Unmanned Systems Technology 006 | ECA Inspector Mk2 USV | Antenna systems | Northwest UAV NW-44 | Unmanned ground vehicles | Navigation systems | Lunar X challenge

39 Antenna systems | Focus rectangular, circular and elliptical, or even a repeating fractal pattern. They have a very low profile, are mechanically robust and can also be shaped to conform to the curving skin of a vehicle for a conformal antenna on the outside of a UAV or UGV, but as they have a smaller area than a dipole type they can be more limited in the range at which they can pick up a signal. They also have to be tuned to a particular frequency band, often using laser cutting to get the exact length of the strip, and there can be a trade- off between using multiple microstrip antennae and more complex multi- element antennae. Microstrip antennae can also provide polarity diversity, as several can be used with different polarities. Fractal designs are used in microstrip antennae to get a longer trace in a smaller area. These make use of the concept of ‘self similar’, where a complex pattern is built from the repetition of a simple shape. This allows a microstrip design to be typically 50-75% smaller than traditional straight-line microstrips, and can allow different antenna designs for different bands to be included in the same system. It is possible to produce fractal versions of all existing antenna types – including dipole, monopole, patch, conformal, bi- conical, discone and helix – to provide more compact versions, although with a higher engineering and tuning cost. Looking first at the use of a helix or spiral coil as the antenna, this can provide a larger antenna in a smaller space compared with a straight rod in a dipole antenna. A discone antenna is a version of a bi-conical antenna, where one cone is replaced by a disc. It is usually mounted vertically, with the disc at the top and the cone beneath. This type is omnidirectional, vertically polarised and with a gain similar to that with a dipole; it also has wideband operation capability, making it suitable for ground station systems for UAVs. The radiation pattern from it in the horizontal plane is quite narrow, making its sensitivity highest in the direction of the horizon and rather less for signals coming from relatively close by, so it avoids local interference. As the name suggests, multi-element antennae combine different antenna elements with different polarities into a single unit. This allows them to pick up a signal from a ground station regardless of the orientation or polarisation, providing a reliable connection. Although microstrip antennae lend themselves to conformal designs, other conformal antennae that use new electromagnetic metamaterials are emerging. These use tunable elements that are arranged in a precisely calculated way so that the RF energy is scattered when the elements are activated, generating a 3D beam from a flat or conformal array. The direction of the beam is defined by the elements, which are electronically activated, allowing for continuous and instantaneous changes in direction. This allows an antenna with no moving parts to lock on to and track a particular satellite, even one in a fast-moving low-Earth orbit. Additional software control of the elements allows the antenna to switch between satellites to maintain a connection. A blade antenna is a monopole type where the traditional dipole is grounded in the centre, and tends to be trapezoid in shape. This allows more effective coupling with a circularly polarised signal, and is generally fitted externally to aircraft for links in the VHF and UHF range. The technology is being used in two working prototypes of Boeing’s X-48B scale-model unmanned airborne craft, in the form of three separate blade antennae for telecommand, telemetry and video links. They weigh less than 20 g each, are robust, weatherproof and measure 105 x 30 x 2 mm; they are mounted on a cross-spar, with each antenna covering a different frequency. Traditional satellite antennae tend to be large and bulky, partly as a result of requiring a mechanism to track a Unmanned Systems Technology | February/March 2016 It is possible to produce fractal versions of all existing antenna types to provide more compact versions, although with a higher engineering cost An omnidirectional antenna (Courtesy of BMS)