Unmanned Systems Technology 038 l Skyeton Raybird-3 l Data storage l Sea-Kit X-Class USV l USVs insight l Spectronik PEM fuel cells l Blue White Robotics UVIO l Antennas l AUVSI Xponential Virtual 2021 report

77 are operating across increasingly broad markets and distances, the need has arisen to transmit across a variety of bands beyond wi-fi, GPS and GLONASS, and to use new technologies including correction services and advanced digital or electronic steering for consistently accurate mission and navigation information feeds. That means new antennas must be engineered to have resonant frequencies in line with their transmitters’ full range of mission frequencies, the required polarisation and possibly correction services too. This is in addition to the requirements on size, weight, power, cost and aerodynamics. Taken together, this makes achieving the perfect antenna an extremely challenging balance from the design stage through to testing and integration. Antenna architectures Historically, antenna designs were based on three key assumptions: that the path between transmitter and receiver would be free from obstructions, that the radio wave between them would be the only one (without any secondary or reflected waves disrupting it), and that the distance and orientation (or polarisation) between them would never change. Under these conditions, a single- element antenna, sized and optimised for peak gain in the correct direction and polarisation, would be ideal for its application and give its user consistently satisfactory reception. These conditions are found in anechoic chambers where antenna r&d and testing are carried out, but almost nowhere else in the autonomous world. Unmanned vehicles and their GCSs can move dynamically in distance and orientation from each other, and the RF spectrum between them is increasingly likely to be congested with data comms and satellite navigation updates. And as with any mobile application, mountains, structures and other vehicles are likely to cross between them, absorbing, reflecting or otherwise weakening radio transmissions. Various antenna technologies are available these days to overcome these problems and enable long-range, consistent data links with high gain and low latency. These are crucial not only for navigation and performance monitoring of unmanned systems but for mission- critical capabilities such as real-time HD video streaming, comms relays and even exploring the Solar System. Broadly, there is a distinction between directional and omnidirectional antennas. As might be guessed, directional antennas produce a concentrated broadcast in a single beam-like direction, while omnidirectional antennas produce a 360 º wave that typically propagates in a doughnut-like or sometimes spherical shape. This means that, conventionally, the GCS will use a directional antenna design such as a horn or sector antenna (pointed in the general direction of where the unmanned vehicle will be operating) with some manner of tracking and steering system to maintain the link with the vehicle. In turn, the vehicle will use an omnidirectional antenna to continue transmitting data and potentially receiving commands as it turns to face in different headings and inclines relative to the GCS. Monopoles (such as whip or blade antennas) and half-wave dipoles represent the oldest and simplest omnidirectional designs in use among unmanned systems these days. They can be improved upon by incorporating additional elements, each one adding to the effectiveness of the others, slightly sacrificing the antenna’s gain and directionality in exchange for greatly expanding its overall beam width. The resulting multi-element antennas can resonate with different signals coming from different angles, phase shifts or refractivities far beyond the capabilities of any single, fixed element. This enables the increasingly popular multiple-input, multiple-output (MIMO) capability that is vital to consistent Antennas | Focus Unmanned Systems Technology | June/July 2021 Monopole antennas can be combined with additional elements or a suitable ground plane to greatly enhance their coverage in terms of frequencies and beam widths (Courtesy of PIDSO)