This chapter mainly summarizes the measurement techniques based on the Time Domain Reflectance (TDR) principle, which are currently mainly applied as follows: ① Cable length measurement technology; ② Soil moisture content; The technology of time domain reflection principle is essentially to calculate the dielectric parameters of the sample in response to the step voltage pulse that excites it. It is essentially a transient method, which can be divided into two types: slow response and fast response techniques: 1) Slow response technique is the capacitive charging and discharging method, which treats the sample to be tested as a capacitor, quickly adds or removes DC voltage, records the time characteristics of charging or discharging current, and uses Fourier transform to convert the time domain data to the frequency domain to obtain the frequency spectrum of complex permittivity. It can be applied as an effective technique for studying the relaxation process of media in very low frequency, ultra-low frequency, and low frequency (10-7Hz~10MHz); 2) Fast response generally requires a high-speed pulse signal generator as the signal source, as well as a sampling oscilloscope as the receiving device and display. A step voltage is commonly used to propagate in a low loss coaxial line. As long as the propagation characteristics of the coaxial line are independent of position and frequency, the shape of the step voltage remains unchanged. When there is a section of dielectric material with different propagation characteristics on the propagation line, the step voltage is reflected and partially transmitted at the interface, and these reflection and transmission information contain the complex permittivity spectrum (dielectric spectrum) information of the sample. Therefore, the frequency spectrum of the complex permittivity can be obtained from time-domain measurement data through Fourier transform. At present, the measurement time resolution range allowed by rapid response technology is about 10us~6ps, corresponding to a frequency range of 10kHz~16GHz.

At present, there are several technical challenges: 1) waveform recognition algorithm. Similarly, for the recognition of the arrival time of reflected waves in soil, pulse signals contain multiple frequency components, and the propagation speed and attenuation of different frequencies are different during propagation, which can cause waveform distortion and time recognition errors. Generally, signal processing and recognition need to be carried out through mathematical methods. In this regard, some researchers are conducting research on recognition algorithms based on foreign TDR hosts; 2) In the development of TDR host hardware (excluding algorithms), the step excitation signal used in TDR only has a rise time of about 300ps, and the upper limit frequency of the signal is greater than 1.2GHz. According to the sampling theorem, the system sampling frequency should reach at least 2.4GHz. Obviously, it is unrealistic to achieve such high-speed sampling under existing technology, which is the key challenge for the widespread application of TDR. If there is a good waveform recognition method, it is mainly due to the lack of high-quality domestically produced high-frequency pulse generator chips and time conversion chips, as well as the lack of equipment and technology for high-frequency measurement verification of the above signal analysis. 3) There is a serious lack of quantitative research on the attenuation and dispersion of signals propagated in the medium after insertion into soil, which leads to nonlinearity and temperature effects between interval measurements and dielectric constants. 4) When applied to high conductivity soil (such as saline alkali land), due to the high conductivity of the soil, the signal attenuation is fast, and the end reflection point of the probe disappears, making traditional methods for analyzing dielectric constant very difficult. 5) Lack of theoretical research on probe structure and material applicability based on TDR principle.