Non-invasive Measurement of Complex permittivity using a Compact Planar Microwave Sensor

This work describes the development of a compact planar microwave sensor for non-invasive complex permittivity measurement of liquids. The sensing element is a quarter wavelength stepped impedance resonator realized on an asymmetric coplanar stripline. To characterize the material, the sensor detects the resonance frequency and phase of the reflection coefficient. Numerical simulations were conducted for non-invasive and invasive measurements and mathematical relations are formulated for extracting the dielectric constant and loss tangent of the sample. The sensor is intended to operate at 5.6 GHz in free space. A prototype of the designed sensor is fabricated and tested with using a modest volume of test liquids as a proof of concept. The actual dielectric properties of the solution are measured using a commercially available dielectric probe. The extracted dielectric measurement acquired with the sensor are very similar to those obtained with the dielectric probe.