## 3 Formulation of a Concept of Radar Antennas for CA Systems
### Identification of a Candidate Architecture  
Typical radar antennas providing \(360^{\circ}\) azimuthal coverage are rotating antennas with narrow beams in azimuth and vertical radiation patterns made of stacked or electronically steered beams [39]. Since those antennas are usually apertures pointing towards the horizon and rotating around the z-axis, the vertical radiation pattern never covers the full range from 0 to \(180^{\circ}\), producing what is commonly known as 'cone of silence' [40]. Modern radars also use fixed antennas where beams are electronically steered or switched to cover the specified volume [39]. This can be accomplished by means of circular arrays or by one or several rectangular arrays properly laid out. A third architecture, the so-called 'ubiquitous radar' was recently proposed and tested [41; 42]. In a ubiquitous radar, energy is transmitted through an omnidirectional pattern, and echoes are received through a number of digitally beamformed patterns and processed in parallel. In recent years, there has been increasing interest in the development of compact, high-gain, omnidirectional antenna systems tailored for radar applications onboard UAS. Various designs have been proposed--particularly in the X band and millimetre-wave ranges--to address constraints related to size, weight, and energy consumption, while aiming to ensure wide angular coverage and reliable target detection. [PERSON] et al. [43] proposed an X-band Doppler radar employing a horn antenna. While functional, this system is limited to detecting intruders approaching from the front of the ownship. Moreover, the radar was tested with targets at a distance of only 3 metres, which is significantly below the detection range required, as defined in Section 2. [PERSON] et al. [44] introduced a 24 GHz monopulse radar offering \(360^{\circ}\) coverage around the ownship. However, the system relies on patch antenna arrays with a maximum power handling of 20 dBm, approximately 30 dBbelow the power requirements specified in Section 2. [PERSON] et al. [45] developed an X-band end-fire antenna array comprising printed Yagi-Uda elements in a \(4\times 2\) configuration. The design achieved a gain of 17 dBi and incorporated mu-negative and metasurface-based decoupling techniques to reduce inter-element coupling below \(-30\) dB. Nonetheless, its strongly directional nature limits its applicability in scenarios requiring omnidirectional coverage. Moreover, being a printed antenna, this design features a limited power handling capability. [PERSON] and [PERSON] [46] proposed a modular conformal antenna array for DAA radars operating in the Ku band. Although this design achieves full azimuthal coverage, it suffers from limited angular resolution due to its use of broad beams (90\({}^{\circ}\) to 110\({}^{\circ}\)) and the absence of monopulse processing. Furthermore, like other patch-based arrays, it is unable to support the transmit power levels required for ATAR radars, as outlined in Section 2. In the broader context of UAS communications for 5G and beyond, antenna systems play a pivotal role in enabling high-data-rate, low-latency, and reliable links. As noted by [PERSON] et al. [47], UAS platforms are emerging as key enablers of future wireless networks due to their flexible deployment and strong line-of-sight connectivity. These characteristics make UAS not only appealing for communication purposes but also highly suitable for advanced sensing and radar applications. The development of compact and efficient antenna designs--particularly at millimetre-wave frequencies, such as 24 GHz--supports this trend by facilitating dual-use systems that integrate both communication and radar functionalities on a single UAS platform. Our proposed cylindrical antenna array is aligned with this vision, offering a lightweight, high-gain solution capable of full 360\({}^{\circ}\) coverage, tailored for UA-borne radar within next-generation integrated sensing and communication (ISAC) scenarios. [PERSON] et al. [48] and [PERSON] et al.