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

78 Focus | Antennas connectivity and throughput amid dynamic operations with unmanned systems as they roll, pitch, yaw, heave and accelerate at increasing distances from their GCSs. Selecting the right antenna for a vehicle requires, at a minimum, an investigation of a few key parameters, as well as an understanding to avoid misinterpreting them. Gain, for example, is in principle a useful parameter for indicating how efficiently an antenna takes its input power and broadcasts it in the intended direction. But integrators will often look merely at ‘peak gain’ and extrapolate an (incorrect) image of how that antenna’s wave propagates. This one- or two-dimensional analysis overlooks areas of where low gain will occur across the radiation pattern. A standard monopole antenna has around 2-3 dBi of gain, in isotropic terms. For one that has 6 dBi – double the power, on paper – the radiation pattern will change, probably from a doughnut shape with notable nulls where no energy is radiated, to more of a disc-like shape that concentrates power in the horizontal axis but produces even larger nulls across much of its sphere. Bandwidth also bears close consideration, as it directly affects the capacity for data to be transported through the propagation channel. As the RF spectrum is closely regulated in terms of which bands can be used for which purposes, less and less bandwidth is available below the 6 GHz level. Most of the ‘free space’ for transmissions is available at higher frequency bands, hence the popularity of broadcasting over 5.8 GHz. This also appeals to integrators, because a higher frequency means both a smaller wavelength and antenna. But higher frequencies also mean much higher losses in propagation and thus range, unless an extremely high-gain antenna is used. And as we have seen, a higher gain is likely to come with a reduction in the antenna’s real omnidirectionality. That creates the potential for severe drop-outs and losses during operations, unless accurate beam steering or beamforming can somehow be installed to track and maintain the link between the vehicle and its GCS. At a product level, many shapes of antenna have emerged to suit different integration, propagation and bandwidth requirements. For example, the aforementioned sector antennas can be stacked radially to approximate an omnidirectional antenna at a GCS, for wide-area control or even swarming of multiple unmanned systems. Similarly, the highly SWaP-optimised patch antenna design can be combined the same way, with multiple units integrated together to produce a single high-gain antenna array or even enable electronic steering, which can give major advantages over mechanically steered antennas. MIMO antennas now come in a wide variety of dome, panel, horn and other shapes to provide integration flexibility as well as bandwidths that are hundreds of MHz across to ensure consistent connectivity amid multi-path and signal congestion. It is worth noting that multi-element antennas, consisting as they do of many smaller internal elements than single- element antennas, can be made 60-80% smaller and thus more aerodynamic than their single-polarised counterparts. That gives further advantages for UAVs and high-speed USVs and UGVs. And while constraints on aerodynamics and weight might seem relatively easier to understand than dynamic data link requirements, in-house integrators should gain a close understanding of what coverage and link budget requirements are achievable, as this can help greatly with matching a platform’s mission and environment to an antenna. For example, in environments with minimal obstructions, reflections and refractivity, single- and multi-element antennas will exhibit fairly similar mean throughputs. In fact, the multi-element antenna could show slightly lower overall reception owing to using smaller individual elements. But as soon as the antennas begin moving or come within distance of materials that cause refractivity and different phase angles of the primary wave to occur, a multi-polarised antenna could be expected to average 20% higher throughputs than a single-element antenna. June/July 2021 | Unmanned Systems Technology Field testing of antennas is crucial to identifying how they will perform during missions; datasheets based on laboratory test results can be insufficient in this regard (Courtesy of Doodle Labs)