A Miniaturized Resonant Moisture Sensor

A novel and very compact resonant sensor is presented. It consists of a resonant metallic helix mounted in a circular waveguide which is operated below cutoff. The resonance frequency is easily adjustable via the helix dimensions. A material sensitive to the environmental properties to be detected is placed next to the helix and alters its resonance. Design and optimization of the sensor are reported with an emphasis on moisture detection. Its application in civil engineering is demonstrated with measurements of the moisture content of cement.