In Situ Soil Moisture Monitoring Using Compact Multiband Sensing System for Smart Agriculture

In this article, a compact dual-band soil moisture sensor (SMS) is presented for precision farming. The presented sensor operates at low resonance frequencies (500 and 900 MHz), allowing it to efficiently cover a substantial volume under test. This ensures cost-effectiveness by minimizing the number of sensors required for the farming area. The higher band is suitable for homogeneous soil, whereas the lower frequency can penetrate more deeply, allowing for in-depth analysis of soil. The proposed sensor consists of a modified patch along with two parasitic patches to achieve dual bands and a three-turn complementary spiral resonator (CSR-3) for miniaturization, with dimensions of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$50 \times 50\,\mathbf{m}{{\mathbf{m}}^2}$</tex-math></inline-formula>. Three varactor diodes have been integrated across the CSR-3 to achieve frequency switching between the two frequency bands. A directional coupler (DiC) is utilized with the SMS to realize a dual-port structure and a 20 dB gain amplifier is also integrated at the input to compensate for DiC coupling and free space losses. The integration of sensor, DiC, and amplifier realizes a dual-band in situ monitoring system. The compact and planar geometry of the proposed structure, together with high sensitivity and low resonance frequencies, makes the design suitable for smart farming.