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

80 Focus | Antennas Testing and simulation Gauging such tangible performance advantages for unmanned system antennas requires a different testing approach than conventional anechoic chamber set-ups. One way of going about this is to perform outdoor tests with an antenna mounted on a rover platform, executing pre-planned distances, velocities and orientations relative to a base station, while a continuous throughput test is carried out in the background to gather data about how the throughput between the rover and base station changes. The same test could also be run with two antennas, such as a single-element and a multi-element, mounted on the rover to clearly define how the two compare in the same conditions. As more UGVs and autonomous aircraft operate in built-up areas, trials of an antenna should include how it performs amid concrete and metal structures, and all the multi-path and nulling issues these create. For example, a multi-element antenna that can broadcast at the lower ends of the RF spectrum is likely to continue transmitting well in alleyways and through walls. Long before testing though, unmanned systems engineers can benefit hugely from simulating an antenna’s radiation pattern, to visualise how it will perform when integrated into different fuselage or radome materials. This is critical, as the antenna is tuned to resonate at a specific frequency band, so changing its surrounding environment changes its resonance output. If simulations show an impedance match between the antenna and its surroundings, that should ensure that all the energy from the transmitter will be transmitted into the air without losing any data packets or reflecting energy back to the transmitter (which can damage it). Some transmitter systems can measure reflected power, enabling integrators to test some of these values in-house. By and large, however, antenna companies are better equipped to model different antenna architectures with specific surrounding materials and environments, in order to simulate how the electromagnetic waves propagate. Pretty much any UAV CAD design and antenna pairing can be evaluated with 3D visualisations in this way. That can be especially useful for integrating cost-effective monopole antennas. While dipole antennas theoretically have twice the coverage by using two conductors rather than one, monopoles can produce a resonance and wave propagation that are just as consistent as dipoles if positioned correctly in the right shape and material of ground plane (for use as a reflector). Having rotating blades nearby can also dramatically change radiation characteristics, so they must be modelled with extreme fidelity so that the antennas can be positioned wherever the beating of the blades has the least possible impact on effective transmission and reception. For example, the recent Ingenuity helicopter flights on Mars would not have been so successful if NASA had not thoroughly evaluated the antennas on its Curiosity rover and the helicopter, as many UAV manufacturers tend to do. From its tests, NASA knew specifically where the directional antenna on Curiosity had to be placed, and also that Ingenuity had to be oriented in specific ways on the ground relative to Curiosity, to compensate for the poor radio coverage at ground level on the Red Planet. GNSS Different GNSS antenna architectures are available depending on the platform and environment. Helical antenna technologies have continued to improve, offering some of the lightest weights relative to gain and throughput, while patch GNSS antennas continue to lead in absolute SWaP minimisation as well as having the lowest drag coefficients. One of the newest designs of GNSS antenna uses crossed dipoles (also known as turnstile antennas for their shape) producing a sinusoidal signal with a 90 º phase shift (also known as being in quadrature) and coupled to wideband radiating elements that are sized and shaped to optimise for low- elevation tracking. This configuration makes it easier than ever to take positioning information from satellites that are low in the sky – a critical advance for autonomous urban road vehicles or farming vehicles, which often have to contend with limited sky visibility amid buildings, trees and rows of crops. Such designs also come with low axial ratios (enabling high multi-path rejection) and minimal phase centre variations (meaning consistently June/July 2021 | Unmanned Systems Technology Some newer GNSS antenna designs enable better low-elevation tracking for unmanned vehicles to navigate more accurately through urban canyons or tree canopies (Courtesy of Tallysman)