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

88 Focus | Antenna systems their various alloys can also be printed to satisfy application-specific requirements for conductivity, strength and workability. Current AM methods are most suited to producing antennas in the 1-100 GHz range with restrictions on size and weight, and with complex structures. For example, a simple horn antenna can be fabricated using a range of processes without trouble, but users seeking a complex array of combined antenna horns might find that AM yields greater value than conventional processes. Other design requirements, such as mono-pulse tracking, filtering, CCA enclosures, thermal dissipation features or mechanical interface features can contribute to the print time, but such projects may still give a reduced total assembly size, weight, lead time and cost compared to traditional fabrication methods. Different antenna components can also be printed together as a single part, instead of having to be machined separately and then assembled together. A monocoque unit made from a structural lattice, an array of antenna elements, a waveguide, multiple connector mounting faces, filters, septum polarisers and separate feeds for RHCP and LHCP could be produced in a single print, saving considerable production time and effort. There are still limitations of course to what AM can achieve. Printers are constrained by the volume and size of the components they can produce, so it is still more cost-effective to manufacture particularly large or simple components using traditional methods. For example, AM is not well-suited to making antennas below 1 GHz. Frequencies below that need particularly large antenna structures, which are better served by conventional manufacturing. Quality control Testing antenna systems becomes more critical as the complexity of their design and niche nature of their use increase. Using an anechoic or semi-anechoic chamber can be crucial during radiative tests in order to distinguish between the signals from the antenna assembly and background noises from outside. The inside of the chambers should also be lined with an RF-absorbent material to minimise the rate of reflected signals, and avoid inaccurate measurements caused by signals from the ceiling and walls. Random batch testing is common with higher volumes of antenna production, to save time compared with routinely testing every batch. This should be avoided wherever possible with unmanned vehicle antennas though, as data link issues can persist – even if vehicle developers follow precise integration and ground testing instructions – owing to otherwise detectable issues going unnoticed in the untested batches. Quality tests tend to focus on a few key variables. The voltage standing wave ratio – the ratio of transmitted waves to received waves in electrical transmission lines – is often one of them. Ideally, this value should be 1:1, which implies no variance in the voltage along the system and therefore no unwanted reflections. Testing the axial ratio – the ratio of vertical to horizontal polarisation – should also be conducted, and should be as close to 0 dB as possible for circularly polarised antennas. For satellite antennas in GNSS and Iridium links, testing the gain and current of the LNA can also be crucial for assuring a functional RF system. Both line-of-sight and satellite antenna systems should benefit from power efficiency testing as well. In addition to size and weight, constraints on power can be detrimental to unmanned vehicle performance, and unnecessary waste of power is naturally undesirable. For unmanned vehicles in general, antenna systems capable of enabling better control, sharper telemetry feeds and BVLOS operations have been around for a considerable time. Advances in awareness may be the most important step towards ensuring that designers pay closer attention to the benefits that smoother – and earlier – integration of antennas can bring. Acknowledgements The author would like to thank Allen Crawford from Tallysman, Rod Waterhouse from Pharad, Vanja Maric from Maxtena, Michael Hollenbeck from Optisys, Adam Krumbein from Southwest Antennas, Sam Pera from Persistent Systems, Tobias Webster from UAV Navigation, Linda Clark from Mobile Mark Antenna Solutions and Martin Newman of Innovelec for their help with researching this article. April/May 2018 | Unmanned Systems Technology RF equipment for GNSS and control is often tested in an anechoic chamber to minimise noise interference (Courtesy of Lockheed Martin)