Antenna design turns entire vehicles into broadcasting equipment

Antenna design turns entire vehicles into broadcasting equipment
UW–Madison engineers built scale models of military platforms to test the efficiency of radio broadcasts. Heat maps of the signal intensity overlaid onto photos of the devices indicate that the low profile platforms transmit communications comparably to existing antennas. Credit: Nader Behdad

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 huge to operate efficiently.

Instead of adding more bulk, University of Wisconsin–Madison engineers are working to increase the effective size of antennas by turning the 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 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 is 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, dissipating as much as 90 percent of input power as useless heat instead of useful broadcast radio signals.

Antenna design turns entire vehicles into broadcasting equipment
The engineers used computers to simulate amphibious assault vehicles acting as antennas. The sphere radiating outward represents the theoretical strength of the radio signals. Credit: Nader Behdad
"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 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," says 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 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 those laws that doesn't rely on any exotic materials. His team has already demonstrated a proof of principle using computer simulations and scale models of simple military platforms. Behdad is recruiting students to further develop practical applications.


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Apr 26, 2016
Hmm..I wonder if one could turn this around and detect vehicles designed in such a way.
Fly a satellite over the area and send a variable frequency pulse down. Analyse which parts are reflected.
The spectrum at the point of the vehicle should then be missing the frequency best suited for the antenna. In conjunction with a shape analysis of the 'missing region' this might give you positive ID of position and vehicle type(?)

Apr 27, 2016
Ham Radio operators abandoned this idea decades ago. No one in his right mind (for very long) wants to sit inside a near-field RF source. Head inside a microwave, anyone?

Apr 27, 2016
Hmm..I wonder if one could turn this around and detect vehicles designed in such a way.
Fly a satellite over the area and send a variable frequency pulse down. Analyse which parts are reflected.
The spectrum at the point of the vehicle should then be missing the frequency best suited for the antenna. In conjunction with a shape analysis of the 'missing region' this might give you positive ID of position and vehicle type(?)


The problem is getting an HF signal from space through the Ionosphere. Terrestrial direction finding efforts are not all that straightforward either. Broadband directional arrays for HF are rather large to put it mildly. Look up Wullenweber array for some idea of what this would be like. You could use orthogonal loop antennas such as what would have been used in aircraft ADF arrays. However those are not particularly sensitive --and these vehicle antennas are not particularly efficient.

Apr 27, 2016
Ham Radio operators abandoned this idea decades ago. No one in his right mind (for very long) wants to sit inside a near-field RF source. Head inside a microwave, anyone?


Uh, no, mobile HF activity is actually quite popular. Okay, it's a bit hard to do this on a Prius, but most SUV and Pickup trucks can handle it quite well.

You need to realize that when you're inside a vehicle where the whole body is the HF antenna you're actually inside a Faraday Cage. You don't get any significant RF radiation exposure. Furthermore, most people don't use all that much power. Near field exposure for even several hundred watts of power are not a significant problem. Keep in mind that this is HF, not microwave exposure.

May 01, 2016
@antialias_physorg : re: "I wonder if one could turn this around and detect vehicles designed in such a way."

You can. Its called Passive Radar and its been around for quite some time. It's basically a kind of radio interferometry that uses a phased array to measure and correlate small changes in the multipath propagation of preexisting terrestrial radio signals such as FM broadcast, caused by vehicles moving between the array and the signal source(s).

From the Wikipedia entry:

"Passive radar systems (also referred to as passive coherent location and passive covert radar) encompass a class of radar systems that detect and track objects by processing reflections from non-cooperative sources of illumination in the environment, such as commercial broadcast and communications signals."

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