Military communications needs high-frequency antennas to transmit radio waves across massive geographical distances. Though the biggest snag found in today’s devices are the right sizes to deliver communicative results as the devices are bulky to operate efficiently.
To get a promising answer, University of Wisconsin–Madison engineers are working to increase the effective size of antennas by turning the military vehicles that carry them into transmitters—using the structures that support the antennas themselves to help broadcast signals.
Troops in remote locations need to communicate by radio and move quickly, unencumbered by heavy and unwieldy equipment. However, antennas need to be at least one-quarter the length of the radio waves they transmit in order to operate efficiently. High-frequency military signals use radio waves ranging from roughly a football field in length to the distance of a successful first down—10 yards. Even at the small end, the ideal size for an antenna is taller than an average adult.
“Unsurprisingly, we don’t use antennas that are that big,” said Nader Behdad, an associate professor and Harvey D. Spangler Faculty Scholar in electrical and computer engineering at UW–Madison. “Putting a big, long antenna on top of an amphibious assault vehicle would be too high-profile.”
Short antennas lighten loads at steep costs to performance. The devices are inefficient, dissipating as much as 90 percent of input power as useless heat instead of useful broadcast radio signals.
“The problem is that antennas that are a small fraction of the wavelength can’t really communicate over long distances, and data rates cannot be as fast as they should be,” said Behdad.
Increasing the size of an antenna without adding to its physical dimensions sounds impossible. However, real-world military antennas are almost always attached to other things—for example, large, metallic objects like trucks and armored transport or amphibious assault vehicles—and Behdad realized that these structures themselves could broadcast radio signals.
“We are proposing to use the platform itself as the antenna,” said Behdad, whose research is supported by a $550,000 grant from the U.S. Office of Naval Research. “It’s a clever way to go around the limitations set by the laws of physics. From a practical point of view, the volume of the object on the military platform is the same, but we’ve effectively achieved a larger antenna.”
Turning trucks into transmitters not only makes antennas more efficient, but also enhances communication in the field by enabling one device to send and receive multiple types of information.
Separate transmitters handle internet data, Bluetooth connections, and cellphone calls, because each signal uses a particular bandwidth. As a result, military vehicles sprout metal forests of ungainly antennas high overhead. Some scientists have speculated that a yet-undiscovered material with unusual properties could pave the way to ultra-wideband antennas that could handle several broadcast types simultaneously, but those predictions remain unsubstantiated.
“It’s pretty clear that we cannot beat the laws of physics, although it’s not for lack of trying,” Behdad said. “With the laws of physics as they are, the only way to increase the bandwidth of ultra-wideband antennas is to increase their size.”
Behdad is recruiting students to further develop practical applications.