## 4 Sensing-Oriented Issues and Solutions
### Defect Types and Antenna Topologies
#### 4.1.1 Strain Detection and Characterization  
Strain sensors (gauges) are required to detect deformations or structural change occurring in our surrounding infrastructures. For this measurand, the antenna sensor design is to be considered so that the mechanical strain is changed into electrical signals and the electrical signals are transmitted to the reader via RFID technique simultaneously. The strain model addresses two factors affecting the measured sensitivity [46]: (i) the efficiency of mechanical strain transfer from the base structure to the top surface of the RFID antenna sensor; (ii) the substrate dielectric constant change because of strain. Strain is denoted as \(\varepsilon=\Delta\)L/L\({}_{0}\), where \(\Delta\)L is difference length because of the strain and L\({}_{0}\) is the initial \"zero-strain\" length. Typically, the strain is unitless and is expressed in percentage or microstrain (\(\mu\varepsilon=\varepsilon\times 10^{-6}\)). Deformation changes the electrical length and therefore the resonant frequency of the antenna. The recent evolution of strain measurement using passive antenna sensors can be summarized in Table 1. A meander-line dipole antenna was designed to measure the strain using a controlled shape factor [40]. Nevertheless, the measurable strain level is low because of its poor mechanical property. To fabricate an efficient strain sensor, researchers are in search of a material that can exhibit a large structural change in response to a small applied strain [106]. Therefore, in conjunction with stretchable substrate and conductive materials, the dynamic range of strain level can reach up to 50% by wirelessly monitoring conductor loss resistance variation of a stretchable dipole on fabric substrate [39]. However, the power variation is used as a feature, which is quite susceptible to wireless channel [43]. Consequently, RFS was extracted as a robustness feature using fabric-based embroidered dipole [107]. In addition, an LC resonator was implemented as a chipless sensor [42]. Unfortunately, vector network analyzer (VNA) is required to monitor the RFS. However, the dipoles antennas are different to be installed with mechanical structures. For this reason, a folded patch antenna was designed to be mounted on metallic surface with a sensitivity of \(-0.7404\) ppm/\(\mu\epsilon\)[46]. In conjunction with turn-on power measurement, patch antenna was developed to increase the read range to 2.1 m [36]. Nonetheless, the above sensors can only detect one directional strain. This motivates the design of a slotted circular patch antenna, which can be used to monitor the omni-directional strain [41]. Because the operating resonance frequency equals approximately the strain sensitivity (Hz/\(\mu\varepsilon\)) of an antenna sensor, the antenna sensor has relatively low strain sensitivities. Hence, a frequency doubling technique was introduced by utilized two radiation patches working at f\({}_{0}\) and its second harmonic frequency, 2f\({}_{0}\), respectively in conjunction with a matching network in serially connected in between. Tensile testing showed an enhanced strain sensitivity of \(-5.232\) kHz/\(\mu\varepsilon\)[38]. Nonetheless, the transmitter and receiver should be customized for this purpose.