Design of CPW fed IDC resonator for non invasive testing of chemical solvents

A microwave planar interdigital capacitor (IDC) based resonator etched on the central conductor of a coplanar waveguide (CPW) structure is presented to measure the complex permittivity of common solvents. The proposed structure is working as a resonator with the resonant frequency of 6.5 GHz under unloaded condition. A sample holder made of borosilicate glass is especially designed to contain the liquid under test. A protective polypropylene sheet is used to cover the CPW to avoid any direct contact between the IDC sensor and the liquid under test thus making the proposed method a non-invasive type. The proposed sensor is highly sensitive as it is observed that there is a shift in resonant frequency of 240 MHz per unit change in permittivity. A number of solvents are tested using the proposed sensor, and a good agreement is achieved between the theoretical and the measured results with a typical error of 2.5%.