Calibration of a waveguide sensor for measuring soil moisture

An open-ended waveguide is calibrated for use as a soil moisture sensor. The calibration procedure models the waveguide as a microwave network and uses an optimization procedure to calculate network parameters which minimize errors between measured and calculated response curves. The test load is formed by terminating the waveguide with a two-layered sand profile whose top layer thickness is incrementally adjusted to vary the output load. The output impedance of the waveguide is approximated from intrinsic impedances of the wave propagation through layered media. This approximation produced excellent agreement between reflection responses calculated from the network calibration parameters and those observed for a wide range of output loads