# A Novel Chipless Hybrid RFID Sensor for Metal Crack Detection
## 3 Results and Discussion
Since the transmitter and receiver antenna both radiate in vertical polarization, the reflections from the vertical setup of the sample are more than that of the horizontal setup. These observations are evident in Figure 14.  
Figure 14 shows the experimental results for three defect depths; the top subfigure is for a horizontal crack and the bottom figure is for a vertical crack. For horizontal cracks with different depths, the RCS is quite noisy, with many frequency notches in the RCS spectrum. It shows that as the depth increases, the resonant  
Figure 12: Experimental results for two widths (2 mm and 3 mm). Figure 13: Experimental setup: The sample with defects in (**a**) vertical orientation; (**b**) horizontal orientation. frequency is reduced from 1 mm to 3 mm, correlated to the concept shown in Figure 9:0 The tag cannot detect the defect with 5 mm depth. However, the noisy RCS spectrum for horizontal cracks with an increased number of other frequency notches is due to the reflections from the sample set in the vertical orientation. For the sample arranged in a horizontal orientation with vertical cracks in Figure 14, the resonant frequency for depth 1 mm and 3 mm is around 4.8 GHz. There is not much shift in the resonant frequency as the depth of the vertical crack increases; this matches with the concept shown in simulation Figure 9:1 The tag cannot detect the defect for a 5 mm deep crack. The above investigation shows that the tag can detect both horizontal and vertical cracks with depths of 1 mm and 3 mm. As the depth further increases, the proposed tag cannot detect the defects. Also, the reflections from the larger metallic samples are dominant. The readability of the tag is affected by the distance. As the distance increases, more ripples are introduced in the RCS. The reflections from the environment become dominant and the tag detectability is reduced. Since the experimental setup is outside an anechoic chamber, it gives a better understanding of the tag performance concerning the read range, readability, and noise. Also, the size of the metallic sample under test impacts the RCS; as the size of the metallic sample increases, more power is reflected from the sample compared to the power absorbed and reflected by the tag. In SHM, the actual size of the metallic objects will be much larger than that of the samples under test. Hence, it is important to analyze how the tag behaves in the table-top setup. However, since the sample and the test setup are in a vertical orientation, the signals reflecting from them and from the vertical polarised antennas cause multipath interference, introducing stronger resonant frequencies, thus reducing the readability of the tag at the vertical orientation of the large rectangular sample setup. The shift in resonant frequency of the tag is also due to the subtle changes in fabrication, the cabling of the VNA, and VNA noise. The experimental results show that the tag is capable of detecting cracks as small as 10 mm long. When the tag is placed on the sample, it is adhered to the sample using tapes; the lift-off due to placement of the tag will change the effective dielectric constant of the tag, thus shifting the resonant frequency of the tag. The tag performance can be improved further as follows: by using directional high gain reader antennas for better transmission and reception of the backscattered signal; embedding of the tag in the sample under test to remove the issue of lift-off, thus maintaining the effective dielectric constant for the tag; the tag performance also depends on the thickness of the substrate and the permittivity of  
Figure 9:2 Experimental results for different depths of (**a**) horizontal crack and (**b**) vertical crack. the dielectric material. To understand the effect of the substrate thickness, an analysis of different substrate thicknesses was performed, as shown in Figure 15; for 0.7 mm thickness, the RCS response of the tag is much better compared to the 1.4 mm thickness and 2.1 mm dielectric substrate thickness of the tag.