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* ''Capacitance probe'', or fringe capacitance sensor. The configuration is like the [[neutron probe]] where an access tube made of [[PVC]] is intalled 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 burried 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 a has a quicker response time. However becuase of the complex electrical field around the probe, the sensor needs to be calibrated for different soil type.
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