流迅速衰减。换言之，当背景磁场与天线平行时，阻抗状态较好，夹角增大时，阻抗迅速降低。角度垂直几乎无法工作。.3）电离层中的VLF辐射源对地传输中，电磁波主要沿地磁场方向传播，电离层在近地部分逐渐电子浓度变低，这使得电磁波在其他方向的传播逐渐增强。进入中性大气层后出现了明显的波导效应。.研究成果可用于分析简单情形的线天线及地电离层波导，预期在未来进一步深入研究后对更复杂情形的天线或电离层问题进行分析。