## 1 Introduction
Because of their intended massive use, sensors do not need to be extremely sophisticated or precise; however, they must satisfy requirements of low cost and acceptable reliability in order to be deployed at a finer granularity than active precise wireless sensors. The ultimate goal is to design \"smart dust motes\", i.e., autonomous sensing, ubiquitous computing, and communication systems small enough to be easily \"dispersed in the environment\" [13]. This motivates the development of low-cost, wireless, and passive sensors for large-scale infrastructure and big data applications. In order to enable such a vision, radio frequency identification (RFID) technology can play a strategic role, thanks to its low-cost, wireless, and \"sensing-friendly\" capabilities [11]. The last decades have witnessed a rapid growth of RFID technology for identification and tracking because of its unique identification (UID). Besides this common usage, an analogue processing of the physical signals related to the reader-tag communication, could permit to achieve much more information about the target without the need for additional electronics or sensors [13]. Enabling the sensing ability into RFID technology can make the system know the state of the real-world objects [14] and seamlessly integrate within the global cyber-physical systems (CPS) and Internet of Things (IoT) [15]. The sensing capabilities provided by RFID tag antennas in the ultra-high frequency (UHF) bands are perhaps an exciting research trend [16], with great applicability to the emerging paradigm of the IoT as a green technology [17]. The key background is a new paradigm of antenna design that merges together the conventional communication issues with more specific requirements about sensitivity to time-varyingboundary conditions [13]. The rationale of this idea lies in the clear dependence of the tag's input impedance and radar cross section (RCS) on the physical and geometrical features of a real target [18]. The RFID technology, which was originally developed for large-scale asset tracking, happens to be a backbone for building low-cost, passive, and large-scale WSNs. This makes deploying massive amounts of sensors possible in reality. Meanwhile, as the RFID platform is widely accepted throughout industry, large-scale WSNs based on RFID technology can be seamlessly integrated into current off-the-shelf RFID systems. For this reason, we seek to highlight this work aimed at enhancing EPC Class 1 Generation 2 (C1G2) standard compliant RFID devices towards the goal of RFID-based sensors and networking. We use the term antenna sensor herein to represent one type of sensor that uses antennas to \"sense\" the things [19]. The operational principle of antenna sensors mounted on conductive surfaces is similar to pulsed eddy current NDT [20], the conductive loss and penetration depth of which is proportional to the operating frequency. With increased operating frequency, the spatial resolution can be continuously enhanced by a corresponding decrease in wavelength [21] thereby the size of the antenna sensor. Because of magnetic resonant coupling (MRC) in the wireless power transmission (WPT) [22], the read range between RFID tag and reader in low frequency (LF) or high frequency (HF) bands is quite short, e.g., in the range of several centimetres. This is due to evanescent (mode) coupling. Resulting from electromagnetic (EM) coupling (propagation mode), UHF and ultra-wide band (UWB) antennas can be used to increase the communication distance [23]. A paradigm of RFID based large-scale passive wireless sensor networks for SHM is described in Figure 1. This paper mainly focuses on the UHF band. Some properties of these passive antenna sensors based on RFID technology can be summarized as the following [24; 25; 26; 27]:  
* _Simple configuration_: The antenna itself can serve the dual function of communication and sensing. Therefore, no external sensor is needed. For chipless tags, there is even no electronic device.