Relationship between the impedance of a multi-pin probe and soil water content
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Research Centre for Precision Farming, China Agricultural University, Qinghua Donglu 17, P.O. Box 63, Beijing 100083, China
Institut für Landtechnik, der Rheinischen Friedrich-Wilhelms-Universität, Nussallee 5, D-53115 Bonn, Germany
Acceptance date: 2003-02-10
Int. Agrophys. 2003, 17(1): 35-40
Based on the theory of the impedance-transform of transmission line, a mathematical model for calculating the impedance of a soil probe with a multi-pin structure was developed. It is found that the impedance of the multi-pin probe may be turned into an inductive-reactance from a capacitive-reactance and vice versa as measurement conditions change. Moreover, the formula for the impedance of a soil probe has a series of periodical discontinuous dots. Three necessary experiments to verify this model were conducted in a laboratory environment. By changing the measurement frequency (50�200 MHz), the length of the pin (5�15 cm) and the number of pins (2�5), some electric properties of the multi-pin probe were confirmed. As an application of the model, a prototype was designed and it achieved a measurement accuracy of volumetric soil water content within ą2% ranging from 0 to 50%.
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