# A Direction-Sensitive Microwave Sensor for Metal Crack Detection
## 4 Experimental Study and Discussion  
To verify the performance of the proposed sensor for metal crack detection, a prototype was fabricated with the dielectric substrate of F4B. Additionally, the dielectric constant is 2.2, and the thickness is 1 mm. The fabricated sensor is illustrated in Figure 13a. Several aluminum plates are machined to generate artificial flaws to simulate real metal cracks, as presented in Figure 13b. Additionally, reflection coefficients of the fabricated sensor are measured with Keysight vector network analyzer N5244A. For scattering parameters used in the experiments, S11 is the reflection coefficient of Port 1, while S22 is the reflection coefficient of Port 2, which can be obtained by vector network analyzer (VNA). The reflection coefficients of the fabricated sensor for crack with different parameters are measured. Figures 14 and 15 show the measurement results for the horizontal and vertical crack with a fixed depth of 2 mm and a varying width of 0.6 mm\(\sim\)1.2 mm. According to measurement results, when there is no crack on the aluminum plate, the measured reference resonant frequencies of port 1 and port 2 are 2.485 GHz and 2 GHz, respectively, which correspond well to the simulation results. From Figure 15a, the resonant frequency of port 1 decreases from 2.4 GHz to 2.34 GHz when the crack width increases. Additionally, according to Figure 14b, the resonant frequency of port 2 shifts from 1.958 GHz to 1.92 GHz when the crack width increases. The sensitivity of port 1 and port 2 for crack width are 100 MHz/mm and 63.3 MHz/mm, respectively. Figure 12: Simulated reflection coefficients of the designed sensor for crack with different directions. (**a**) Port 1. (**b**) Port 2. Figure 14: Measured reflection coefficient results of fabricated sensor for vertical crack with different widths. (**a**) Port 1. (**b**) Port 2. Figure 13: Fabrication and measurement. (**a**) Fabricated sensor. (**b**) Metal crack on aluminum plate. (**c**) Reflection coefficient measurement by VNA. Next, when the metal crack width is set as 0.6 mm, the measurement results at each port with different crack depths are as Figure 16 shows. It can be seen that the measured resonant frequency shifts toward lower frequencies as the crack depths increase. Additionally, the sensitivity of the sensor for depth detection is about 20 MHz/mm and 14 MHz/mm for port 1 and port 2, respectively. Then, the crack width is fixed at 0.6 mm, and the crack depth is fixed at 2 mm; the measurement results of the reflection coefficient with different crack directions are shown in Figure 17.  
The measurement results show that the resonant frequency of port 1 drops from 2.491 GHz to 2.4 GHz as the crack direction angle varies from 0\({}^{\circ}\) to 75\({}^{\circ}\), while the resonant frequency of port 2 shifts toward an opposite direction, from 1.971 GHz to 2 GHz. Additionally, the sensitivities of port 1 and port 2 for crack direction detection are 6.07 MHz/5 degrees and 1.93 MHz/5 degrees, respectively. The results of simulation and measurement are listed in Table 2. Through comparison, the measured reference frequency and crack width detection sensitivity agree well with the simulation results. Additionally, the experimental sensitivity of crack depth detection is larger than the simulation. For the sensitivity of crack direction detection, there is a minor discrepancy between the experiment and simulation. Combing the results above, it can be  
Figure 16: Measured reflection coefficient results of fabricated sensor for crack with different depths. (**a**) Port 1 to horizontal cracks. (**b**) Port 2 to vertical cracks. Figure 15: Measured reflection coefficient results of fabricated sensor for horizontal crack with different widths. (**a**) Port 1. (**b**) Port 2.  
concluded that the proposed sensor has a favorable performance for the crack dimensions and direction detection.