Submitted to: Agriculture
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/5/2016
Publication Date: 11/11/2016
Citation: Pelletier, M.G., Schwartz, R.C., Holt, G.A., Wanjura, J.D., Green, T.R.Frequency domain probe design for high frequency sensing of soil moisture. Agriculture. 6(4):60-72. 2016.
Interpretive Summary: This research reports on the development of an embedded microwave moisture sensing probe that is designed to provide high accuracy results to aid in the establishment of laboratory grade reference standards. The work established that the new probe has superior performance with regard to preservation of the probe's signal integrity, which allows the sensor to extend the typical operating from below 1GHz to above 6 GHz. This wider frequency range opens up the potential for using the probe in low permittivity media, such as seedcotton and cotton lint, that previously didn't provide a strong enough response at the lower frequencies the previous probe design was limited to. In the development of the probe, new techniques were also developed to remove the effects of impedance mismatch from the readings. The correction of impedance mismatch further improves the accuracy of the obtained measurements which is significant for materials that cover a wide span of permittivities as the moisture content changes from dry to saturated conditions that cause the impedance to exhibit a strong impedance mismatch across a significant portion of the moisture regime.
Technical Abstract: Accurate moisture sensing is an important need for many research programs as well as in control of industrial processes. This paper covers the development of a frequency domain sensing probe for use in obtaining measurements of material properties suitable for work ranging from 0 to 6GHz. The probe was developed to cover a wide range of permittivities ranging from a low permittivity of 2.5 to elevated permittivities in excess of 40. Due to such a wide range of permittivities, a 50 Ohm probe was impossible to design, so algorithms were developed to correct for the impedance mismatches along with calibration techniques to obtain an accurate measurement of material that is independent from the probe design.