Fig. 4 Permittivity, , as function of water content, , for fine sand (curve c and e, and measurements g and h) and for glass beads (curve a, b, and d and measurements f). The curves a, b, and c are according to Topp et al. (1980). Curves d and e are calculated according to Eq. [16]. The points indicated by f and g were measured using a frequency domain sensor at 20 MHz. The points indicated by h are measured using a time domain reflectometry sensor