# Microwave NDT of Smart Composite Structures with Embedded Antennas
## 2 Detection Concept and Simulation  
As explained earlier, CLAS refers to antennas sandwiched between multiple layers of composite. A schematic depicting a multilayer composite with a patch antenna fabricated on one of the layers is shown in Figure 1. As can be seen from the figure, a patch antenna operating in the C-band is sandwiched between a paper-based honeycomb substrate and a GFRP sheet from above. The planar resonator probe (UHF probe) is used for inspecting the antenna embedded inside (patch in this case) and placed at a standoff distance \(d\) from the surface of the top cover. The probe radiates the structure under test, such that it excites current on the antenna surface underneath it. The UHF probe is very sensitive to current distributions and any deformity that represents a structural anomaly can easily be detected. Moreover, a larger structure can be interrogated by the probe owing to its greater wavelength, avoiding the possibility of false detection. This is highly likely for probes operating at a higher frequency and shorter wavelength, where the null in the electric field can easily be mistaken for structural deformity in the SUT. The potential utility of microwaves in the non-destructive evaluation of composites is well known. However, the selection of the microwave probe for the detection of defects in a given application is critical. While near-field aperture probes have been demonstrated to be effective for inspecting composites, the distinct advantage of using a planar resonator probe, or a UHF probe, is explained herein. The planar resonator probe introduced in [30] is highly sensitive to discontinuities in current distribution and operates at a low frequency of 300-900 MHz. For simulation studies and analysis, a representative CLAS based on the structure shown in Figure 1 was designed and simulated using CST Studio Suite. The antenna was sized appropriately to operate in the C-band (4-8 GHz), which is commonly used in practical applications. A copper-grounded square patch of dimensions 27 mm by 25.88 mm was created on an air substrate with a dielectric value similar to a paper-based honeycomb substrate. A 15 mm long and 4 mm wide quarter wavelength transformer, in addition to a feed measuring 15 mm in length and 14.75 mm in width, was also made. The width of the feed was selected to ensure 50-ohm matching. Finally, a 1.5 mm thick substrate of GFRP was used to cover the antenna, as depicted in Figure 1. The structure was simulated as built and is referred to as \"healthy antenna\". Subsequently, a 2 mm wide cut was introduced in the transformer line, in addition to a diagonal slit on the patch itself to emulate a \"damaged antenna\". The reflection coefficient of the healthy and damaged antenna obtained from simulation is presented in Figure 2. It is evident from the figure that the antenna resonates sharply at the intended frequency band ~4.6 GHz with a narrowband response expected from a simple square microstrip patch antenna. However, the damaged antenna renders a less efficient response and barely resonates at ~10 dB. To ensure and further investigate the operability of the damaged antenna, the radiation pattern and electric field distributions were plotted. Figure 3a,b show the simulated antenna gain radiation patterns in the two principal planes (E, H) for the healthy and damaged scenarios, respectively. The farfield radiation pattern is another important parameter that is used to characterize an antenna. As can be seen in Figure 3a, the simulated healthy microstrip patch antenna radiates unidirectionally towards the broadside as expected, with minimal back lobes and a symmetric pattern in the E-plane. While the damaged version has similar characteristics, the maximum gain is significantly reduced (~5.8 dBi). Figure 3b shows the orthogonal polar cut (H-Plane) of the gain pattern for both antenna states (healthy and damaged) as annotated in the figure. Figure 1: A schematic showing the antenna embedded in a composite structure, and a UHF probe used for inspection of the structure.