# A Multi-Band Circularly Polarized-Shared Aperture Antenna for Space Applications at S and X Bands
## 1 Introduction  
At the beginning of space exploration, satellites were large and heavy and delivered a multi-functional payload. These satellites used high power and needed a long development time and a huge budget. Satellite development skills were inaccessible to students and researchers in most cases. Also, there were many scientific questions that required attention but could not justify the large cost of satellite development to resolve them. The technological advances in low-power microelectronics, micro-electromechanical systems, and digital signal processing paved the way for the development of small satellites with a low cost and short development time yet with a lot of advanced functionality [1; 2; 3]. These small satellites are lighter and cheaper compared to conventional satellites, which makes them suitable for applications in academia and amateur projects. CubeSat, a special type of small satellite made of one or multiple 10 \(\times\) 10 \(\times\) 10 cm\({}^{3}\) cubic structures, rapidly gained popularity in different sectors due to its flexible design and capacity for custom development. A CubeSat with a dimension of 10 cm on each side is called 1 U, which is the standard unit of the CubeSat size. A CubeSat can be 1 U, 2 U, 3 U and even increases up to 12 U and 16 U if required by the mission. CubeSat was first developed and standardized in 1999 by [PERSON] and [PERSON] at Stanford University's Space Systems Development Laboratory (SSDL) [4]. The goal behind CubeSat's development was to provide a platform to help students and researchers develop skills related to the design, manufacture, and testing of small satellites at a lower earth orbit for various science missions while reducing the cost and development time. With time, CubeSats gained a lot of attention and found its way into academia, industry, and government projects. A CubeSat comprises different subsystems (e.g., electrical power, communication, attitude determination and control, propulsion, etc.) with numerous components. Antennas are integral and one of the most important passive components of a CubeSat communicationsubsystem. The antenna design for CubeSats originated from analyzing the link budget of the communication system and identifying antenna requirements, such as gain, bandwidth, polarization, etc. The operating frequency of the antenna depends on frequency licensing and the availability of the ground station. The required bandwidth from the antenna depends on modulation and the data rate of the communication system. Before designing an antenna, requirements for gain, bandwidth, polarization, and other antenna performance metrics must be identified. Designing an antenna for CubeSats is challenging due to space limitations and mission requirements. Reliability is the most important aspect of a CubeSat antenna as it cannot be changed once the CubeSat is launched. CubeSat antennas must also survive mechanical vibration, radiation, out-gassing, atomic oxidation, and wide temperature variations. Like other satellite communication systems, CubeSats prefer circularly polarized antennas [5]. Linearly polarized antennas radiate along a single axis and require the strict orientation matching of the transmitting and receiving antennas. On the other hand, circularly polarized antennas do not require the orientation matching of transmitting and receiving antennas. It also reduces the fading effect caused by the Faraday rotation effect in the ionosphere [6]. Linearly polarized antennas also experience the same Faraday rotation effect, as the polarization vector is rotated by charged particles in the ionosphere. Consequently, keeping strict orientation matching between linearly polarized receiving and transmitting antennas becomes difficult. The CP antennas are not prone to this Faraday rotation effect, as the strict orientation of transmit and receive antennas is not required. Antennas of different sizes, shapes, and performance have been used in CubeSats.