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United States Department of Agriculture

Agricultural Research Service

Research Project: SENSING METHODS & INSTRUMENTATION FOR RAPID DETERMINATION OF MOISTURE CONTENT... AND OTHER QUALITY ATTRIBUTES OF GRAINS, SEEDS, & NUTS

Location: Quality and Safety Assessment Research Unit

Title: Measuring the complex permittivity of grain by the free-space technique with a thru-reflect-match calibration procedure

Authors
item Roelvink, Jochem
item Trabelsi, Samir

Submitted to: National Radio Science Meeting
Publication Type: Abstract Only
Publication Acceptance Date: March 9, 2012
Publication Date: April 8, 2012
Citation: Roelvink, J.T., Trabelsi, S. 2012. Measuring the complex permittivity of grain by the free-space technique with a thru-reflect-match calibration procedure. National Radio Science Meeting.Proceedings, P.75.

Interpretive Summary: The moisture content of grain is one of the most important quality indicators for this produce. The complex permittivity of grain is known to correlate strongly with moisture content. By measuring the dielectric properties of grain, the moisture content can be determined. Many techniques have been developed for material permittivity determination.Some examples of these are: resonance techniques, which are based on the measurement of the change in the resonant properties of a cavity with and without the material under test, and transmission line techniques, which are based on the measurement of the material/line scattering parameters. Free-space transmissionbased microwave sensors are known to be capable of accurate, nondestructive,measurements of low- to medium-permittivity agricultural products, such as grain (S. Trabelsi, A. Kraszewski, and S. O. Nelson, IEEE Trans. Instrum. Meas., 47, 1998). These sensors determine the grain permittivity from measurements of the transmission coefficient of a sample holder with and without the grain sample.The measurement of the empty sample holder is akin to a ‘response’ calibration, which is known to be less accurate than full two-port calibration procedures,particularly for low-loss transmission lines and reflection coefficient measurements. It is of interest to compare moisture content predictions from permittivity measurements obtained with a simple response calibration to those obtained from a full two-port calibration. In this paper, we use a vector network analyzer and a free-space thru-reflectmatch(TRM) calibration procedure presented in (N. Gagnon, et al, IEE Proc.Microw. Antennas Propag., 151, 2004) to measure the two-port scattering parameters of grain samples for a range of moisture contents and frequencies. The permittivity of the grain sample is determined using a numerically stable procedure (J. Baker-Jarvis, E. Vanzura, and W. Kissick, IEEE Trans. Microwave Theory Tech., 38, 1990) that relates the measured scattering coefficients to the propagation coefficient within the material. A method for resolving the phase ambiguity in the propagation coefficient is presented. Results for the permittivity calculated using the two-port approach are compared to those calculated using the simplified transmission-only measurement, and it is shown that the full two-port procedure produces accurate results for a wider range of grain moisture contents.

Technical Abstract: The moisture content of grain is one of the most important quality indicators for this produce. The complex permittivity of grain is known to correlate strongly with moisture content. By measuring the dielectric properties of grain, the moisture content can be determined. Many techniques have been developed for material permittivity determination.Some examples of these are: resonance techniques, which are based on the measurement of the change in the resonant properties of a cavity with and without the material under test, and transmission line techniques, which are based on the measurement of the material/line scattering parameters. Free-space transmissionbased microwave sensors are known to be capable of accurate, nondestructive,measurements of low- to medium-permittivity agricultural products, such as grain (S. Trabelsi, A. Kraszewski, and S. O. Nelson, IEEE Trans. Instrum. Meas., 47, 1998). These sensors determine the grain permittivity from measurements of the transmission coefficient of a sample holder with and without the grain sample.The measurement of the empty sample holder is akin to a ‘response’ calibration, which is known to be less accurate than full two-port calibration procedures,particularly for low-loss transmission lines and reflection coefficient measurements. It is of interest to compare moisture content predictions from permittivity measurements obtained with a simple response calibration to those obtained from a full two-port calibration. In this paper, we use a vector network analyzer and a free-space thru-reflectmatch(TRM) calibration procedure presented in (N. Gagnon, et al, IEE Proc.Microw. Antennas Propag., 151, 2004) to measure the two-port scattering parameters of grain samples for a range of moisture contents and frequencies. The permittivity of the grain sample is determined using a numerically stable procedure (J. Baker-Jarvis, E. Vanzura, and W. Kissick, IEEE Trans. Microwave Theory Tech., 38, 1990) that relates the measured scattering coefficients to the propagation coefficient within the material. A method for resolving the phase ambiguity in the propagation coefficient is presented. Results for the permittivity calculated using the two-port approach are compared to those calculated using the simplified transmission-only measurement, and it is shown that the full two-port procedure produces accurate results for a wider range of grain moisture contents.

Last Modified: 11/22/2014
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