Small Distance Increment Method for Measuring Complex Permittivity With mmWave Radar

Measuring the complex permittivity of material is essential in many scenarios, such as quality checks in material manufacturing. Generally, measurement methods for characterizing the material are based on the use of a vector network analyzer (VNA), which is large and not easy for on-site measurement, especially in high-frequency range such as millimeter wave (mmWave). In addition, some measurement methods require the destruction of samples, which is not suitable for nondestructive inspection. In this work, a small distance increment (SDI) method is proposed to nondestructively measure the complex permittivity of a material. In SDI, the transmitter and receiver are formed as a monostatic radar, which is facing toward the material under test (MUT). During the measurement, the distance between the radar and the MUT changes with small increments, and the signals are recorded at each position. A mathematical model is formulated to depict the relationship among the complex permittivity, distance increment, and measured signals. By fitting the model, the complex permittivity of MUT is estimated. To implement and evaluate the proposed SDI method, a commercial off-the-shelf (COTS) mmWave radar is utilized, and the measurement system is developed. Then, the evaluation was carried out on the acrylic plate. With the proposed method, the estimated complex permittivity of the acrylic plate shows good agreement with the literature values, demonstrating the efficacy of the SDI method for characterizing the complex permittivity of the material.