## 2 Experimental Setup
### Design, Realization, and Characterization of the Dielectric Waveguide  
Electromagnetic waves (EWs) can propagate in 3D open space or in a volume constrained by metallic boundary conditions. Metallic boundary conditions cause electromagnetic fields to disappear at the edges, resulting in complete reflection. EWs propagating in so-called waveguides are denoted as guided EWs. Resonators are a special case in which metallic boundary conditions exist in all three spatial dimensions. Cavities indicate that the dielectric consists of a gas or vacuum. On the other hand, dielectric resonators can also be filled by a dielectric medium [18]. Concerning electrodynamics, solving [PERSON]'s equations results in characteristic resonant frequencies that can be calculated analytically for plane-parallel, rectangular, and cylindrical waveguides [19]. For other cross-sections and non-metallic boundary conditions, such as those present in omega stringers, only numerical calculations are possible [7]. For the proof of concept in this work, a rectangular dielectric resonator was filled with polypropylene with dimensions of 200 mm \(\times\) 100 mm \(\times\) 3.3 mm, as shown schematically in Figure 2. A numerical model in CST Microwave Studio is depicted in Figure 3a. Four scattering parameters with the two-port setup can be measured when electromagnetic energy is coupled into and out of the waveguide through the K-connectors. The metallic housing made of aluminum is used to fulfill the three-dimensional metallic boundary conditions for the lowest possible transmission loss. In a previous study, [PERSON] et al. [18] used this waveguide under laboratory conditions with a vector network analyzer (VNA) for ultra-wideband stimulation from 20 GHz to 40 GHz. To set the resonant frequency to the center frequency of 30 GHz, a thickness of 3.3 mm was determined for the polypropylene matrix. However, the VNA cannot be used for in situ demonstration due to its large dimension and significant weight. Together with an aluminum test plate, which is shown in Figure 3b, the waveguide is completed. The test plate is screwed to the substrate and consists of seven holes (numbered consecutively) with diameters ranging from 2 mm to 8 mm, which can be closed with screws. Figure 2: Schematic representation of the waveguide with a propylene matrix in two spatial dimensions. The waveguide covers 200 mm of the DUT for determining its structural health. The direction of EM wave propagation is denoted as \(z\). Starting with the reference state, which represents the intact structure of the DUT, the screws are unscrewed one after the other in order to measure structural changes systematically. Preliminary numerical and experimental results with the VNA from 23.5 GHz to 26 GHz are shown in Figure 4. \(S_{11}\) stands for the reflected signal at port 1 and \(S_{21}\) for the transmitted signal from port 1 to port 2. Due to the symmetry of the waveguide, \(S_{12}\) and \(S_{22}\) are similar. More transmission is given with a higher frequency up to the excitation of the first mode at 30 GHz. The trends of simulated and experimental \(S_{11}\) are comparable. The proposed device benefits from the metallic housing so that microwave radiation remains inside the cavity. This means that the immunity and susceptibility of the UAV is not affected [20]. The same shielding protects the device from electromagne  
Figure 4: Comparison of numerical and experimental signals in frequency domain from 23.5 GHz to 26 GHz for the case of reflection (\(S_{11}\)) and transmission (\(S_{21}\)). Figure 3: **(a)** Complete rectangular waveguide design containing two K-connectors, a polypropylene substrate, metallic housing, and an aluminum test plate as the DUT. **(b)** The bolts (numbered) of the test plate are removed one by one to reduce electromagnetic energy inside the waveguide. the outside, for example, coming from criminals and the environment [21].