# A Multi-Band Circularly Polarized-Shared Aperture Antenna for Space Applications at S and X Bands
## 1 Introduction
Commonly used CubeSat antennas are wire antennas (dipole/monopole/quad-pole) [7; 8], reflector antennas [9], reflect-array antennas [10], membrane antennas [11], microstrip patch antennas [12], horn and waveguides [13], etc. These antennas have their advantages and disadvantages based on performance, ease of integration, shape and size, mass, deployable mechanisms, etc. Developers select antennas from these options and customize the design to suit the purpose of the mission. Moreover, the widespread practice is to use dedicated antennas on CubeSats to serve the following purposes:  
* Telemetry, tracking and command (TT&C) for uplink and downlink communications with the ground station;
* High speed downlink communication for large payload data when downloaded;
* GNSS (GPS, Galileo, etc. );
* Inter-satellite communication. Normally, CubeSats are equipped with 3-4 antennas or even more if required by the mission. The placement of these antennas is also challenging due to several issues. CubeSats must fulfill mass and size requirements and pass the vibration test before launching. The designed antennas must withstand shock and vibration during launch and deployment. Most of the CubeSat's antennas are placed on the outside surface of the structure. CubeSats also have solar cells placed on the outside surface of their structure for power generation from solar energy. If multiple antennas with a large form factor are placed on these sides, CubeSats suffer from a lack of space for solar panels. Developers then struggle to accommodate enough solar cells to maintain power positivity in the total period of the satellite operation. A reduction in size is one of the major challenges faced by the antenna designers of CubeSats. An advancement in commercial-off-the-shelf products, rapid prototyping, and testing have made it possible to develop high-frequency radios with a high data rate for application in small satellites with low power. As a result, CubeSats are collecting more scientific payload data than ever before. However, CubeSats have a very short ground pass duration while orbiting at lower earth orbits. This means that the time for downlink payload data using a line-of-sight communication channel to the ground station is very short. For the fast and efficient downlink of the payload data from a CubeSat, a transmitter system with large bandwidth and high data rate is required. Hence, CubeSat's payload data downlink system requires an antenna with a large bandwidth and high gain [14; 15]. Shared aperture antennas (SAA) have the potential to resolve the current issues and future challenges of recent CubeSat antennas [16; 17; 18; 19]. The concept of structure reuse can reduce the size of the antenna and combine multiple antennas in one single structure. Moreover, multiple antennas can be fabricated into a single structure and make a multiband antenna system. This could play a vital role for CubeSats as they struggle to accommodate multiple antennas due to lack of space and volume. In addition to this, planer antennas do not need any deployment. In 1991, when the STS-37 launch of the Gamma Ray Observatory was happening, the high gain antenna did not deploy as per the initial command [20]. The proposed high-gain antenna has an upper hand in reliability in addition to saving more space. In this article, a microstrip patch-based and shared aperture antenna is proposed for multiband operation. A dual-band-shared aperture antenna operating in L1 and X bands is presented in [21]. The antenna discussed in this paper has two microstrip patches that operate at S-band and X-band frequencies. It has two printed elements that share the same ground plane and dielectric aperture. The antenna system uses a single aperture yet operates in two frequency bands and, thus, saves space on the CubeSat's sides. The sense of polarization of the proposed antenna is Right Hand Circularly Polarized (RHCP). The performance of the patches is further improved by placing an additional dielectric layer that acts as a partial reflective surface (PRS) [22].