## 4 Antenna Prototype Fabrication and Test Results
### OCROP Antenna Prototype  
A prototype of the differentially-fed, dual-polarized OCROP antenna, as shown in Figure 9, was fabricated and tested. Printed circuit board (PCB) substrates with copper on one side were used for constructing the ground cavity, as this had the dual benefit of being easy to solder and having low mass (compared to a solid piece of the copper plate). Since the dielectric material of the PCB substrate was behind the copper ground cavity, its material properties had minimal effect on the antenna performance. Due to the material available at the fabrication facility, DiClad 527 (\(\varepsilon_{r}\) = 2.5 and \(\tan\delta\) = 0.0018) with a thickness of 0.762 mm was used. As described earlier, the open prism is a 51 mm \(\times\) 51 mm \(\times\) 51 mm bottomless hollow cube. The supporting substrate for the feedlines and the open prism is 0.762 mm thick Rogers CuClad 217. For better alignment and easier assembly of the prism, a 51 mm \(\times\) 51 mm relief was routered in the center of the substrate. The depth of the relief is half the substrate thickness. The open prism was then placed in the square relief. The open prism was milled from an aluminum cube and had a wall thickness of 0.4 mm. The supporting substrate was suspended 15 mm above the ground plane. i.e., g = 15 mm. The prism was mechanically attached to the supporting substrate with a small stainless-steel screw in each bottom corner. The ground plane length and sidewall height were 165 mm and 80 mm, respectively. Detailed antenna dimensions are given in Table 2. The reference dimensions can be seen in Figure 1.  
To evaluate the differential reflection coefficients of the antenna, we conducted two types of measurements with a two-port VNA. The first type measured single-ended S-parameters at all antenna ports. The differential reflection coefficients were obtained from the post-processing of these data by assuming differential signals for one differential pair and matched loads for the other pair. In simulations, the reflection coefficients were calculated using the same post-processing approach (an already established function in HFSS) but with the single-ended S-parameters obtained numerically in HFSS. Both simulated and measured differential reflection coefficients using this method are shown in Figure 10a. S\({}_{\text{dd11}}\) and S\({}_{\text{dd22}}\) are Y- and X-polarized differential reflection coefficients, respectively. As \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \(L\) = \(W\) = \(H\) & \(L_{g}\) & \(H_{sw}\) & \(g\) & \(s\) & \(l\) & \(w\) & \(l^{\prime}\) & \(w^{\prime}\) \\ \hline
51 & 165 & 80 & 15 & 3 & 53 & 30 & 1 & 3.6 \\ \hline \end{tabular}
\end{table} 
Table 2: OCROP prototype dimensions (all dimensions in mm). Figure 8: Schematic configuration of the differential feed network. can be seen, there is good agreement between the simulated and measured results in terms of the operating frequency range where S\({}_{\text{dd11}}\) and S\({}_{\text{dd22}}\)\(\leq\)\(-\)10 dB. To provide differential feeding to the antenna, we used the feed network described in Section 4.1. The 3 dB 180\({}^{\circ}\) hybrid coupler divides the input signal from the differential port (\(\bigtriangleup\)) into two output signals with equal amplitude and opposite phases at port 0\({}^{\circ}\) and port 180\({}^{\circ}\) of the coupler. In the differential reflection coefficient measurement, the port \(\bigtriangleup\) of the coupler was connected to the VNA, and the coupler ports 0\({}^{\circ}\) and 180\({}^{\circ}\) were connected to the antenna ports 0\({}^{\circ}\) and 180\({}^{\circ}\). The measured results showed a 0.5 dB magnitude imbalance and 176.6\({}^{\circ}\) phase difference at the two differential ports for 1.5 GHz. Figure 10b shows the reflection coefficient measured at the input to the combined feeding network and antenna. Several additional notches appeared in the measurement, which was attributed to the effects of the hybrid coupler.