Fig. 1. (a) The multisection transmission line model of the time domain reflectometry (TDR) measurement system, in which each uniform section is characterized by the geometric impedance (Z_p), resistance loss factor (_R), complex dielectric permittivity (_r*), and waveguide length (L). The transmission line is driven by a source voltage (V_s) with a source impedance (Z_s) and terminated in a load (Z_L). The input impedance (Z_in) is defined as the ratio of line voltage (V) to the line current (I). (b) The associated direct current circuit model, in which R_s is the inner resistance (equal to the source impedance Z_s), R_cable is the cable resistance, R is sample resistance, v_s is the source voltage (in time domain), and v is the TDR steady-state voltage. (c) A typical TDR waveform showing definition of reflection coefficient (), where t₀ is the roundtrip travel time in the probe section, and t_cable is the roundtrip travel time in the cable section.