# A Novel Chipless Hybrid RFID Sensor for Metal Crack Detection
## 3 Results and Discussion  
The proposed tag is printed on the Fr4 substrate with a permittivity of \(\epsilon=4.4\) and substrate thickness of 1.4 mm, as shown in Figure 10. Steel samples are used for testing. Figure 10 shows the printed proposed tag, which is quite small in size (16 mm \(\times\) 16 mm \(\times\) 1.4 mm), and Figure 10 shows the defect profiles used for the experimental setup. Two types of defect profiles are used. * Defects with two widths 2 mm and 3 mm are tested. * Defects with three different depths, 1 mm, 3 mm, and 5 mm, are tested (Figure 10). * The tested defects are in the horizontal and vertical orientation (Figure 10,c). Figure 10,c have vertical defects and horizontal defects, each with dimensions of 10 mm \(\times\) 2 mm \(\times\) 2 mm (length \(\times\) width \(\times\) depth), respectively. Figure 10 shows the different depths of the defect profile. The length of each defect is 10 mm and the width is 2 mm, while the depths vary. The defect with 1 mm depth is a little blurry as shown in the picture. Figure 9: Table shows the crack dimension, its resonant frequency, and \(\Delta\)Fr. The experimental setup consists of a VNA that acts as a reader and two Vivaldi antennas are connected to the VNA; one is a transmitter and the other is a receiver. Both antennas are set up to radiate and receive vertically polarized EM waves. The test setup is arranged in a table-top setup environment to imitate the real environment and hence there will be reflections coming from the objects in the testing area. The main goal is to observe the tag performance in a real-time test setup environment and make the system more feasible. The transmitter and receiver antennas are kept at 5 cm distance apart. The tag is attached to a steel sample of 15 cm \(\times\) 15 cm \(\times\) 0.5 cm with transparent duct tape. The power delivered by the VNA is set as 0 dBm in the entire frequency band with an averaging function activated for 100 measurements to increase the signal-to-noise ratio. The tag to be tested is kept 10 cm away from the reader antennas, as shown in Figure 11.  
Figure 12 shows the experimental results for two widths (2 mm and 3 mm) of defects; the top figure is for a horizontal crack, and the bottom figure is for a vertical crack. The resonant frequency with no crack is 3.9 GHz. For 2 mm width, the resonant frequency is reduced to 3.78 GHz, and for 3 mm width, it is further reduced to 3.72 GHz. As the width of the horizontal crack is increased, the resonant frequency is reduced, thus matching with the simulation results in Figure 5. The bottom subfigure of Figure 12 shows for the vertical crack that as the width is increased from 2 mm to 3 mm, the resonant frequency is increased to higher values, which matches with the concept shown in the simulation Figure 7. A wide notch observed in the vertical crack of 2 mm is due to the background reflection; however,  
Figure 11: Experimental setup of the tag. Figure 10: (**a**) Printed proposed sensor tag. Defect profiles for (**b**) vertical defect; (**c**) horizontal defect; (**d**) three defects with different depths (5 mm, 3 mm, 1 mm). it is reduced to 3 mm width. Thus, a reduction in the resonant frequency of the tag can indicate a horizontal crack, and an increase in the resonant frequency can indicate a vertical crack. Also, the RCS spectrum includes noises and reflections from the background; despite this noise, the spectrum shows the shift in resonant frequencies with different crack profiles. The defects with three depth profiles are created on a larger rectangular steel sample of dimensions 31 cm \(\times\) 10 cm \(\times\) 0.5 cm. This is done to investigate two things:  
* RCS with different defect depths. * RCS from a large metal sample with defects. Figure 13a shows the vertical rectangular sample with horizontal defects with different depths and Figure 13b shows the horizontal rectangular sample with vertical defects with different depths. The tag is placed on each defect and the RCS is measured.