Capacitance can be measured from the change in frequency of a reflected radio wave or resonance frequency (Kelleners 2004). This method of measurement uses an oscillator to propagate an electromagnetic signal through a metal tine or other wave guide. The difference between the output wave and the return wave frequency is measured to determine soil moisture.
FDR probes are considered accurate but must be calibrated for the type of soil they will be buried in. They offer a faster response time compared to Time Domain Reflectometer (TDR) probes.
These sensors are often referred to as frequency domain reflectometers (FDR), however the term FDR is often misused because most frequency sensors are using a single frequency and not a domain of frequencies. Other capacitance probes and amplitude impedance-based probes are often mistakenly referred to as “FDRs”.
FDR sensors need to have good contact with the medium they are measuring with no air gaps. The volume of measurement is dependent on sensor size with most sensors in the order of 5cm to 10cm in length but one sensor is 3m in length. The field of influence is greatest at the sensor to substrate interface and declines rapidly from there. Generally, the field of influence is approximately 1cm distance from the sensor. Given a 3m length sensor with 1cm distance into the surrounding medium, extreme caution needs to be taken with installation to ensure there are no air gaps.
FDR probes need to be calibrated for the specific soil in which they will be placed in.