# Exploring CP Array Antenna Characteristics and Its Applications to a Deployable Structure
## 1 Introduction  
Circularly polarized (CP) antennas play a critical role in modern communication systems due to their capability of minimizing polarization mismatch and maintaining signal quality. Unlike linearly polarized (LP) antennas, which require precise alignment between transmitting and receiving antennas, CP antennas provide realiable performance by accommodating various relative orientations between the two. This makes them indispensable in applications such as satellite communications, global navigation satellite systems (GNSS), and unmanned aerial vehicles (UAVs), where maintaining consistent communications despite movement is critical [1, 2, 3]. In addition to the above-mentioned conventional communication applications, CP antennas are particularly advantageous in wireless power transmission (WPT) systems. In WPT, efficient energy transfer depends on maintaining a stable link between the transmitter and receiver, even when the receiver's orientation varies due to movement or deployment constraints. CP antennas address this challenge by ensuring that the received power remains relatively constant regardless of polarization alignment. This feature of CP wave propagation is especially beneficial for space-based WPT systems, where transmitting power to a CubeSat or other spacecraft demands resilience to unpredictable rotationsand misalignments [4; 5]. In [6; 7; 8; 9; 10], various resonator-type-based CP array antennas are reported. The PIN diodes are implemented to achieve the polarization reconfigurable characteristic in [6]. The multilayered substrate-integrated resonators are sequentially fed and realize more than 20% in axial ratio (AR) bandwidth at 60 GHz of frequency band [7; 8]. In addition, three-dimensional microstrip-based patches or dielectric resonators are fed by microstrip line structure for the required AR bandwidth characteristics. However, in this manuscript, such circularly polarized propagation characteristics can be achieved using a simple microstrip-patch-based resonator and the sequentially rotated microstrip feed line structures. Figure 1 illustrates an example of 27U CubeSat bus with the four panels of a solar cell and three different possible mounting configurations for WPT application. Considering the gain and efficiency characteristics of the transmitting antenna, both the array and panel sizes of the proposed design are optimized for integration with the 27U CubeSat bus. Please note that the example photograph of the CubeSat bus is sourced from Hex20 and the satellite platform and its electronics subsystems are commercially available from [11]. When the output signal from a power amplifier (PA) module placed in the satellite bus is applied to the transmitting antenna, the interface between them should be taken care of as indicated by the solid white lines in the left image of Figure 1. In our approach, the proposed antennas are assumed to be stacked beneath the solar cell panels, providing greater flexibility for both WPT scenarios: satellite-to-satellite power transmission and satellite-to-ground power transmission. Based on this assumption, we propose three different applicable mounting configurations, as shown in the right image of Figure 1. In the first configuration, a single \(8\times 8\) CP antenna array panel is placed beneath the solar cell panels. The second configuration divides the \(8\times 8\) array into four separate units, each with an independent input cable connected to PA modules. The third configuration combines two \(8\times 8\) array panels using deployable hinges. In this study, the CP array antennas for these mounting configurations were designed and their performance characteristics were evaluated. In the following sections, we briefly introduce the PA module and its characteristic, which were designed by KERI for testing the WPT system performance in both ground and airborne environments. Subsequently, we provide a detailed designs of the CP array antenna.