# Exploring CP Array Antenna Characteristics and Its Applications to a Deployable Structure
## 2 Design and Experimental Results
### Circularly Polarized Antenna Design  
#### 2.2.1 Single Radiator and Feed Line Designs  
The CP array antenna is designed based on a circular microstrip patch as a single radiator, as shown in Figure 3a[12]. To enhance gain performance, an air gap is introduced between the feed line and the circular patch, improving radiation efficiency compared to previous designs [13]. This air-gap configuration minimizes substrate losses and enhances impedance matching, leading to noticeable radiation efficiency and gain improvement, as shown in Figure 3b. Based on the simulation results, an optimal air-gap thickness of 1.5 mm between the coupling slots and the radiating element is selected. This air gap is implemented using a fixture between the substrates, ensuring structural stability. The  
Figure 2: **(a)** Photograph of fabricated power amplifier module on evaluation board, and **(b)** frequency response at 5.8 GHz. detailed dimensions of the single radiator for the simulation are given in the caption of Figure 3. Figure 3c,d present the simulated design results of the single radiator. Note that all simulation results in this manuscript were obtained using Ansys Electronics Desktop 2021 R1. The results demonstrate that the circular-shaped single radiator patch is optimally designed to operate at a center frequency of 5.8 GHz, achieving excellent impedance matching, AR, and circular polarization gain characteristic. Figure 4 shows a schematic of 1:4 power divider and its S-parameters simulation results. The circularly polarized wave propagation of the proposed antenna design can be achieved by determining a physical rotation, \(\phi_{pm}\), and an electrical rotation, which signifies the phase change, \(\phi_{cm}\), when \(m\)-number of radiating units are implemented [14; 15]. In this work, four numbers of the circular-shaped radiating unit given in Figure 3 will be attached at port 2 \(\sim\) 5 shown in Figure 4a for the unit CP antenna design. In other words, the integer, \(p\), and the number of radiating number, \(M\), \(M\) in the following equation are 2 and 4, respectively. \[\phi_{cm}=\phi_{pm}=(m-1)\frac{p\pi}{M},(1\leq m\leq M). \tag{1}\]  
Figure 4b,c show the magnitude of the S-parameters and the phase differences between ports. For more details, the one-quarter of power division ratio can be obtained at each output port (port 2\(\sim\)5) and the excellent input impedance-matching characteristic can also be obtained by fine tuning the width of microstrip feed line. The 90\({}^{\circ}\) phase difference between output ports can also be precisely realized by changing the length of microstrip feed lines and its results coincide with the phase requirement for CP propagation calculated from the equation above. In summary, the detailed dimensions of the single radiator and the associated feed line for the 2 \(\times\) 2 array module are provided in Table 1.  
In the following section, we provide a detailed performance evaluation of the proposed CP array antennas, which are designed based on the single radiator and feed line design presented in this section. First, a single large panel is designed with an 8 \(\times\) 8 array configuration, consisting of 64 radiating elements. Second, four separate units, each containing a 4 \(\times\) 4 array, are also fabricated and its radiation characteristics are measured to compare with  
Figure 4: (**a**) Feed line configuration, (**b**) magnitude of simulated S-parameters, and (**c**) phase of simulated S-parameters. Figure 3: (**a**) Single radiator configuration, (**b**) gain variation with different air-gap thickness, (**c**) simulated return loss, and (**d**) simulated AR and gain. (\(l_{slot1}\) = 16.2, \(l_{slot2}\) = 8.5, \(D_{patch}\) = 17.6, all in [mm]). the single large-panel configuration. This comparison ensures that the proposed antenna structure maintains its performance regardless of the interface between the PA modules and the power-transmitting antenna.