Design and fabrication of Open-ended Co-axial probe for complex permittivity measurement of liquids using CST Microwave Studio

Abstract

An Open-ended co-axial probe technique is widely used for measuring the complex permittivity of material under test (MUT) over a broad range of radio and microwave frequencies. Dimension, structure and material of the co-axial probe vary with the measurement range of frequency and the magnitudes of the properties of the MUT. Therefore, it is required to customize the probes depending on the type of the material to be investigated and on the range of frequency of measurement. A co-axial probe, to measure the complex permittivity of the liquids in the frequency range 10 MHz to 3 GHz, was simulated and tested for simulated test liquids, using CST Microwave Studio software. A real probe, of same design as simulated probe, was also fabricated. Complex permittivity measurements of some test liquids were carried out using Vector Network Analyzer (VNA) and the real probe. Varying degree of agreement between theoretical and experimental complex permittivity spectra in the different frequency segments was observed. Study suggested that the open ended co-axial probe of required dimension and structure can be designed and tested through simulation.