A novel approach was developed to improve communications range and allow for covert behavior using a team of robots for future multi-domain operations. Specifically, researchers demonstrated an approach for enabling targeted wireless communication by exploiting miniature antennas and coordination of intelligent ground robots, each of which has a mounted antenna.
The role of antennas in the communication system is to provide a way to efficiently couple energy between a radio and the environment. Key antenna parameters include the radiation efficiency — which measures how well the antenna couples energy to and from the radio — and the directivity that measures how well the radiation can be focused in a direction of interest.
At lower frequencies, such as very high frequency (VHF) band or lower, existing electrically small antennas (ESAs) are very inefficient and other conventional designs are prohibitively large, limiting their application for low-power mobile robotic networking. Furthermore, the ESAs radiate in nearly all directions due to their small aperture, which is undesirable since this makes the transmitted signal easily detectable by potential adversaries.
Building on recent advances in developing highly miniature and efficient small antennas and their integration on small mobile agents, along with software-defined radios, researchers developed an approach that enables directional links at lower frequencies for targeted and robust communications at low frequencies.
Unlike conventional multi-antenna systems that either deploy large directional antennas — such as large dish or horn-type antennas or static antenna arrays, which are not appropriate for integration on small agents with limited battery power — the team used an ensemble of small, low-power ground robots that coordinate and adaptively re-configure their locations and antenna element sizes to create an adaptive and re-configurable parasitic array.
An advantage of deploying robots for this task is that they can perceive and act in the physical and electromagnetic domains, which is very difficult for humans. The team created a parasitic array with these robot-mounted antennas. This means that passive antennas in the array parasitize the energy of the active antenna connected to a radio to redirect the overall energy of the array.
Unlike conventional phased arrays, parasitic arrays are significantly less complex since the various nodes need not be synchronized and calibrated. The proposed system, which is inspired by a Yagi-Uda-type antenna, consists of a single excited ESA integrated on one of the robots and a group of robots equipped with parasitic antenna elements that are passive — their inputs are shorted and not connected to a radio.
The researchers developed an adaptive design strategy where the robots coordinated their inter-element spacing and parasitic element height to adapt to the ground conditions. The resulting design provides an optimal design that provides similar performance as the free-space case.