High-Sensitivity and Compact Time-domain Soil Moisture Sensor Using Dispersive Phase Shifter for Complex Permittivity Measurement

TL;DR

This paper introduces a highly sensitive, compact time-domain soil moisture sensor using a dispersive phase shifter, capable of accurately measuring soil permittivity and moisture content for precision agriculture and IoT applications.

Contribution

The paper presents a novel soil moisture sensor based on a dispersive phase shifter with improved accuracy and sensitivity, along with a detailed design guide and validation through simulations and experiments.

Findings

Accuracy better than ±1.2% at 30% VWC for sandy soil

Sensor exhibits low profile, low power consumption, and high sensitivity

Validated theoretical analysis with measurement results

Abstract

This paper presents a Time-Domain Transmissometry Soil Moisture Sensor (TDT-SMS) using a Dispersive Phase Shifter (DPS), consisting of an interdigital capacitor that is loaded with a stacked 4-turn Complementary Spiral Resonator (S4-CSR). Soil moisture measurement technique of the proposed sensor is based on the complex permittivity sensing property of a DPS in time domain. Soil relative permittivity which varies with its moisture content is measured by burying the DPS under a soil mass and changing its phase difference while excited with a 114 MHz sine wave (single tone). DPS output phase and magnitude are compared with the reference signal and measured with a phase/loss detector. The proposed sensor exhibits accuracy better than +-1.2 percent at the highest Volumetric Water Content (VWC=30 percent) for sandy-type soil. Precise design guide is developed and simulations are performed to achieve a highly sensitive sensor. The measurement results validate the accuracy of theoretical analysis and design procedure. Owning the advantages of low profile, low power consumption, and high sensitivity makes the proposed TDT-SMS a good candidate for precision farming and IoT systems.