【作者】 陈蕊；

【导师】 卢智远；

【作者基本信息】 西安电子科技大学 ， 生物医学工程， 2009， 硕士

【摘要】 近年来,随着微波遥感技术和微波电路的日益发展,材料介电性能参数的研究和测量成为人们关注的课题。介电常数的精确测量能够给科学家和工程师提供非常有价值的信息,其测量方法很多,本论文重点介绍谐振腔微扰法。本论文介绍了介质材料的两个电磁特性参数—介电常数与磁导率,并讨论了几种常用的测量方法,谐振法中的谐振腔微扰法测量介电常数具有比较高的测量精度,且使用样品少,但不适宜宽频带测量和损耗较大的介质材料测量;传输线法限于低、中损耗介质材料的测量,测量精度不如谐振腔法;自由空间波法适用于介电常数的宽频带测量,可完成非损伤性和非破坏性的测试,与上述几种方法比较,自由空间波法更适宜实地测量。通过分析矩形波导的波动方程和边界条件,得到矩形谐振腔中电磁场分量的表达式,从而为分析和测量介质材料的介电常数奠定了理论基础。谐振腔的主要指标参数是谐振频率f₀、谐振波长λ₀、品质因数Q₀和等效电导G₀ ,在此基础上,做出了测量系统选用的三公分矩形谐振腔的模式图,使系统的设计工作清晰、便捷。论文着重阐述了如何利用动态扫频谐振腔微扰法,实施一腔多模的技术,拓宽频带测量介电常数。本论文选用高校试验中常用的矩形谐振腔,设计建立了由扫频信号源、检波器以及示波器组成的测试系统,并运用Matlab强大的计算绘图功能做出矩形谐振腔的谐振模式图,为了保证测量精度,谐振腔在各个频点上应是单模谐振的,结合谐振模式图,选用了谐振腔TE_{10 p}的前五个模式,即p = 1,2,3,4,5。在谐振腔微扰法的理论基础上,将样品放在TE_{10 p} （为奇数）矩形谐振腔的中央,该点电场最强而磁场为零,这样就可以获得电效应,用来测量介电常数;若将样品放在TE_{10 p} （为偶数）谐振腔的中央,该点磁场最强而电场为零,这样就可以获得磁效应,用来测量磁导率。本论文主要测量介质材料的介电常数,所以只需要观察动态扫频显示的奇数模即可。在以上的研究基础上对树枝的介电常数进行测量,并对结果进行了讨论,介电常数能部分反映生物的电磁特性。结果表明,本论文的测量系统是准确的,测量的结果对研究树木的生态特性和生长状态提供了科学的依据,也为其他的研究人员提供了可靠的数据。同时,也拓展了高校的试验研究范围,为高校学生的研究试验提供了切实可行方案。更多还原

【Abstract】 In the last year, with the rapid development of remote sensing technique by microwave and microwave circuit, the study on the measurement and mechanics of microwave dielectric constant of material has become a concerned topic. Precise measurement of dielectric constant could afford much useful information to scientists and researchers. Among many measurement methods, resonant cavity is mainly discussed in the paper.In this paper, firstly, two important parameter-dielectric constant and magnetic constant are introduced. There are many measurement methods ofε, such as the method of circuit, the method of transmission line, the method of resonant cavity and the method of space wave in microwave frequency. The method of circuit is suitable for measuring materials with low frequencies and high loss. The method of resonant cavity, especially perturbation method can perform well when measuring materials with low loss and small samples but not high loss and wide frequency band. The method of transmission line can just measure materials with low and suffer loss. The method of space wave in microwave frequency can be used for the measurement of dielectric constant and wide frequency band which can complete tests with nondamnification and no destruction. Comparison among these methods above all, the method of space wave in microwave frequency is the best one for real measurement.The expression of electromagnetic component of rectangular resonant cavity can acquire by analyzing the equations of rectangular wave guide and conditions of boundary, which lays the foundation of measurement. There are four main parameters of resonant cavity, resonant frequency f₀, resonant wave lengthλ₀ , quality factor Q₀ and equivalent conductance G0. Based on these, the mode chart of rectangular resonant cavity can be plot, which makes the design of system more clear and convenient.This paper states how to use dynamic frequency sweep-resonant cavity perturbation method to measure dielectric constant and actualize one cavity-many modes technique. The rectangular resonant cavity which is usually used in college is chosen to build a testing system which consists of the sweeper, detector, and oscilloscope and so on. The resonant frequencies of varies modes in the resonant cavity have been drawn by using Matlab software, which makes our work more rapid and convenient. In order to ensure the high precision of measurement, resonant cavity works at single mode, the first five modes of TE_10p（p = 1,2,3,4,5） have been chosen based on the modes chart. According to resonant cavity perturbation theory, the electric effect can be obtained to measure the dielectric constant when the sample is set in the middle of rectangle resonant cavity of mode of TE_10p（ p is the odd number）; in which electric field is the strongest and the magnetic field is zero. The magnetic effect can be obtained to measure the magnetic constant when the sample is set in the middle of rectangle resonant cavity of mode of TE_{10 p} （ p is the even number）, in which electric field is zero and the magnetic field is the strongest. We only need observe the resonant curve of odd mode since measuring the dielectric constant of material.Based on above, electromagnetism characters of biology can be reflected by the dielectric constant. The dielectric constant of tree is measured and analyzed in this paper, the results tested under the system show that the system works at high accuracy, and provide scientific basis for tree research and data for others. At the same time, it has developed experiment in college and provided the practical and feasible plan for students.更多还原