This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination. IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS 1 Novel Improved Sensitivity Planar Microwave Probe for Adulteration Detection in Edible Oils Nilesh Kumar Tiwari , Surya Prakash Singh , and M. Jaleel Akhtar , Senior Member, IEEE Abstract—Novel design of the complementary split ring res- onator (CSRR) loaded planar microwave probe with improved sensitivity is proposed for liquid adulteration detection. As com- pared to the conventional planar resonant sensors, the proposed single-port structure is quite advantageous for detecting adulter- ation in edible fluids since it can directly be immersed inside the liquid specimen. The flared microstrip line of the proposed probe facilitates the etching of two CSRRs on the top side at the electric field maxima resulting into the compact design with improved sensitivity. The parameters of the probe are optimized using the CST-MWS, and its prototype is fabricated, which possesses the ability to easily distinguish two materials having close dielectrics values. Finally, the proposed probe is successfully employed for adulteration testing of various edible oils where the presence of common adulterants such as mineral oil inside high-quality olive oil and mustard oils can be clearly detected. Index Terms— Adulteration detection, complementary split ring resonator (CSRR), flared microstrip line, planar microwave probe, sensitivity. I. I NTRODUCTION M ICROWAVE resonant sensors have been used for var- ious industrial and biomedical applications in the past decades due to their higher measurement accuracy [1]–[7]. During the past few years, the compact low-cost resonant sensors have been mainly realized using the metamaterial inspired structures [3]–[7]. The metamaterial inspired split ring resonator/complementary split ring resonator (SRR/CSRR) possesses various advantages such as low profile, compact size, and their ease of integration with the other existing planar structures thus making them popular in recent times. However, most of the SRR and CSRR-based sensors available in the literature are of nonsubmersible types which seem inappropri- ate for testing liquids. It is to be noted that the conventional coaxial probe sensor has quite often been used for testing liquids accurately by immersing the probe inside the test liquid [8]. Recently, the SRR-based-planar structure has been proposed as the submersible sensor to characterize solid and liquid samples [7]. However, the measured sensitivity (shift in the resonant frequency/unit change in dielectric constant) of the submersible sensor in [7] is relatively low (3.04%), due to the lower dielectric sensitivity of the SRR [5]. Manuscript received September 14, 2018; revised November 2, 2018; accepted November 29, 2018. This work was supported by DST India under Grant DST/TSG/ME/2015/97. (Corresponding author: Nilesh Kumar Tiwari.) The authors are with IIT Kanpur, Kanpur 208016, India (e-mail: nileshkt@iitk.ac.in). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LMWC.2018.2886062 The sensor in [7] is also not compact because of its two- port design topology and lower operating frequency. The lower sensitivity of the sensor [7] makes it a somewhat unsuitable for the adulteration detection of edible oils, where both parent oil and mineral oil having dielectric constant values close to each other. In the present scenario, the detection of mineral oil mixed with edible oils is a serious challenge in various parts of the world, as it directly affects human health by causing serious diseases such as liver damage, cancer, paralysis, and cardiac arrest [9]. In this letter, a novel CSRR-based-open-ended microstrip structure with improved sensitivity is proposed to implement a compact submersible sensor. As compared to the earlier reported conventional CSRR loaded microstrip-based sensors, the proposed sensor is different since, in the proposed struc- ture, the CSRR is directly etched on the top side of the open ended flared microstrip line. This fact makes the proposed sensor quite compact along with some advantages as compared to the conventional CSRR-based sensors. The first advantage is that the single port of the proposed structure makes it convenient for detecting adulteration in edible oils since it can directly be immersed inside them. Second, the proposed sensor shows improved measured sensitivity of ∼9% because of the fact that the two CSRRs are etched on the main line laterally near the open end of the structure where the maximum electric field region is observed. To show the applicability of the proposed sensor, the adulteration of various edible oils with mineral oil is mainly tested as it can be considered as the most common adulterant in various edible oils [9]. The minimum deducible frequency shift corresponding to 10% adulteration of mineral oil in the edible oils is found to be ∼14 MHz using the designed probe sensor. II. DESIGN OF THE PROBE SENSOR The proposed single-port flared microstrip line loaded with optimized CSRR resonators shown in Fig. 1 is presented as a submersible probe sensor. As the flared microstrip structure is kept open at the other end, it behaves like an open-ended line thus facilitating the formation of a standing wave pattern along the line as depicted in Fig. 2(a). To obtain the better interaction with the electric field, the CSRRs are loaded near the flared end. For observing the actual electric field around the sensing element at the resonant frequency, its plot is shown in Fig. 2(b), where the strong electric field localization in the electrically small sensing region can be observed. Based on the electric field distribution, it can be ascertained that the 1531-1309 © 2018 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.