The pulse propagates along a coaxial cable and enters the TDR probe, which is traditionally a pair of parallel metallic rods inserted into the soil. Part of the incident EM waves of the pulse is reflected at the top of the probe because of the difference in impedance between cable and probe. The remainder of the wave propagates through the probe until it reaches the end, where the wave is reflected back to its source. The transit time of the pulse for one round-trip, from the beginning to the end of the probe is measured with an oscilloscope branched on a cable tester. For a homogeneous soil, volumetric water content, ��v (m3 m?3), is calculated by using a calibration curve which is normally established empirically with the desired material.
One of the first and still widely accepted calibration functions for soils was established by Topp et al. at the beginning of the 1980s :��v=?5.3��10?2+2.92��10?2��b?5.5��10?4��b2+4.3��10?6��b3(1)In Equation (1):��v is volumetric soil water content [m3 m?3]��b is bulk soil dielectric Brefeldin_A permittivity [-].3.?Sensor Developments and Applications of CMM3.1. FD Sensor ��LUMBRICUS��One of the first developments of the former Soil Moisture Group (SMG), from which today��s CMM emerged, was a FD type sensor technique which is used by Meteoloabor AG in the LUMBRICUS SM device . The portable moisture measurement system consists of a glass fibre access tube which will be inserted into the soil prior to the measurement and in which an antenna (resonator) can be moved up and down (Figure 1).Figure 1.��LUMBRICUS�� FD sensor with antenna, access tube and sealing.
The field of the antenna penetrates the tube walls into the soil and is influenced by its dielectric properties which lead to a shift in resonance frequency as well as a change of the amplitude and bandwidth. A voltage controlled oscillator controlled by a monoboard computer sweeps a frequency range of 100 MHz to 300 MHz. Attenuators improve the adjustment and reduce the noise of the signal and a diode detector measures the performance. The resonator-type antenna is coupled with this transmission path and in the case of resonance it acts as an absorption circuit and detracts energy. The remaining power and the corresponding frequency are measured and recorded by the computer. The resonator has a resonance frequency of 230 MHz for air and 170 MHz for saturated soil. Since the resonance curves are not only influenced by the real part of the dielectric permittivity but also by the quality, it is possible to determine the complex dielectric properties . Figure 2 shows the commercial design of LUMBRICUS. On the right hand side the probe head with the integrated antenna on top of an installed access tube can be seen.