This paper presents a system architecture to accomplish over-the-horizon communications with medium-sized unmanned aerial vehicles (UAV) in order to enhance its autonomy and maximum range. This architecture is based on using a second UAV as a communication relay with a compact active circular array with separated transmitting and receiving elements at C-band. The designed active feeding network provides switching capabilities in order to selectively activate these elements depending on the

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... ed direction of the antenna beam, achieving maximum equivalent isotropic radiated power (EIRP) in transmission and maximum antenna gain in reception. In this way, a bidirectional long-range link for telemetry, telecommand, and high-resolution video can be deployed between UAVs providing high quality of service, reliability, and moderate data throughput at an affordable cost. Based on the measurements of a manufactured prototype, the maximum range of the deployed air-air link using the designed system is estimated in 20 km, considerably increasing the performance of current systems based on the use of omnidirectional antennas onboard UAVs. INDEX TERMS Antenna array, beam steering, beam-switching, microwave circuits, over-the-horizon communications, patch antennas, relay architecture, switched circuit, unmanned aerial vehicle (UAV).