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Frequency domain (FD) sensor is an instrument developed for measuring soil moisture content. The instrument has an oscillating circuit, the sensing part of the sensor is embedded in the soil, and the operating frequency will depend on the value of soil's dielectric constant.

There are two types of sensors:

• Capacitance probe, or fringe capacitance sensor. The configuration is like the neutron probe where an access tube made of PVC is installed in the soil. The probe consists of sensing head at fixed depth. 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.

• Electrical impedance sensor, which consists of soil probes and using electrical impedance measurement. The most common configuration is based on the standing wave principle (Gaskin & Miller, 1996). The device comprises a 100 MHz sinusoidal oscillator, a fixed impedance coaxial transmission line, and probe wires which is buried in the soil. The oscillator signal is propagated along the transmission line into the soil probe, and if the probe's impedance differs from that of the transmission line, a proportion of the incident signal is reflected back along the line towards the signal source.

Compared with time domain reflectometer (TDR), FD sensors are cheaper to built and have a faster response time. However because of the complex electrical field around the probe, the sensor needs to be calibrated for different soil types.

• Gaskin G.J., Miller J.D. 1996. Measurement of soil water content using a simplified impedance measuring technique. Journal of Agricultural Engineering Research 63, 153-160.