The sensing head consists of an oscillator circuit, the frequency is determined by an annular electrode, fringe-effect capacitor, and the dielectric constant of the soil.
Each capacitor sensor consists of two metal rings mounted on the circuit board at some distance from the top of the access tube.
The oscillating electrical field is generated between the two rings and extends into the soil medium through the wall of the access tube.
The capacitor and the oscillator form a circuit, and changes in dielectric constant of surrounding media are detected by changes in the operating frequency.
The capacitance sensors are designed to oscillate in excess of 100 MHz inside the access tube in free air.