Study On The Measuring Theory & Method Of Dielectric Characteristics For Unsaturated Soil

The measurement for the behavior of soil-water-system dielectrics has been increasingly gaining attentions over the past decade because precision-farming technology, which is becoming a new highlight in modern agriculture, relies on accurate field maps of soil characteristics. It is found that the behavior of soil-water- system dielectrics furnishes plenty of useful information concerned with soil physical and chemical properties. This thesis focused on the measurement of the behavior of unsaturated soil-water-system dielectrics using a series of theories and experimental methods referring to dielectric physics, radio engineering, and optical measurement. Furthermore, some important conclusions have been drawn out. In general, the study which is reported in this thesis, can be briefly summarized in the follows: 1. The measuring principle for the determination of soil water content based on standing-wave ratio (SWR) method was approached theoretically as well as experimentally. In particular, a prototype with a novel structure of soil probe used for determining soil water content was developed. Measurement error was limited to % within the O-50m3m3 range of volumetric soil water content. Since the cost of a SWR instrument is considerably lower than that of a time domain reflectometry instrument, it would allow installation of the SWR sensors in large quantities to monitor soil moisture at field scales. 2. The multi-pin soil probe can be viewed as a kind of irregular transmission lines and it is critical to calculate the impedance of this probe in order to get the measurement results with high accuracy. On the other hand, the calculation of the impedance for this probe was a thorny problem due to its odd-shaped geometry. Referring to the theory of the impedance transformation of transmission line a mathematical model has been established. By computing of this model it is proven there exist infinitive break points with a deterministic cycle in the impedance calculation. Moreover, it is found that the characteristic of the impedance for multi- pin probe can act as either a capacitance or an inductance under the different measuring frequencies as well as the pin lengths of the sensor. Therefore, a class of measurement errors, which is caused by so-called multiple-to-one mapping, should be avoided for TDR (Time Domain Reflectrometry), FD (Frequency Decomposition) and SWR (Standing-Wave Ratio) instruments. 3. An optical measurement method, which depends on the principle of the polarization of reflect light and belongs to non-contact methods, was also discussed to determine the dielectrics of soil-water-system in this thesis. An important formula for calculating the polarization of reflect light under the continuous measurement process was obtained. In addition, an experimental device based virtual instrument was designed and a new algorithm with Fourier spectrum decomposition was employed as a powerful tool to filter all kinds of noises during the test process. Unfortunately, the experimental results show that there are some unavoidable defects, such as the scatter and the penetration limit for incident light, in this measurement method. 4. As for the measurement of dielectric loss of soil-water-system, the four-wire method was analyzed theoreticall...