# Microwave NDT of Smart Composite Structures with Embedded Antennas
## 3 Measurement Results
The probe was held stationary, while the composite sample was moved in controlled but predetermined step sizes using an automated scanning table equipped with motorized linear stages. The sample was moved with a step size of 1 mm along each axis. During each step, the complex reflection coefficient (S\({}_{11}\)) was recorded in and saved as a two-dimensional matrix. The scan data thus saved were then processed in Matlab software (R2022 a version) by indexing the complex data for all frequency swept points over the total scan area. Subsequently, the complex data for a particular frequency point (of interest) were then selected and a magnitude image was created by plotting the absolute value of the complex data over the scan area. Similarly, the angle of the complex data was plotted over the scan area to obtain the phase image. Two different scans were performed for the healthy and the intentionally damaged antenna-embedded composite. The images of both samples generated by the UHF probe are shown in Figure 9. The magnitude and phase images are placed side by side in each case. The images of the healthy antenna embedded in the composite are depicted in Figure 9a,b while the images for the damaged antenna-embedded composite (magnitude and phase) are presented in Figure 9c,d. The UHF probe produced a sharp depiction of the patch antenna structure with clear distinction from the background. The transformer line and the 50-ohm feed are also clearly visible in the image. On the other hand, magnitude and phase images of the damaged antenna also portray the patch antenna outline sharply. Moreover, the 2 mm wide cut and the diagonal slit in the patch are conspicuous in the image. The gap  
Figure 8: Photograph of the imaging setup. Figure 7: Measured magnitude of reflection coefficient of the patch antenna before and after inflicting damage. created in the transformer line due to the cut manifests itself in both magnitude and phase images. Furthermore, the diagonal slit on the patch is distinguishable from the intact area with clear demarcation, particularly in the magnitude image. To further investigate microwave NDT for antenna-embedded composite structures, a patch antenna operating in the C-band was fabricated using a highly conductive copper mesh with a surface resistance <0.03-ohm. The same mesh was utilized as the ground plane for the antenna. It is to be noted that in actual applications the antenna will be meshed. Microwave NDT methods have been used extensively to detect cuts in sheets, but not metallic meshes. Therefore, the imaging performance was initially demonstrated on the copper foil to establish a reference to something common (sheet/foil) before introducing the mesh application. Similarly to the previous experimental procedure, two scenarios representing a healthy and damaged antenna were considered. For each imaging scan, the antenna was covered with the 1.5 mm thick GFRP sheet. A higher magnification picture of the healthy, damaged and GFRP-covered antenna is shown in Figure 9(a)-c respectively. The UHF probe employed in the previous experiment was used to image the two sets of SUT. The magnitude and phase images thus produced by the probe for the healthy antenna are shown in Figure 9(a),b, respectively. As evident from the images, the probe produced  
Figure 10: Photographs of (**a**) healthy and (**b**) damaged patch antenna fabricated using metal mesh, and (**c**) damaged antenna covered with a GFRP sheet. Figure 9: Magnitude and phase images of the (**a,b**) healthy and (**c,d**) damaged C-band patch antenna fabricated using copper foil, produced by the UHF probe. very sharp depictions of the patch, the transformer and the feed. The background in the image is also very distinct and clear, particularly in the magnitude response. Subsequently, the mesh was then intentionally damaged by severing the transformer line with a 2 mm wide cut, and a diagonal slit was made across the patch, as illustrated in Figure 10b. The damaged patch antenna-embedded composite structure was then imaged using the UHF probe.