Amplitude domain reflectometry
Many soil moisture measuring instruments are based on the principle of Amplitude Domain Reflectometry . This method measures the electrical impedance. Electromagnetic waves traveling along Transmission Lines enter in soil medium whose impedance is different from TL a part of the energy is reflected back to transmitter. The reflected wave interferes with incident wave and produces a standing wave along the TL, this changes the amplitude of wave along the TL. The impedance can be measured from difference in amplitude. The impedance has two components: electrical conductivity and dielectric constant. The effect of conductivity can be minimized by selecting an appropriate frequency.
