Unmanned Systems Technology 006 | ECA Inspector Mk2 USV | Antenna systems | Northwest UAV NW-44 | Unmanned ground vehicles | Navigation systems | Lunar X challenge

36 Focus | Antenna systems travel underground, and the design of the antenna system is critical to ensuring a reliable link. One way to address the polarisation mismatch, particularly in UAVs, is to have a circular polarisation match at both ends (right circular to right circular, or left circular to left circular) so that the link is maintained irrespective of the position of the antennae. This will maintain the link, but circular polarisation antennae can have a large diameter and therefore be difficult to mount on an unmanned platform because of their weight, size and inferior aerodynamics. The compromise solution is to have linear polarisation (usually vertical) on the unmanned platform, for wide-angle coverage, and circular polarisation on the antenna at the ground station. Larger unmanned aircraft can also make use of satellite links. These use much bigger dish antennae to communicate with a satellite using L-band or Ku-band frequencies (see below) and can be housed in the UAV’s tail section. This avoids some of the polarisation and multi-path issues of ground-to-air links. Importance of frequency bands A wide range of frequency bands are used for control and data links for unmanned systems. These vary from the long range, unlicensed bands at 915 MHz in the US and 868 MHz in Europe up to satellite systems in the Ku and Ka bands. Different antenna technologies are used for the different frequencies, partly for size but also for range, data rate and robustness. Higher frequencies support higher data rates with smaller antennae but have a shorter range, so the choice of antenna technology depends very much on the requirements of the craft’s design. The industrial, scientific and medical (ISM) unlicensed band, which covers the 868-915 MHz band, offers opportunities for longer range but need larger antennae, with a ground plane typically a quarter to half a wavelength. For a 5 GHz antenna, this ground plane is typically 2.5-5 cm (1-2 in) in diameter or less, while at 900 MHz it is 25 cm (10 in). There are ways around this challenge though, by using the aircraft frame itself as the ground plane for the antenna. The implementation then depends on the material used for the aircraft: conducting materials such as aluminium, carbon fibre and magnesium have been used, and it is even possible to use foil as the lining in a fibreglass airframe to provide an effective reference ground plane for such systems. Navigation Receivers for GNSS are a key area for antenna designs, as they have to be able to detect signals from multiple satellites that can be anywhere in the sky. This leads to a variation in the signal from less than 0.5 dB at zenith to 2 dB at the horizon, which has to be taken into account in the antenna’s design. The system also has to support a high receive gain over the full GNSS spectrum, from the Low GNSS band (1164-1300 MHz), the L-band correction services (1525-1559 MHz) and the High GNSS band (1559-1610 MHz). For smaller systems using a single band and one set of satellites, a microstrip or fractal antenna can be used to reduce the size and weight of the receiver, as the antenna can be tuned to the exact band requirements. February/March 2016 | Unmanned Systems Technology Satellite signals can vary from less than 0.5 dB at zenith to 2 dB at the horizon, which has to be accounted for in antenna design Conformal antennae can be moulded to the shape of the vehicle, as on this Toyota concept car with a satellite antenna on the roof (Courtesy of Kymeta)