Military robots of the future will provide a wireless, reliable, and stealthy communication capability for Soldiers using new technology pioneered by Army researchers.Scientists at the U.S. Combat Capabilities Development Command Army Research Laboratory, the Army's corporate research laboratory, developed a novel approach to improve the communications range and allowing for covert behavior using a team of robots for future Army multi-domain operations.Specifically, the researchers proposed and 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," said ARL electronics engineer Dr. Fikadu Dagefu. "Some 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, Dagefu said existing electrically small antennas, or ESAs, are very inefficient and other conventional designs are prohibitively large, limiting their application especially for low power mobile robotic networking. Furthermore, these ESAs radiate in nearly all directions due to their small aperture, which Dagefu said is undesirable since this makes the transmitted signal easily detectable by potential adversaries."The challenge of designing systems with small form-factor that can create directional radiation pattern has been a long standing and very challenging problem," he said. "Building on recent advances in developing highly miniature and efficient small antennas and their integration on small mobile agents along with software-defined radios, we 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 researchers took a different approach. The research team envisioned using 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."One of the advantages 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," said ARL researcher Jeffrey Twigg. "This is why antenna array design is traditionally conducted in a lab with specialized equipment."He said they 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 re-direct the overall energy of the array."Unlike conventional phased arrays, one advantage parasitic arrays have is that they are significantly less complex since the various nodes need not be synchronized and calibrated," Dagefu said. "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, meaning their inputs are shorted and not connected to a radio."Dagefu said some of the key steps in this research effort include investigation of the effect of inherent position and orientation uncertainties as well as the effect of the ground electromagnetic characteristics on the performance of the mobile parasitic array.Realizing conventional free-space designs are not valid when the above-mentioned uncertainties are present; 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.The results of the research are in a paper titled "Directional Communication Enabled by Mobile Parasitic Elements." It received best paper award based on its content, presentation and military relevance at the 2019 International Conference on Military Communications and Information Systems Conference, sponsored by NATO agencies, in Budva, Montenegro.-------------------The CCDC Army Research Laboratory (ARL) is an element of the U.S. Army Combat Capabilities Development Command. As the Army's corporate research laboratory, ARL discovers, innovates and transitions science and technology to ensure dominant strategic land power. Through collaboration across the command's core technical competencies, CCDC leads in the discovery, development and delivery of the technology-based capabilities required to make Soldiers more lethal to win our Nation's wars and come home safely. CCDC is a major subordinate command of the U.S. Army Futures Command.