Measuring the moisture content of the sown wheat soil using a radio pulse at a frequency of 630 MHz

This article presents the results of measuring the moisture content of 6 cm topsoil using a radio impulse with a central frequency of 693 MHz, emitted and received by a log-periodic antenna the radiation pattern of which is oriented on the nadir. Measurements of the reflection coefficient at the center frequency of the radio impulse were carried out from May 25 to July 8, 2020 in an agricultural field sown with wheat in the area of Minino village (56.08676 North Sh., 92.67944 East D.), Krasnoyarsk region. In the course of the experiments, the height of the wheat was from ~ 10cm to ~ 60cm. Radio impulses were formed and recorded using an Agilent N9918A FieldFox vector network analyzer connected to the antenna output. The method for estimating the reflection coefficient consisted in measuring the maximum of the envelope of the radio impulse reflected from the soil cover relative to the maximum of the envelope of the radio impulse reflected from the metal sheet placed on the soil cover. It has been experimentally established that neglecting the influence of wheat vegetation up to 60 cm in height leads to an underestimation of the retrieved soil moisture by 0.05 cm3/cm3. It is shown that a simple model describing the attenuation of a radio impulse in the vegetation cover according to an exponential law, with a determination coefficient of 0.976 and a standard deviation of 0.03 cm3/cm3, makes it possible to retrieved soil moisture in the surface layer of 0-6 cm thick at various stages of wheat growing no more than 60 cm high. The author sees the indisputable advantage of using ultra-wideband pulsed signals, the spectrum of which is concentrated in the megahertz frequency range, for the further development of technology for remote measurement of moisture in the arable layer of agricultural soils.