Humidity in sandy soil measured by passive, wireless, and resonant sensor with bifilar coil

Abstract

In this paper a passive, wireless, and resonant (PWR) sensor for monitoring soil humidity is presented. This sensor employs a bifilar coil and it is compared with the classical PWR sensor with interdigital capacitor. Besides that, a theoretical model is developed to evaluate the influence of medium in sensor's response. A resonant circuit is used to detect the humidity variation. In a setup for 300 g of fine sand, where sensor's responses were evaluated for dry and wet sand, tests show a linear correlation of 0.7195 and a sensitivity of 252 kHz / % between resonant frequency and fresh water-to-sand mass ratio.