## 1 Introduction  
The importance of sensors is undeniable in today's world of technology and innovation. The advent of the Internet of Things and automation has raised their significance to unprecedented levels [1]. With advancements in the areas of electronics, photonics and materials, new types of sensors are introduced every year [2; 3; 4; 5; 6; 7; 8]. Recent research has focused on developing sensors with high accuracy, compact size and high sensitivity [9; 10; 11; 12; 13; 14; 15]. Microwave sensors employ an electromagnetic field for sensing purposes, and operate in a frequency range of 300 MHz to THz [1]. Microwave sensors have several advantages over their counterparts including low cost, compact size, accuracy, ease of fabrication, and testing [16; 17; 18; 19; 20; 21]. Due to these obvious advantages, microwave sensors play a vital role in different fields of life such as healthcare, agriculture, the defense sector, and industry [9; 10; 11; 22; 23; 24]. Microwave sensors can be categorized into two types: (i) broadband sensors, and (ii) resonance-based sensors. Resonance-based sensors operate on a narrow frequency range with high precision [10] and utilize a highly confined and dense electric field for sensing. The resonance frequency is extremely sensitive to dielectric loading in this region, and any minute change can result in a shift in resonance frequency. The use of metamaterial structures such as split rings (SRR) or complementary split rings (CSRR) in microwave sensors has further enhanced the accuracy of measuring dielectric permittivity [25; 26; 27; 28]. Furthermore, different oil molecules have different rotational dynamics and correlation times, and can achieve resonance with the applied external electric field at different frequencies. The detection of adulteration in food has remained a key application of microwave sensors. For example, the purity of edible oils plays an important role in human health. Contamination in edible oils could be due to intentional adulteration, or as a result of factors such as aging or improper packing. Adulterated oil can harm human health by causing different types of diseases, such as cardiovascular and throat diseases [29]. Dielectric spectroscopy using microwave sensors has been widely employed to detect adulteration in edible oils. Cavity resonator-based sensors for liquid characterization have been reported in the literature [30]. However, planar microwave sensors are preferred over large bulky cavity resonators due to advantages such as low cost, easy use, easy fabrication, and the benefits of integration with other microwave devices. A high-sensitivity sensor with a non-destructive technique for the detection of complex permittivity of liquids was presented in [31]. The sensor was mounted on a Rogers RT6002 substrate and the sample testing was carried out inside a polydimethiolsilicon (PDMS) microchannel. High sensitivity was achieved by incorporating only a single capacitor in the LC resonator circuit. A CSRR-based planar microwave sensor for adulteration detection in edible oils was proposed in [32]. This sensor was designed on a Roger 5880 substrate and could detect adulteration of up to 20%. The sensor presented in [33] employed a resonator on FR4 substrate to detect adulteration in oils, but the sensitivity and quality factors of the proposed sensor were not calculated. These two parameters are the most crucial parameters to determine the performance of a sensor. In [34], the proposed sensor could detect oil adulteration only in the range of 50% and above. One of the drawbacks in the existing literature includes the use of expensive substrates such as Rogers, which will become critical at the mass production stage. Sensors fabricated on low-cost substrates have either low sensitivity, or cannot detect low levels of adulteration. Moreover, the use of the PDMS microchannel also makes the testing process complicated.