New approach for dielectric constant detection using a microstrip sensor Sohrab Majidifar a , Gholamreza Karimi b,⇑ a Department of Electrical Engineering, Kermanshah University of Technology, Kermanshah, Iran b Department of Electrical Engineering, Faculty of Engineering, Razi University, Kermanshah, Iran article info Article history: Received 26 September 2015 Received in revised form 7 March 2016 Accepted 22 June 2016 Available online 23 June 2016 Keywords: Dielectric constant Microstrip resonator Resonant frequency LC model abstract A novel dielectric constant detection method using a microstrip sensor is presented in this paper. The aim was to estimate the dielectric constant in the space above a resonator which was the basic part of the sensor. Variation in resonant frequency of the resonator was considered as the criterion for detection of the dielectric constant variation. The microstrip resonator was designed using coupled stepped impedance open stubs. The circuit model of the resonator was then extracted and the resonant frequency as a function of the dielectric constant was obtained. Applicability of the proposed method was verified in experimental studies. Ó 2016 Elsevier Ltd. All rights reserved. 1. Introduction Investigation of dielectric constant of various materials has been performed in many cases as an index of their physical and chemical changes. Among the existing applications of this approach, design of gas sensors, humidity detection, and also detection of type and quality of vegetable oil using dielectric con- stant [1–5] are notable. There are various methods for measure- ment of dielectric constant of materials and its changes including [6–8]. Despite the achievements so far in terms of both applica- tions and measurement methods of dielectric constant, there seems to be a huge potential for further developments. One area where limited contribution has been made is designing microwave sensors. Due to the advantages of microwave devices and possibil- ity of their utilization in wireless systems, they have been vastly considered for building diverse sensors. Among many industrial applications of microwave sensors [9,10], an example is a micro- wave level sensor for molten glass able to operate in an industrial furnace which was proposed and tested in [9] while in another application [10] a new microwave sensor for the detection of cracks in metallic materials was presented. Materials in forms of gas, liquid and solid have been investigated using microwave sensors [5,11–13] which indicates to their vast applicability. Microstrip resonators are common components in design of micro- wave sensors which provide small size and low fabrication cost. Moreover, these resonators are capable of being connected to other parts of a wireless monitoring system. Split-ring resonator, parallel ring resonators, quarter and half wavelength resonators and inter- digital coupled lines have been used for developing microwave res- onators in [3,4,10–14]. A change in property of the substrate of the resonator as a result of proximity to different materials has been investigated in a number of microstrip sensors [2]. This is while in some other cases, the substrate properties do not change and circumstances governing the resonator vary instead [11,12]. In this work, the aim is to develop a new method for detection of dielectric constant of materials using a microstrip-based micro- wave sensor. A change in the resonant frequency of the microstrip sensor resulting from a change in its effective dielectric constant is considered as the index for defining the dielectric constant of the sample material. In order to illustrate the method, a coupled stepped impedance resonator that was developed and used as the sensor and its circuit model was extracted. Using this model the relationship between the dielectric constant of the tested materials and the resonant frequency of the sensor was obtained. Simple structure, well known governing equations and flexibility in design are the advantages of the proposed resonator against the common resonator such as split-ring resonator and parallel ring resonators. The method was successfully examined in experi- ments using the sensor. In the following sections, the sensor, extraction of its model and governing equations are presented first receded by results of the experiments and their discussions. 2. Microstrip sensor structure, modeling and analysis Variation of the resonant frequency of a resonator based on the environmental changes is the basic concept behind the proposed http://dx.doi.org/10.1016/j.measurement.2016.06.051 0263-2241/Ó 2016 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. E-mail addresses: s.majidi@kut.ac.ir (S. Majidifar), ghkarimi@razi.ac.ir (G. Karimi). Measurement 93 (2016) 310–314 Contents lists available at ScienceDirect Measurement journal homepage: www.elsevier.com/locate/measurement