A Low Cost Planar Resonant RF Sensor for Permittivity Measurement of Solid and Liquid Materials

Abstract

A low cost planar resonant RF sensor for permittivity measurement of solid and liquid materials is proposed in this paper. A quarter wavelength resonator realized using microstrip line technology acts as a quasi static near field sensor. The resonant frequency of the sensor varies with the dielectric properties of the test sample in contact. A mathematical model based on finite element method is developed to determine the permittivity of dielectric test samples. The proposed sensor is designed to operate in 2.42 GHz ISM band. As a proof of concept, a prototype is fabricated on FR4 substrate of 1.6 mm thickness. The performance of the fabricated prototype is validated by measuring the permittivity of some commercially available substrates and vegetable oil samples. The measurement results showed a good agreement with the data available in the literature. The single port architecture of the planar sensor facilitates direct immersion in liquid samples. Moreover, this technique does not pose any limitations on thickness of solid test samples. The proposed submersible sensor is simple, compact, low cost and easy to fabricate which promotes its wide spread usage for microwave material characterization.