# Design of Dual Circularly Polarized Sequentially-Fed Patch Antennas for Satellite Applications
## 3 Dual-CP 2-by-2 Array Antenna for Satcom Applications  
Once we evaluated the radiation performances and the polarization purity of a single sequentially-fed patch antenna loaded by a fence of passive monopoles, the antenna was properly scaled to be used as a building block for the design of a dual-CP 2-by-2 array, operating in the higher UHF band at \(f=2.2\) GHz. In this case, the antenna was loaded at its center and around it by passive monopoles to increase the antenna gain close to end-fire direction. In Figure 4a the array geometry is presented by highlighting the relevant parameters for antenna optimization. To enhance the antenna compactness, the circular patches composing the array were cut in correspondence of \(\phi=0^{\circ},90^{\circ}\) array symmetry axes, without significantly affecting the radiation pattern due to the two-folded symmetry preservation. In addition, the presence of four passive monopoles at the antenna center helps to compensate tip/edge effects due to the cuts. The final parameter list used to fabricate the antenna prototype is shown in Table 3.  
Moreover, in Figure 4b, the simulated reflection coefficient (\(S_{11}\), \(S_{22}\)) versus frequency, obtained by alternatively feeding both RHCP and LHCP ports, is shown, thus demonstrating a good array impedance matching. Instead, in Figure 4c the antenna gain at \(\theta=0^{\circ}\) is shown over all the considered bandwidth, clearly demonstrating a satisfactory gain for a feeder of reflectors in Satcom applications. On the other hand, in Figure 4d, the simulated broadside AR versus frequency is presented in the bandwidth [2; 2.3] GHz, by considering both presence (traces with filled markers) and absence (traces  
Figure 3: Simulated RHCP (\(G_{R}\)) and LHCP (\(G_{L}\)) gain patterns versus elevation angle with (filled markers) and without (empty markers) monopoles around the single radiating patch, for the cut-planes at (**a**) \(\phi=0^{\circ}\) and at (**b**) \(\phi=90^{\circ}\). The results are provided at the central frequency \(f=2.2\) GHz. (**c**) Simulated (empty triangular/squared markers, dashed lines) and measured (filled triangular/squared markers, continuous lines) reflection coefficients at RHCP (\(S_{\text{RHCP}}\)) and LHCP (\(S_{\text{LHCP}}\)) input ports, together with simulated (empty circular markers, dashed line) and measured (filled circular markers, continuous line) isolation between RHCP and LHCP ports (\(S_{\text{ISOL}}\)), in the considered bandwidth [8.1, 8.4] GHz. (**d**) Measured co-polar and cross-polar gain patterns at \(\phi=0^{\circ}\) versus elevation angle at the operating frequency \(f=8.25\) GHz. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Parameter** & **Description** & **Value [mm]** & **Value [\(\lambda_{\phi}\)]** \\ \hline \(h_{d}\) & Dielectric thickness & 2.57 & 0.019 \\ \(h_{a}\) & Air gap thickness & 9 & 0.07 \\ \(R_{p}\) & Patch radius & 29 & 0.21 \\ \(R_{m}\) & Monopole radius & 1 & 0.007 \\ \(h_{m}\) & Monopole heights & 24 & 0.20 \\ \(l_{c}\) & Patch cut length & 41 & 0.30 \\ \(d_{m}\) & Adjacent monopole distance & 4 & 0.03 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: The 2-by-2 array geometric parameters (\(f=2.2\) GHz). with empty markers) of monopoles. As it is apparent, monopoles are able to greatly increase antenna polarization purity at high elevation angles (\(\theta>45^{\circ}\)), since the simulated AR can be decreased up to 1 dB over almost all the operating bandwidth. Moreover, to highlight the effect of monopoles also on antenna gain elevation pattern, in Figure 5, the simulated RHCP (\(G_{R}\)) and LHCP (\(G_{L}\)) gain patterns versus elevation angle are shown, with (filled markers) and without (empty markers) monopoles around the 2-by-2 array, for the cut-planes at (a) \(\phi=0^{\circ}\) and at (b) \(\phi=90^{\circ}\).