DARPA has awarded HRL Laboratories its AMEBA project, an initiative to develop low-frequency radio transmitters that are vastly more compact and efficient than the massive antennas used to communicate in traditionally radio-denied conditions.
Ultra-low frequency (ULF) signals have very large wavelengths that can penetrate areas usually prohibitive to radio signals within caves or underwater. Commonly used radio wave frequencies and radar bands do not penetrate water or the ground due to their electrical conductivity and in the case of the earth, iron ores that strongly attenuate radio signals.
While ULF wavelengths do not carry large amounts of data, they can be used for sending short encoded messages - and enable communication that is impossible with typical radio equipment, such as with divers, troops in caves or difficult terrain, or personnel housed underground.
Most RF systems for consumer electronics, such as cell phones, operate at GHz frequencies. 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 thousand hertz and is man-portable. This would enable communications deep underwater or underground, with a backpack-based/sized 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.
Typical antennas are physically sized to resonate with the electromagnetic wavelength, which is convenient for portable communications at common radio and cell phone bands with wavelengths of a meter or so. At ULF, the low frequency and the high speed of light combine to create a very long city-sized wavelength. HRL’s proposed antennas are also resonant, but use resonant acoustic waves, which travel about million times slower than radio waves, to dramatically shrink the antenna size, weight, and power.
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. Their approach is different because instead of physically spinning a magnet, their 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 magnetized just like iron, but unlike iron, when magnetized this material elongates. A reciprocal effect is that mechanical stress can be used to control the direction of the magnetization inside the material. By vibrating the material, elongating and compressing, the magnetic field oscillates within the antenna without physically spinning it.
One of the keys to transmitting signals with the antenna is the ability to modulate a 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.
