Tham khảo tài liệu 'electromagnetic waves part 12', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Time Domain Reflectometry Temperature-dependent Measurements of Soil Dielectric Permittivity 375 increase from 278 K to 338 K increases the relaxation frequency of water molecules located 2 to-10 m away from the solid phase from GHz to GHz. As a result this temperature increase also increases the effective value of the dielectric permittivity of adsorbed water. Or and Wraith 1999 recognized the frequency limit of 1 GHz of the TDR method as the cut-off frequency f that distinguishes water molecules between free and bound. Water layers having dielectric relaxation frequencies lower than the cut-off frequency were considered as bound water having smaller dielectric permittivity than for free water. It was shown Dirksen Dasberg 1993 that the quantity of bound water increases with the volume of clay fraction in the soil due to large specific surface area of clay. Fig. 2. A - relaxation frequency for water particles bound to solids according to 8 and related to its distance x from the solid phase B - example relation between dielectric permittivity of water molecules and relaxation frequency for bound and free water particles 3. Materials and methods TDR measurements were performed in the laboratory on mineral soils with negligible organic content collected from Lublin region Poland Table 1 mainly from the topsoil layer. The soils were spread in layers of about 1 cm thickness on a flat surface in laboratory for drying at room temperature and then they were grinded to destroy big aggregates and passed through 2 mm sieve. Dry soils were mixed with appropriate amount of distilled water to obtain five soil samples for each analyzed soil with moisture values from air dry to saturation with regular differences in moisture content taking care to get homogeneous distribution of water in the soil samples. Eight containers with soil samples covered with plastic foil to minimize evaporation were placed in a specially constructed rack. The volume of the soil containers was .
