## 1 Introduction  
The number of space missions that are based on CubeSat is continuously increasing [1, 2, 3]. Indeed, this class of nanosatellites is particularly attractive because it enables access to space at a low cost, allowing small countries, universities, or even minor private companies to gain experience in the aerospace sector. In addition, thanks to modern technologies, relatively complex missions can be planned, e.g., for earth observation, remote sensing, communication technology experiments, hardware validation, and other scientific missions, as well as educational purposes. These satellites must satisfy strict requirements not only to ensure their successful operation in the harsh environment of space but also to comply with the safety requirements imposed by the launcher company or organization. In the case of CubeSat nanosatellites, they also must comply with the CubeSat standard [4], which imposes stringent limitations on satellite dimensions and weight, which makes the design of each subsystem a challenge [5]. In this scenario, INTA (National Institute for Aerospace Technology) has launched ANSER, an ad hoc constellation of clusters, where each cluster is formed of several CubeSat nanosatellites flying in formation, with one Leader (which could not be launched due to technical issues with the launcher) and two Followers (currently in orbit since 2023 and operational) [6]. The objective of ANSER is to develop and demonstrate technologies that will enable efficient earth observation missions in the future, making use of four main novel concepts: constellations of nanosatellites, formation flying, fractionated instruments, and miniaturized technologies. The ANSER flight segment consists of three 3U CubeSats (a Leader and two Followers), each with nearly identical physical characteristics, i.e., size, mass, moment of inertia, etc. There was an attempt to launch the three 3U CubeSats, using a Vega-C rocket, into an orbit at an altitude of 550 km in October 2023. Once there, the satellites, which do not have an on-board propulsion system, should maneuver themselves to establish and maintain a formation using the aerodynamic drag generated by the Earth's atmosphere. But an anomaly occurred during the launch of ANSER, preventing the deployment of the Leader satellite. The two mission satellites successfully deployed by the launcher, Follower 1 and Follower 2, are currently operating normally, with their subsystems already validated in orbit. Consequently, the decision has been made to reassemble the lost satellite, now called Leader-S, and seize the opportunity to introduce some enhancements to the system, given the validated in-orbit performance of the remaining subsystems of the Follower satellites, which are identical to those of the Leader [7]. Leader-S relies on UHF communication systems with deployable monopole antennas for low-bit-rate uplinks and downlinks (telecommands and telemetry), while, for high bit rates, the S-band is among the best choices [8]. As a result, one of the key components of the Leader-S is the design of a new S-band communication subsystem, as this ensures a link with the ground station for the uplink of telecommands and the downlink of telemetry and payload data. In particular, the design of this new antenna system is a fundamental step as it must take into account aspects of the mission (e.g., satellite attitude) [9; 10] and comply with the CubeSat size constraints while, of course, ensuring good performance [11]. In this scenario, the S-band antenna to be included in Leader-S and in the future ANSER-AT satellites can provide gain, versatility, and reliable up/downlinks and inter-satellite links in a wide operating bandwidth, with its focus on aspects such as miniaturization, radiation efficiency, bandwidth, and improved polarization capability using dual circular polarization [12].