Transmission Line Based Wideband Microwave Sensor for Determination of Biodiesel Purification

The main objective of this study is the application of the microwave measurement technique to determine electrical characteristics of material with complex permittivity values in a frequency range from 100 MHz - 10 GHz. The measurement results represent the purification level of biodiesel compared to the results analyzed by Gas Chromatography (GC) technique (modified EN14103). The microwave sensor was created based on a simple strip line structure of a microwave transmission line which is filled with the liquid under test (LUT) samples. Scattering parameters have been measured using a vector network analyzer (VNA). After that, the complex permittivity values and impulse response using inverse Fourier transformation of the propagation coefficient have been calculated and compared with the results from GC analysis. The results of complex permittivity can be used to describe the behavior of LUT in terms of relative permittivity related to the stored energy and the dissipation of energy within the sample. The levels of purification of biodiesel are represented by the relative complex permittivity of the LUT. Another way to identify the level of purification uses the complex value of inverse Fourier transformation of the propagation coefficient. By doing this, the stored energy and the dissipation of the LUT can also be analyzed in time domain. Four biodiesel samples have been analyzed using the proposed sensor. The purification level can be obviously identified especially in a frequency range from 2 to 4 GHz.