## 4 Realization and Measurements  
As stated, the FFF technology has been used to realize the main element of the DRA by using a lab-made printable filament with enhanced dielectric properties. It has been printed with a Prusa MINI+ desktop 3D printer [26], using the printing settings listed in Table 2.  
Conversely, for the substrate hosting the microstrip feed slot, it has been realized by using a commercial PREPERM PPE1200 substrate of 60 \(\times\) 60 \(\times\) 2 mm\({}^{3}\)[27], while the metallic parts have been shaped onto a copper adhesive tape, through a Graphtec CE 6000-40 cutting plotter [28]. Some pictures of the realized prototype can be seen in Figure 4, where both the resonator and the microstrip feed slotted substrate are shown before and after the assembling procedure. The two elements have been locked together by means of a very thin layer of silicone glue. The so-realized device has been measured by using a Siglent SVA1075X Vector Network Analyzer (VNA) and calibrated with a Siglent F604 FS kit to determine the S\({}_{11}\) curve. The comparison between measured and simulated results is shown in Figure 5, where a picture taken during the measurements is also reported. As can be seen, there was a very good agreement between simulations and measurements, and the overall bandwidth of the realized prototype exhibited values even larger than the simulated ones, which led to a fractional bandwidth (FB) slightly less than 46%. The maximum gains for both 2.45 GHz and 3.6 GHz (which are the center frequencies of the two band of interest) were 5.45 dBi for the former and 6.38 dBi for the latter, as can be seen in Figure 6, where the co-polarized and cross-polarized gain radiation patterns are shown in both E-plane and H-plane. For the former (Figure 6a), the main lobe directions were \(-3^{\circ}\) and \(+3^{\circ}\) for the two frequencies, while the 3 dB angular width was 98.8\({}^{\circ}\) at 2.45 GHz and 73.4\({}^{\circ}\) at 3.6 GHz, respectively. For the latter (Figure 6b), the main lobe directions were 0\({}^{\circ}\) for the two frequencies, while the 3 dB angular width was 94.3\({}^{\circ}\) at 2.45 GHz and 75\({}^{\circ}\) at 3.6 GHz, respectively. The low value of cross-polarization indicates, as expected, that the prototyped antenna was linearly polarized. For an easier visualization, \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Extrusion Temperature** & **Bed Temperature** & **Infill** & **Speed** \\ \hline
210 \({}^{\circ}\)C & 60 \({}^{\circ}\)C & 100\% & 60 mm/s \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Printing settings used for the PLA-BaTiO\({}_{3}\) (17.5% Vol.) filament. Figure 3: Rendered view of the DRA (on the **left**). Technical views of the DRA with dimensions in mm (on the **right**). the 3D versions of the antenna radiation gain patterns, are shown in Figure 7, as well as the maximum gain and the values of total and radiation efficiency over frequency. These values can allow researchers to calculate an interesting metric that can easily give an idea of the miniaturization goodness achieved by the antenna prototype. This is the so-called miniaturized antenna figure of merit (MAFM), which was defined in [29] by the equation:  
\[\mathrm{MAFM_{dB}}=10\log_{10}\biggl{(}\eta\cdot\frac{\Delta f}{f_{c}}\cdot \frac{\lambda}{D}\biggr{)}, \tag{1}\]  
where \(\eta\) is the antenna efficiency; \(\Delta f/f\) is the FB; \(\lambda\) is the free-space wavelength, and \(D\) is the largest linear dimension of the antenna. The proposed prototype exhibited a \(\mathrm{MAFM^{2.45 GHz}}=-0.053\) dB if evaluated at 2.45 GHz and a \(\mathrm{MAFM^{3.6 GHz}}=-2.56\) dB if evaluated at 3.6 GHz, which are quite good results if it is considered that a simple half-wavelength dipole with a 99% efficiency and a narrow band of 5% has a MAFM of about \(-10.04\) dB. Figure 4: Top view of both the resonator element and the slotted ground plane disassembled (on **top left corner**). Top frontal views of the assembled DRA (on **bottom left corner**).