# Design of Dual Circularly Polarized Sequentially-Fed Patch Antennas for Satellite Applications
## 2 Design and Characterization of a Dual-CP Sequentially-Fed Single Patch Antenna  
In this Section, the design and fabrication of a dual-CP sequentially-fed patch antenna are presented, extending the results shown in [21] through the prototype fabrication and experimental characterization in an anechoic chamber. Let us consider a circular metallic patch suspended in air to minimize losses due to the dielectric (Figure 1). It is fed in such a way to radiate a broad beam in the boresight direction (\(\theta=0^{\circ}\)) by exciting the fundamental \(TM_{110}^{2}\) mode, according to the cavity model in [22]. Such a circular patch is sequentially-fed by using three pins rotated of \(120^{\circ}\) each other [15], in such a way to generate a CP with a considerable AR bandwidth. The proper phase-shift was achieved by designing the feeding network in microstrip technology on the antenna back. With reference to Figure 2a, the microstrip line impedances \(Z_{i}\) and lengths \(L_{i}\) were properly synthesized according to the systematic methods explained in [23; 24; 25; 26; 27; 28], and for the sake of convenience are reported in Table 1. The substrate used to realize the feeding network was a ROGERS TMM4 of thickness \(h_{d}=0.76\) mm, relative electrical permittivity \(\epsilon_{r}=4.5\) and loss tangent \(\tan\delta=0.002\). In such a network, the two input ports allow dual-CP operation mode, by simply alternatively feeding the two ports. To increase the antenna gain close to the end-fire direction (\(\theta=90^{\circ}\)), a fence of eight passive monopoles, connected to the antenna ground plane, was placed. This solution mitigates the unavoidable cross-polarization at high elevation angles [21], and is lightweight relative to cavity-backed antenna solutions [29]. The antenna geometry was parametrically studied by using the full-wave commercial software Ansys HFSS, by acting on its relevant parameters, whose optimized values are summarized for the sake of conciseness in Table 2. It is worth mentioning that the antenna was optimized by means of physical considerations and not by using blind multi-variable optimization algorithms. The dominant physical effects associated to most relevant geometrical parameters on final antenna radiation pattern. For instance, monopole heights \(h_{m}\) were tuned from \(\lambda_{0}/5\) to \(\lambda_{0}/4\) to achieve satisfactory gain compensation at high elevation angles. Moreover, patch radius \(r_{p}\) was chosen to set antenna central frequency, according to the general considerations in [22]. Finally, the patch height \(h_{0}\) was also used for tuning purposes. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Microstrip Line** & **Impedance [\(\Omega\)]** & **Length [mm]** \\ \hline \#1 & 90.2 & 8.19 \\ \#2 & 75.1 & 6.48 \\ \#3 & 86.4 & 5.39 \\ \#4 & 55.6 & 5.78 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Dual-circularly polarized (CP) single patch feeding network parameters. Figure 1: Side-view of the proposed sequential-fed patch antenna loaded by monopoles properly designed to increase the antenna gain close to end-fire direction, in which the relevant geometrical parameters are highlighted. Figure 2: (**a**) Schematic of the feeding network in microstrip technology. By alternatively feeding the antenna at ports RHCP or LHCP, dual-CP operation mode is enabled. (**b**) Prototype of sequentially-fed patch antenna surrounded by a fence of passive monopoles and operating at \(f=8.25\) GHz. Furthermore, to verify the correctness of the design procedure, a prototype of sequentially-fed circular patch antenna was fabricated for operating over the bandwidth \([8.1,8.4]\) GHz, as shown in Figure 2b. The effect of passive monopoles on the radiated field pattern was investigated by simulating the resulting antenna both in presence and in absence of monopoles, as shown in Figure 3a,b, both in \(\phi=0^{\circ},90^{\circ}\) cut planes.