# Microwave NDT of Smart Composite Structures with Embedded Antennas
## 3 Measurement Results
The magnitude and phase images (c.f. Figure 11c,d) depict the induced damage very distinctly. The cut in the transformer line is apparent in the magnitude image. Furthermore, the diagonal slit on the patch is also faithfully rendered in both magnitude and phase images. In order to benchmark the imaging capability of the UHF probe, the damaged patch antenna was imaged by a standard K-band rectangular aperture probe. The purpose of benchmarking the imaging results to a K-band rectangular aperture is because it is widely used in microwave NDT applications. It is noted here that the standard rectangular waveguide aperture probe is linearly polarized, while the UHF probe is not (it is polarization-independent). For this imaging experiment, the K-band rectangular aperture was positioned such that its polarization was along the direction of the patch antenna feed. The magnitude and phase images obtained from the rectangular aperture probe are depicted in Figure 11e,f. While the cut on the transformer line is visible in the magnitude image, the diagonal slit is not discernible. The target is almost missed by the rectangular aperture probe. Figure 5:0 Magnitude and phase images of the (**a,b**) healthy and (**c,d**) damaged C-band patch antenna fabricated using metal mesh, produced by the UHF probe, and (**e,f**) damaged patch produced by K-band rectangular aperture probe. This experiment highlights the sensitivity of the UHF probe to the presence of thin cuts/slits, as it is particularly sensitive to changes in current distributions. To further evaluate the imaging performance of the probe, the signal-to-noise ratio (SNR) values of the images produced by the UHF probe and the K-band rectangular aperture probe were calculated. Table 2 shows the SNR values of the images, and it can de deduced that the UHF probe image has a significantly high (more than 8 times) SNR in contrast to the K-band rectangular aperture probe image. The qualitative as well as quantitative metrics therefore highlight the proficiency of the UHF probe in terms of the NDT of the antenna-embedded composites. It is further remarked that the lateral spatial resolution of the UHF probe has been established to be 3 mm [30], while that of the K-band rectangular aperture probe is limited to half the broad dimension, around 5.35 mm in this case. Additionally, using high- frequency probes for inspection in this application may not necessarily yield the best results, since the antenna under test (patch in this case) might be a good portion of the operating wavelength. Hence, by employing a low-frequency probe, such as the UHF probe (used herein), the structure under inspection is much smaller than the wavelength and thus better imaging results are obtained. To summarize, the planar resonator probe utilized herein offers significant advantages over the conventional rectangular aperture probe. Firstly, it is polarization-independent and can detect cuts/slits regardless of their orientation, which is not the case for rectangular apertures. Secondly, it is highly sensitive to current distributions and operates at a low frequency thereby having a larger wavelength. This feature is considered to be favorable in these applications, since it limits false detections, as explained earlier. Thirdly, the probe renders high-resolution images with relatively better SNR at such low frequency that are comparable to images generated by probes operating at much higher frequencies. Overall, the effectiveness of microwave NDT as a modality for the in situ inspection of composite smart structures is shown in this work.