  Measuring the complex permittivity of material is essential in many scenarios
such as quality check and component analysis. Generally, measurement methods
for characterizing the material are based on the usage of vector network
analyzer, 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 non-destructive inspection. In this work, a small distance increment (SDI)
method is proposed to non-destructively measure the complex permittivity of
material. In SDI, the transmitter and receiver are formed as the monostatic
radar, which is facing towards the material under test (MUT). During the
measurement, the distance between radar and 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 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 acrylic plate shows good
agreement with the literature values, demonstrating the efficacy of SDI method
for characterizing the complex permittivity of material.
