With mobility and flexibility in non-terrestrial environments, unmanned aerial vehicles (UAVs) have shown the potential for emerging applications, such as remote surveillance and mobile base stations. This work raises the emerging need to chain UAV relays in support ofon-demand connectivityto faraway users. However, the ideal isotropic antennas are too simplified to ensure connectivity among UAVs. Since antennas used in realistic communication systems including UAVs are non-isotropic, and the angle of radiations (AoR) between a pair of transmitter and receiver significantly influences received signal strengths (RSSs). Therefore, this work takes the AoR into account to deploy UAV relays in a 3D environment. Not only positions but also the headings of UAVs are modeled in the AoR-based deployment problem to minimize the number of UAVs in the relay chain, while the end-to-end RSSs are guaranteed. We prove the NP-hardness of the AoR-based deployment problem. Then, theAngle-of- Radiation-based Deployment (AIM) algorithmis proposed to solve the problem. The extensive simulation results show that the AIM algorithm reduces the number of UAVs used in the relay chain by 59.6% compared to the baselines when the heading of each UAV is well-adjusted. Also, the proposed AIM algorithm outperforms the baselines by 61.2% in terms of the average number of UAVs and demonstrates adaptability to various terrains.

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