## 5 Discussion and Conclusions  
In this paper, we have introduced and validated a monopulse antenna concept designed for Collision Avoidance (CA) systems aimed at preventing collisions between Unmanned Aircraft (UA) with a maximum dimension of less than 25 feet (typically used in UAS operations within the specific category, as defined by the European Regulation), with the Remain Well Clear (REC) function managed by ground-based U-space/UTM services. This antenna enables the detection of large intruders (GA and UAM/AAM aircraft) at approximately 2000 feet from the ownship, and small UAs (sUA) at around 200 feet (which are the RWC limits defined by State-of-the-Art applicable standards), using CW-LFM radars implemented with COTS components at 24 GHz. We employed a well-established full-wave electromagnetic simulator (CST) to design a prototype, which was fabricated using Selective Laser Melting (SLM) 3D printing technology with AlSi10 Mg. The prototype features a low aerodynamic profile that does not compromise the ownship's airworthiness, weighs 163 g, and costs approximately 200 euros. We measured the antenna's performance in an anechoic chamber, where the results closely matched the simulated design and met the specified requirements. The findings presented in this paper demonstrate that the proposed concept currently holds a Technology Readiness Level (TRL) of 3, according to the scale used in European R&D framework programs [53] (i.e., experimental proof of concept). This makes it a strong candidate for radar antenna development for CA systems  
Figure 16: Comparison of the simulated and measured \(\Delta_{\theta}\) patterns on the vertical plane.  
Figure 15: Comparison of the simulated and measured \(\Sigma\) patterns on the vertical plane.  
intended for small, unmanned UAS. However, further research is needed to evaluate the effect of the radome and potential interferences with other airborne systems, as well as to explore alternative technologies such as groove waveguides, which could lead to lighter antennas with even lower aerodynamic load.  
Conceptualization, J.V.B.; methodology, M.F.-B. and J.R.A.; validation, J.R.A.; formal analysis, M.F.-B.; resources, J.V.B.; writing--original draft preparation, J.R.A.; writing--review and editing, J.V.B. and M.F.-B.; supervision, M.F.-B. and J.V.B. All authors have read and agreed to the published version of the manuscript.  
This research was conducted within the framework of the CREATE U-space project. The CREATE U-space project (CIAICO/2022/044) has received funding from the Conselleria de Innovacion, Universidades, Ciencia y Sociedad Digital of the Generalitat Valenciana.  
Not applicable.  
The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding authors.  
We kindly appreciate the support of [PERSON] from UPV's APL in conducting the experimental characterisation of the prototype.  
The authors declare no conflicts of interest.  
The following abbreviations are used in this manuscript:  
AAM
Advanced Air Mobility  
ACAS
Airborne Collision Avoidance System  
AoA  
Angle of Arrival  
APL  
Antennas and Propagation Lab  
ARC
Air Risk Class  
ATAR  
Air-to-Air RADAR  
CA
Collision Avoidance  
ConOps  
Concept of Operation  
COTS  
Commercial-Off-The-Shelf  
CW-LFM  
Continuous Wave, Linearly Frequency Modulated,  
DAA  
Detect and Avoid  
EEA  
European Economic Area  
EU  
European Union  
EUROCAE  
European Organisation for Civil Aviation Equipment  
FFT  
Fast Fourier Transform  
FOV  
Field of Vision  
GA  
General Aviation  
GRC  
Ground Risk Class  
ICAO  
International Civil Aviation Organisation  
IFR  
Instrumental Flight Rules  
ISM  
Industrial, Medical, and Scientific  
MAC  
Mid-Air Collision  
MOPS  
Minimum Operational Performance Standard  
NMAC  
Near Mid-Air Collision  
OSED  
Operational Services and Environment Description  
RPAS  
Remotely Piloted Aircraft Systems  
RTCA  
Radiotechnical Commission for Aeronautics  
RWC  
Remain Well Clear