# Microwave NDT of Smart Composite Structures with Embedded Antennas
## 3 Measurement Results  
To demonstrate the utility of microwaves in inspecting antennas embedded in composites, a representative sample was made. A photograph of the various layers constituting the composite structure is shown in Figure 6a. The lowermost layer is a 0.1 mm thick copper foil, which also serves as the ground plane for the antenna. The middle layer is an aramid paper-based honeycomb substrate of thickness 3 mm. To ensure the smooth laying of the antenna, the honeycomb substrate was covered with a 0.15 mm thick paper tape. A simple patch antenna operating in the C-band was then fabricated and sandwiched between the honeycomb substrate and a GFRP sheet. A rectangular patch antenna was chosen for the demonstration, since it is the most common in such applications. Initially, the patch was made out of copper foil, as depicted in Figure 6b. \begin{table}
\begin{tabular}{c c} \hline
**Probe Type** & \(\Delta\)**1**S**1**1 **(Linear)** \\ \hline UHF Probe (at 510 MHz) & 0.72 \\ \hline K-band rectangular aperture probe (at 25 GHz) & 0.27 \\ \hline \end{tabular}
\end{table} 
Table 1: Change in magnitude of reflection coefficient of imaging probes. Figure 5: Picture of the simulation setup to assess probe sensitivity for damaged patch antenna. The patch antenna dimensions measured 27 mm by 25.88 mm, with a quarter-wavelength transformer width of 4 mm and length of 15 mm, in line with the simulations. The 50-ohm feed of the antenna is also 15 mm long with a width of 14.75 mm to ensure 50-ohm matching. An SMA connector was then soldered onto the patch and connected simultaneously with the ground plane. Finally, a 1.5 mm thick GFRP sheet was used to cover the antenna from the top, as illustrated in Figure 6c. Subsequently, to emulate a damaged antenna, a 2 mm wide cut was deliberately introduced in the transformer line. Additionally, a diagonal cut (slit) was also made on the patch itself, as annotated in Figure 6b. The antenna was then characterized pre- and post-damage using a Keysight N5225A performance network analyzer (PNA). The measured antenna response, in terms of the reflection coefficient, was plotted against the simulated response and is shown in Figure 7. It is evident that a close match between simulation and measurement was obtained. The minor differences in the response, particularly the reflection coefficient of measurement not at zero, could be explained due to the fact that calibration was performed only up to the cable and not up to the SMA connector. The slight shift in resonance frequency of the damaged antenna could be attributed to fabrication tolerances and human error while introducing the damage into the patch antenna. In line with the simulation results, the antenna was radiating in the post-damage state. After ensuring that the antenna was working even after being damaged, the structure was then inspected using the planar resonator probe to non-destructively test and portray any damage/irregularities in the structure through high resolution images. Figure 6: (a) Picture of the individual layers of the composite structure, (b) the C-band patch antenna fabricated on a honeycomb substrate and (c) the embedded antenna covered with a GFRP sheet. From an NDT point of view, the detection of such minor damage in the antenna, which does not hinder its performance significantly, is crucial. It is indeed very difficult to determine the extent of damage merely from the reflection coefficient response of the antenna. Unless detected during an early stage, this may lead to the complete failure of antennas embedded in composites, and consequently the smart structure. To this end, the GFRP-covered composite with an embedded antenna was imaged using the UHF probe. The PNA mentioned before (Keysight N5225A) was used as a source/detector. The microwave imaging setup employed for all imaging experiments is shown in Figure 8. For all imaging experiments, the UHF probe was fixed at a liftoff of 1 mm from the topmost surface of the composite sample.