Unmanned Systems Technology 019 | Navya Autonom Cab | Batteries | UGVs Insight | UAV Factory UAV28-EFI | Swiss Aerobotics Hummel | UMEX 2018 report | Antennas | Oceanology International 2018 report

82 Focus | Antenna systems surface. Underwater systems also have to withstand often significant pressures, and antennas are not always tested for such a capability; denser, more pressure- resistant materials also tend to have worse conductivity and experience greater losses. Vehicle design considerations The assembly and design of an antenna’s element structure can also have major ramifications on the same aspects of the aircraft or surface vehicle. As mentioned, a monopole antenna is simple and requires a counterpoise known as a ground plane, but that often conflicts with a vehicle designer’s choice of materials for the hull or fuselage. The use of carbon fibre-reinforced plastic has become almost ubiquitous in UAVs owing to its high strength-to- weight ratio, but it is prone to absorbing signals instead of reflecting them, and is undesirable as a ground plane. Polyethylene, polycarbonate and other plastics are also used in small unmanned aerial systems and to a greater extent in commercial USVs, but they will not work well with monopole antennas unless a metal plate is fitted or they come with a self-contained antenna set-up. That may not be a problem for larger unmanned aerial systems and UGVs with metallic bodies, so long as the metal serving as the ground plane is reasonably conductive. However, the size and shape of the effective ground plane might not correspond with the propagation requirements for the antenna and its application, and coverage may suffer as a result. Ground plane sizes need to be at least one-quarter of the wavelength they are intended to serve. Dipole antenna set-ups on the other hand work independently of ground planes, as they consist of two connectors or ‘poles’ with RF voltage channelled to the mid-section between them. These types of antennas are increasingly popular among unmanned systems designers as they do away with having to install a ground plane. However, size and weight may still be a concern compared with monopoles, as both ‘halves’ of the antenna are designed into the assembly. The most common dipoles measure half the length of the device’s intended wavelength. UGVs make especially effective use of vertically polarised omnidirectional dipoles, often with spring bases that allow the antenna to flex and bend after an impact with an obstacle to prevent breakage. However, there is an increasingly common trade-off during antenna development when designers of very small UGVs (as well as unmanned systems in other domains) seek to operate at lower frequencies than the L band, such as the 433 MHz ISM or 900 MHz bands. These inevitably require larger, more unwieldy antennas, and ground planes if present, than the UGV designers anticipated installing, to suit the longer wavelengths relative to the 2.4 GHz or 5.8 GHz wireless bands that are more common among UAVs. Unmanned vehicle operators also increasingly want features such as live video or telemetry feeds, in addition to semi-autonomous waypoint-oriented command and control. These may require a MIMO-type system with multiple antennas on the same vehicle, or a single multi-band antenna set-up in order to cover the different frequencies required for each data link. For example, control directives and flight information might be carried between a UAV and its GCS over the 900 MHz band, with an accompanying video downlink over a 2.4 GHz channel. If though the relevant size, weight and balance considerations for integrating multiple lower-frequency antennas have not been taken into account then prototype completion and flight testing could be delayed as developers revisit the airframe body’s design to accommodate the data link necessities. For UAVs, the aerodynamics of the antenna and its enclosure should also be considered. The use of thin micro-strip antennas, such as rectangular patch types, can be integrated without any practical effect on wind loading, while also providing polarisation diversity. They are inexpensive to manufacture owing to their simple geometry in a printed circuit board. That comes at the cost of signal strength, however, as such antennas tend to be more susceptible to noise, interference and an incompatible mounting April/May 2018 | Unmanned Systems Technology There is a range of antenna design options for UAV designers seeking to match their RF set-up to their chosen hull material, while minimising changes to drag and airflow around the aircraft (Courtesy of Pharad)