# Meta Surface-Based Multiband MIMO Antenna for UAV Communications at mm-Wave and Sub-THz Bands
## 4 Conclusions  
The article evaluates the potential of millimeter-wave (mm-wave) communication using unmanned aerial vehicles (UAVs) by developing and testing an MIMO antenna system with circular polarization. It is a conformal antenna which is designed on a flat, broad space and consists of two individual antennas which work together as a single antenna to transmit or receive radio waves. This system utilizes a coffee bean metasurface structure to achieve low loss, high efficiency, and easy integration with mm-wave circuits. The antenna's radiation properties were analyzed, considering the drone frame's impact on signal coverage. Testing covered multiple frequency bands: X-band, K-band, Ku-band (8.1-24.7 GHz), 5G mm-wave bands (66-68 GHz, 74-78 GHz), and sub-THz bands (80-140 GHz). Results showed an axial ratio below 3, a maximum gain of 13.8 dBi, and radiation efficiency over 85% for frequencies below 80 GHz, with low cross-polarization levels. The antenna demonstrated significant diversity gain and a low envelope correlation coefficient (ECC), which is crucial for MIMO applications. Both simulations and measurements were consistent, confirming the antenna's wide bandwidth, high gain, and flexible radiation patterns, making it suitable for UAV-assisted 5G mm-wave and sub-THz wireless communications. It also included practical evaluations on a 3D-printed drone model, with future plans to test the antenna on an actual Parrot drone to further validate our design methodology. The proposed antenna setup facilitated the investigation of two potential arrangements for positioning antenna arrays on a UAV, providing insights into hemispheric coverage, gain distribution, and received power across different frequency bands. Leveraging UAVs as aerial wireless communication platforms presents a promising alternative to traditional cellular systems, though energy constraints often limit their operational duration. Addressing these challenges, our antenna design emphasizes miniaturization and lightweight construction, which are crucial for reducing energy consumption and improving UAV endurance. Our results underscore the advantages of employing multi-array systems, particularly over extended distances, and highlight the potential for indoor communication and tracking within large structures like factories. This work contributes to implementing multiband antennas with adaptable radiation patterns. It provides valuable insights into the landscape beyond 5G and sub-THz wireless systems for UAV-enabled communications. The proposed high-performance multi-functional antenna was designed for UAV-assisted wireless communication, enhancing D2G, D2D, and D2S applications. The associated features and benefits are as follows. (1) D2G communications: These operate at X-band (8-12 GHz) and mm-wave (30-300 GHz) frequencies, providing high data rates for HD video streaming, real-time telemetry, and sensor data transfer. The obtained operating bands can reduce interference, improve reliability, and enhance communication range and signal quality through their high directional gain. The related applications for this type of UAV communication include surveillance, high-speed sensor data transfer, and reliable command/control links. (2) D2D communications: These can support efficient swarm communication with high data rates at mm-wave and sub-THz frequencies. This ensures low latency for time-sensitive operations and robust directional links between drones. The related applications are synchronized search-and-rescue operations, environmental monitoring, and networking. (3) D2S communications: These can provide long-range communication with high bandwidth and directional gain. X-band and higher frequencies enable high bandwidth communication, and circular polarization improves satellite signal quality. The related applications are beyond visual line of sight (BVLOS) operations, data relays in remote areas, and global positioning/navigation support.