1558-1748 (c) 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. This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/JSEN.2018.2826982, IEEE Sensors Journal 1 A Contactless Dielectric Constant Sensing System Based on a Split-Ring Resonator-Loaded Monopole Javier Carnerero-Cano, Gabriel Galindo-Romera, Jos´ e Juan Mart´ ınez-Mart´ ınez, and Francisco Javier Herraiz-Mart´ ınez, Member, IEEE Abstract—In this work, a low-cost contactless passive sensor is designed and manufactured. The structure consists of a short-circuited printed monopole antenna coupled to two split- ring resonators. The permittivity of the materials under test is characterized within a near-field link between the sensor and a contactless reader. Concretely, the reader has been implemented by using a broadband patch antenna. The sensing principle relies on the reader detection of the notch introduced by the resonators in the power reflected by the sensor. Specifically, when a sample is placed over the sensor, the change in its effective permittivity produces a frequency shift of the notch detected in the reflection coefficient of the reader. A complete equivalent circuit model of the whole system is proposed. Moreover, the results are corroborated through full-wave simulations. Finally, the whole system is manufactured and measured. It is shown that the system can reliably detect the permittivity of the materials placed over the sensor at a distance between the reader and the sensor equal to 9.52 mm. Index Terms—contactless system, near-field link, passive electromagnetic sensor, permittivity characterization, printed monopole antenna, split-ring resonator (SRR). I. I NTRODUCTION In recent years, the scientific community has made a great effort to develop electromagnetic or radiofrequency sensors (for instance, [1]–[10]). A significant benefit that they present is that they can be easily merged with antennas, result- ing in wireless devices [6]–[13]. The use of radiofrequency identification (RFID) chips and schemes for sensing is also very common [14]–[17]. From another standpoint, despite its drawbacks, e.g., the shorter range and the more limited number of bits than in chip-based sensors, the deployment of chipless passive sensors implies an important diminution of costs and complexity [18]–[21]. These schemes [6]–[10], [15], [18]–[20] typically follows a similar approach. In this scheme, a signal is first sent wirelessly with a transmission antenna to interrogate a sensor or RFID tag. Then, this signal is modified by these elements and an answer signal (with the required information) is backscattered to a receiver antenna. With respect to the sensor technology, most of sensing systems based on passive electromagnetic sensors typically codify the sensing information in frequency domain or time domain signals. Usual frequency domain chipless systems take advantage of resonant elements in order to encode the information (spectral signature) [22]–[26]. On this subject, split-ring resonators (SRRs) are planar structures that have been widely used in the last decade in metamaterials and left-handed structures design [27]. They were introduced by Pendry in 1999 [28]. In the case of antennas, these particles allow the appearance of new resonance frequencies [29], [30] and additional functionalities. However, their high quality factor limits their working bandwidth and their application in many communication systems. Nonetheless, printed circuits based on SRRs are becoming interesting structures for sensing applications due to their low-cost, quick time response, high sensitivity and selectivity [4], [5], [31]–[35]. Particularly, these structures can be used to characterize substances in real time [34], [35], with appealing applications in industry [36], biology or medicine [37]–[39], among others. In this manner, the SRRs exhibit a strong localization of electromagnetic fields, that makes possible to improve the sensor selectivity for detecting samples [40]. This can be done by relating the resonance frequency of the SRRs to the real part of the permittivity of the material under test (MUT). Inductively coupled resonant sen- sors [41]–[43] represent another interesting approach. These kind of sensors are LC resonant circuits which are measured via inductive coupling between the sensor and a reader coil. The magnitude under test affects to the resonance frequency of the circuit due to a change in the inductance or capacitance. However, in spite of its simplicity this reading technique can lead to additional measurement errors (e.g. in the resonant frequency) depending on the position of the reader coil. Nevertheless, in laboratories or industrial environments, the measurement process is often harmful to the user health, causing damage when the MUT is in direct contact with the human skin, or when the gas under analysis is breathed. Concerning this, it is interesting to develop techniques that enable to read these sensors at a safe distance, allowing the monitoring of the measurement process. In this regard, most of the previous passive electromagnetic sensors offer a wired or far-field radiation approach, but it is difficult to find near-field disposable sensors. For instance, the printed sensor for permittivity characterization proposed in [34] features a real-time response and high sensitivity, but was directly wired to complex and expensive laboratory instruments. On the contrary, near-field passive sensors would present great advantages, as they would be quick and simple to use (even in massive applications). Furthermore, they would be more secure due to their short reading range (on the order of mm), as opposed to far-field links [7]–[10], [14], [15], in which the information is not coded in a secure way, and avoiding external interferences and undesired radiations. In this proof of concept, a SRR-loaded printed antenna working as a contactless passive sensor is proposed for first time, up to the authors’ knowledge. The structure consists of a printed monopole antenna ended by a short-circuited