Affordable microwave soil moisture detector

Monitoring the content of soil moisture is crucial in defining soil conditions and irrigation that affect agricultural efficiency. Accordingly, inexpensive and reliable devices are essential. Electromagnetism could be exploited for quantification of soil moisture, and in particular the real and imaginary part of the scattering parameter (S11). Several sensors have been developed, but some limits were encountered, such as interference due to soil constituents and a cost that is not always affordable. We developed an inexpensive cheap and rapid instrumental chain based on the response of a cavity antenna in the frequency range from 1.5 to 3 GHz. Very low-cost components were assembled and used: a miniaturized commercial vector network analyzer and a homemade cavity antenna. The antenna should be placed in contact with soil without being inserted. Tests were carried out directly in the field with a moisture content ranging from 1% to 45%. Starting from spectral variables (real and imaginary part of S11), partial least square regression models have been developed for estimation of moisture content (%). The main results show R2 values of 0.872 with root mean square error) of 4.1% (imaginary part, segmented validation). In light of the results obtained, the user-friendly device developed can be very attractive for final users also in terms of affordability.