2-port MIMO antenna system for High Frequency data link with UAVs using characteristic modes

In the last two decades a trend toward the new type of aircraft design has developed. The Unmanned Areal Vehicles (UAVs) and further Unmanned Combat Areal Vehicles (UCAVs) are supposed to support and ultimately may supersede manned aircraft in the future missions. The wireless data link with these vehicles uses High Frequency (HF) band (from 3 MHz to 30 MHz) due to atmospheric low-attenuation window that exists in this frequency range. It allows to use ionosphere as a reflector to allow non-Line-of-Sight (LOS) communication beyond the Earth's horizon. However, typically the antennas for HF suffer from low efficiency and narrow bandwidth as they are electrically small. In the present paper the whole airframe of the craft is used as a radiator which significantly increases the bandwidth and efficiency of the resulting antenna. Moreover, the analysis of characteristic modes on an example UAV airframe led to the concept of incorporating two couplers into design of the aircraft wings. Both couplers excite mutually orthogonal characteristic modes on the UAV's airframe which enable realization of an 2-port Multiple Input - Multiple Output (MIMO) antenna system for High Frequency (HF) band.