Magnetic appeal of ULF antennas

ULF antennas made of magnetoelastic material could enable radio communication through water or under the ground.

Project AMEBA, the US Defense Advanced Research Project Agency (DARPA) initiative, aims to develop low-frequency radio transmitters that are far more compact and efficient than the large antennas used to communicate in traditionally radio-denied conditions.

While ultra-low frequency (ULF) wavelengths do not carry large amounts of data — typically short encoded messages — they could enable communication that is impossible with typical radio equipment, such as with divers, troops in caves or difficult terrain, or personnel housed underground.

“That’s why people trapped in mines must communicate with the surface by tapping on pipes, because typical radio communication cannot be used,” said Geoff McKnight, co-lead researcher on the project from HRL Laboratories’ Sensors and Materials Laboratory. HRL Laboratories has received a contract to participate in project AMEBA.

The goal of the AMEBA project, which stands for A Mechanically Based Antenna, is to enable a communications system that transmits at less than a 1000 Hertz and is man-portable. This would enable communication deep underwater or underground, with the ease of a backpack-based system.

“For those people in mine disasters, or in buildings collapsed after earthquakes, a portable low-frequency beacon could also make a dramatic difference in search and rescue,” said Walter Wall, project co-lead from HRL’s Advanced Electromagnetics Laboratory.

“Mobile low-frequency communication has been such a hard technological problem, especially for long-distance linkages, that we have seen little progress in many years,” said program manager Troy Olsson of DARPA’s Microsystems Technology Office.

“With AMEBA, we expect to change that. And if we do catalyse the innovations we have in mind, we should be able to give our warfighters extremely valuable mission-expanding channels of communications that no-one has had before.

“If we are successful, scuba divers would be able to use a ULF channel for low bit-rate communications, like text messages, to communicate with each other or with nearby submarines, ships, relay buoys, UAVs and ground-based assets. Through-ground communication with people in deep bunkers, mines or caves could also become possible,” Olsson added.

And because of the atmospheric waveguide effect, VLF systems might ultimately enable direct soldier-to-soldier text and voice communication across continents and oceans.

Magnetic antennas

Typical antennas are physically sized to resonate with the electromagnetic wavelength, which is convenient for portable communications at common radio and mobile phone bands with wavelengths of a metre or so.

“At ULF, the low frequency and the high speed of light combine to create a very long city-sized wavelength,” McKnight said.

HRL’s proposed antennas are also resonant, but use resonant acoustic waves, which travel about a million times slower than radio waves, to dramatically shrink the antenna size, weight and power, McKnight added.

“Other teams working on this problem are attempting to achieve a low-frequency wave by taking a permanent magnet and rotating or oscillating it. The mechanical motion of that magnetic moment is equivalent to a traditional antenna, which achieves an oscillating magnetic moment by oscillating large amounts of electrical current,” McKnight said.

“Our approach is different because instead of physically spinning a magnet, our device is magnetoelastic, meaning the magnetic field oscillates within the material in response to acoustic stress waves, created through structural vibrations.”

The HRL team’s antenna will use materials that possess a quality called magnetostriction. This enables the material to be magnetised just like iron, but unlike iron, when magnetised this material elongates.

A reciprocal effect is that mechanical stress can be used to control the direction of the magnetisation inside the material. By vibrating the material, elongating and compressing, the magnetic field oscillates within the antenna without physically spinning it.

“We’re just vibrating a stack of magnetic material and the magnetisation is flipping back and forth in the material,” Wall said. “These elastic forces allow us to control the magnetism.”

One of the keys to transmitting information with the antenna is the ability to modulate the signal frequency. A physically spinning antenna begins to act like a flywheel and store energy due to inertia. High inertia makes such devices inherently frequency-stable, in turn making signal frequencies very hard to modulate. Vibrating systems are also very stable, hence their use in clocks.

But HRL recently discovered a mechanical way to rapidly shift the resonant frequency and the researchers propose to use that mechanism to rapidly modulate the transmitter frequency with relatively little electrical power.

“This is a pretty exotic project for us,” Wall said. “When you consider that most wireless devices operate at a billion cycles per second, this system will work at a thousand cycles per second.

“It is a part of the spectrum most people don’t think about much, but it is pushing HRL research in completely new directions, which is a nice thing as well.”

Image courtesy DARPA.

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