Invasive and Noninvasive Measurement of Complex Permittivity Using ACS-Fed Stepped Impedance Sensor

Abstract

Compact asymmetric coplanar strip (ACS) fed stepped impedance sensors for complex permittivity measurement of liquids are presented in this article. The step impedance discontinuities in the feed line improve the sensitivity and compactness of the sensor. Two sensors operating at 5.6 GHz with different impedance discontinuities are designed on a low-cost substrate with an overall size of 11 × 10 × 1.6 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> . Invasive and non-invasive sensing techniques are simulated using 3D electromagnetic simulation software and experimentally validated using the fabricated prototype. The change in reflection characteristics of the sensor with varying dielectric constant and loss tangent are analyzed for the extraction of complex permittivity. The proposed sensors’ extracted permittivity values strongly agree with the Keysight dielectric probe measurement. Invasive sensing is carried out by submerging the sensor in liquid samples, whereas non-invasive sensing is carried out by holding the sensor at a varying distance from the samples. The sensor’s single port and compact topology necessitate a minimal volume of liquid samples for measurements. According to the results of the experiments, the ACS-fed stepped impedance resonator (SIR) sensor has a maximum sensitivity of 6.6 % for invasive sensing and 4.6 % for non-invasive sensing. In contrast, the ACS fed extended stub SIR sensor shows a maximum sensitivity of 10.3 % and 6.2 % for invasive and non-invasive sensing approaches, respectively. The sensor measures permittivity values ranging from 2 to 78.