Measurement And Research Of The Permittivity Based On Dynamic Frequency Sweep

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.