# Exploring CP Array Antenna Characteristics and Its Applications to a Deployable Structure
## 2 Design and Experimental Results
### Circularly Polarized Antenna Design
#### 2.2.2 Array Antenna Characteristics for Various Mounting Scenarios  
Figure 5 presents photographs of the fabricated CP antenna along with the measurement setup in an anechoic chamber. The circularly shaped radiating patches are arranged with equal spacing, set at a distance of 0.74\(\lambda\), which corresponds to 38.25 mm in terms of the element spacing, d\({}_{elem}\). The specific value of the radiating element spacing is selected considering the size of a commercially available substrate suitable for the mounting configuration. In addition, the chosen spacing helps reduce mutual coupling between elements and improves sidelobe characteristics [16]. On the rear side of the proposed CP array, the feed network is implemented using microstrip lines. The fundamental structure of the feed line, originally designed to sequentially rotating radiation elements shown in Figure 4a, is expanded to accommodate all 64 elements in the array. To evaluate the radiation characteristics, the fabricated antenna is tested in an anechoic chamber using a linearly polarized standard gain horn antenna as the reference. The axial ratio and gain characteristics of the proposed antenna are determined while accounting for the polarization mismatch between the antenna under test (AUT) and the horn antenna. A standard gain horn antenna (QRH-002G-018G, MTG, Chungnam, Republic of Korea), operating over the 2-18 GHz frequency range, is employed for the radiation pattern measurements. The anechoic chamber used for the measurement has dimensions of 11 m \(\times\) 8 m \(\times\) 8 m, which are sufficient for accurate measurement of the radiation patterns at the center frequency of 5.8 GHz. The proposed CP array antenna achieves a gain larger than 20 dBi at the center frequency of 5.8 GHz while demonstrating excellent return loss and axial ratio characteristics over a broad bandwidth. This radiation characteristic is attributed to the unique sequentially rotated feed line, which effectively cancels out reflections near the center frequency at the input port, thereby enhancing the operational bandwidth, as illustrated in Figure 6. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline Description & \
\begin{tabular}{c} Patch diameter, \\ \(D_{patch}\) \\ \end{tabular} & Slot length, \(l_{slot1}\) & Slot length, \(l_{slot2}\) & Slot width, \(w_{slot}\) &
\begin{tabular}{c} Microstrip line \\ for matching \(l_{m}\) \\ \end{tabular} \\ \hline Value & 17.6 mm & 16.2 mm & 8.5 mm & 1.5 mm & 4.5 mm \\ \hline Description & 80 \(\Omega\) line width & 96 \(\Omega\) line width & 100 \(\Omega\) line width & 120 \(\Omega\) line width & 128 \(\Omega\) line width \\ \hline Value & 0.838 mm & 0.570 mm & 0.514 mm & 0.287 mm & 0.270 mm \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Detailed dimensions of the single radiator and the feed line structure. Figure 5: (**a**) Fabricated 8 \(\times\) 8 array CP antenna for configuration \(\sharp\)1 and (**b**) measurement setup in an anechoic chamber. The simulation results agree well with the measured data, validating the proposed antenna design. Note that the return loss was measured using Keysight PNA E8363B. The simulated and measured radiation patterns at various frequency points are shown in Figure 7. The simulation tool provides the ideal CP radiation patterns and plotted at 5.45, 5.8, and 6.15 GHz. Both ends of the frequency points are selected, for which the sidelobe level is 10 dB smaller than the peak of the pattern and corresponds to about 12% bandwidth. The half-power beamwidth varies between 8\({}^{\circ}\) and 9\({}^{\circ}\) in the operational bandwidth with an extremely small axial ratio less than 0.3 dB. The radiation pattern of the proposed CP antenna is measured using a conventional linearly polarized standard gain horn antenna and the spinning radiation patterns are obtained [17]. Measurement results shown in Figure 7d-f provide the agreeable characteristics in terms of the sidelobe level and half-power beamwidth with the simulation results.