# An IoT Reader for Wireless Passive Electromagnetic Sensors
## 2 Materials and Methods  
### Sensing System and IoT Reader Definitions  
The general sketch of the sensing system is shown in Figure 1. It is based on two elements: a passive wireless electromagnetic sensor and an IoT-R. The sensor is capable of backscattering a wireless electromagnetic signal which depends on the magnitude under measurement. The sensor is interchangeable. Thus, the proposed system can be used to remotely sense different magnitudes by changing the sensor. Moreover, the sensor can be easily replaced in case of failure or damage. In the present work, the system will be used to obtain the dielectric permittivity of different liquids by immersing the sensor in each of the liquid under test (LUT). The sensor is based on printed metamaterials particles without additional electronics, leading to a low-cost, replaceable, battery-free and fully passive wireless solution. The description of the sensor used in this work is provided below. The system also contains an IoT-R, which generates the radiofrequency signal to interrogate a wireless sensor, receives the sensor responses, digitizes that signal and acts as the IoT interface of the laboratory. The presence of the IoT interface implies that the control of the system is made remotely by a computer or handheld device (e.g., smartphone or tablet) connected to the reader through the Internet. Thus, the working principle of the proposed system is as follows: the laboratory technician inmres the sensor in a LUT; then, the reader takes a wireless measurement from the sensor when it receives a sensing request from a client through the Internet. Finally, the IoT-R sends the result of the measurement back to the client through the Internet. This operation protocol is valid for the characterization of different LUTs, but it is possible to remotely monitor the changes of the same LUT (e.g., changes with time or temperature) by maintaining the sensor immersed in the same LUT and remotely sending periodic requests to the reader. The final objective of this article is the development of a functional demonstrator including the passive wireless sensors, the IoT-R and remote control and acquisition functions through the Internet. In order to show the validity of the demonstrator, it is devoted to a particular application: the characterization of the relative permittivity of different liquids. It is important to note that the proposed paradigm may be applied to the detection of other physical magnitudes by changing the sensors, as commented before. In order to implement the proposed application, a submersible electromagnetic printed sensor presented by the authors in [30] is used. The sensor is based on a pair of printed rectangular split-ring  
Figure 1: Schematic of the proposed IoT sensing system. resonators (SRRs) coupled to a microstrip line. The SRRs introduce a notch in the transmission coefficient (\(S_{21_{sens}}\)) between the ports of the microstrip line. The sensing principle relies on the frequency shift of the notch when the SRRs are submersed into different LUTs. In [30] it has been demonstrated that the sensor notch varies between 1.6 and 1.9 GHz, corresponding to a relative permittivity range between 1 and 9. In this work it has been tested with permittivities between 1 and 6, due to the impossibility to find non-polar liquids with relative permittivities out of that range. In order to make a wireless system, the sensor ports are connected to two patch antennas which are presented in Section 2.2. The authors are working on the integration of the sensor and its antennas, but this topic is out of the scope of the present article. The IoT-R follows the diagram of Figure 2. It is based on four functional layers: the radiating layer, the RF interface, the IoT mini-computer and the power unit. The functions of the RF interface are the generation of a periodic frequency sweep to interrogate the sensor and the detection of the signal modified by the sensor.