High-frequency antennas transmit radio waves across vast distances and even over mountain ranges using very little energy, making them ideal for military communications. These devices, however, have one big problem: they need to be physically huge to operate efficiently.
Instead of adding additional bulk, University of Wisconsin-Madison engineers, with support from a $550,000 grant from the U.S. Office of Naval Research, are developing strategies to increase the effective sizes of antennas by turning military vehicles into transmitters—using the structures that support the antennas themselves to broadcast signals.
Troops in remote locations need to rapidly communicate by radio, without being weighed down by extraneous equipment. However, antennas’ dimensions 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. Even at the smallest end, the ideal size for an antenna would be taller than an average adult.
“Unsurprisingly, we don’t use antennas that are that big,” says 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—as much as 90 percent of input power is dissipated as useless heat instead of being sent as 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,” says 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, armored transports, or amphibious assault vehicles, and Behadad realized that these structures themselves could broadcast radio signals.
“We are proposing to use the platform itself as the antenna,” he says. “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 send Internet data, Bluetooth, and cell phone calls because each signal uses a particular bandwidth. Similarly, military vehicles sprout miniature metal forests of ungainly antennas sticking high in the sky because fundamental properties of electromagnetic waves limit the range of signals available to each device. Some scientists have speculated that a yet-undiscovered material with unusual properties could pave the way to ultrawideband antennas, but, so far, 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 says. “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’s practical approach to increase the size of antennas by using the platform they stand on as broadcasting equipment finds a loophole in the laws of physics that doesn’t rely on any exotic materials. His team has already demonstrated a proof of principle using scale models of simple military platforms in the lab and computer simulations. Behdad is currently recruiting students to further develop practical applications.
Author: Sam Million-Weaver