Modern day radio transmitters and receivers, the devices that uses electromagnetic wave signals to communicate through cellphones, radios and television, seem to be present everywhere. The propagation of these electromagnetic waves has some limitation too, Key among these is that radio frequency signals hit veritable and literal walls when they encounter materials like water, soil, and stone, which can block or otherwise ruin those radio signals. This is why scuba buddies rely on sign language and there are radio-dead zones inside tunnels and caves.
US Militaries are reportedly constructing of numerous Top Secret underground and undersea bases, as well as secret tunnels, all over the world. “Many governments have contracted with major civil and marine engineering firms to construct massive installations underground and undersea. These bases are found in many countries, including the USSA, Russia, China, Switzerland, Norway, Israel, Saudi Arabia, Australia, Canada, Great Britain and more,” Martin. Communications is a vital component of such facilities.
Ultra-low frequency (ULF) signals have very large wavelengths that can penetrate areas usually prohibitive to radio signals such as 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 – 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’s Sensors and Materials Laboratory.
A nearby band of very-low-frequency (VLF) signals (3 KHz to 30 KHz) opens additional communications possibilities because for these wavelengths the atmospheric corridor between the Earth’s surface and the ionosphere—the highest and electric-charge-rich portion of the upper atmosphere—behaves like a radio waveguide in which the signals can propagate halfway around the planet.
Currently U.S. ground forces employ PRC 117 SATCOM and PRC-150 high frequency radios for over-the-horizon communications but with substantial drawbacks, according to Olsson. While High frequency radios require the transmitter to know the precise location of the receiver, and the operator must change antenna construction for night and day operations to match the lowered ionosphere. SATCOM radios are vulnerable to Jamming attacks by sophisticated state agents such as China and Russia.
ULF and VLF can also be utilized as a search and rescue tool for buried miners or victims trapped in earthquake rubble because of its ability to penetrate rocks and building materials. “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. And because of that atmospheric waveguide effect, VLF systems might ultimately enable direct soldier-to-soldier text and voice communication across continents and oceans.
However, the free-space wavelengths of electromagnetic fields at ULF and VLF frequencies measure tens to thousands of kilometers in length, resulting in either very large or severely inefficient transmitter structures when constructed using conventional antenna approaches.
And since longer wavelengths have required taller antennas, communications in these frequency bands have entailed the construction of enormous and costly transmitter structures. A VLF antenna that the Navy built on a remote peninsula in Cutler, Maine, in the heat of the Cold War just to send a trickle of data to submarines makes the point: the gargantuan transmitter complex occupies 2,000 acres, features 26 towers up to 1,000 feet high, and operates with megawatt levels of power. Such transmitters are impractical in many operational scenarios, especially those requiring mobility.
DARPA’s Microsystems Technology Office, with his newly announced A Mechanically Based Antenna (AMEBA) effort, is betting on a little-exploited aspect of electromagnetic physics that could expand wireless communication and data transfer into undersea, underground, and other settings where such capabilities essentially have been absent. The basis for these potential new abilities are ultra-low-frequency (ULF) electromagnetic waves, ones between hundreds of hertz and 3 kilohertz (KHz), which can penetrate some distance into media like water, soil, rock, metal, and building materials.
With the AMEBA program, Olsson aims to develop entirely new types of VLF and ULF transmitters that are sufficiently small, light, and power efficient to be carried by individual warfighters, whether they are on land, in the water, or underground. The transmitters developed in AMEBA will consume less than 20 W of power and weigh less than 10 kg, making them suitable for man-portable wireless communications.
“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 said. Low frequencies can allow underwater communications at distances to hundreds of meters and through-earth communications at distances of hundreds of meters though soil and rock of heterogeneous composition and moisture content.
AMEBA Program Description
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 communication deep underwater or underground, with the ease of a backpack-based system
Technical Area 1: Penetrating RF (< 3 kHz).
There are many DoD-relevant applications within this frequency range that can benefit from the penetrating properties of low-frequency EM fields. Examples include, but are not limited to: Underwater communications at distances to hundreds of meters; Through-earth communications at distances of hundreds of meters though soil and rock of heterogeneous composition and moisture content
Rather than relying on electronic circuits and power amplifiers to create oscillating electric currents that, when driven into antennas, initiate radio signals, the new low-frequency VLF and ULF antennas sought in the AMEBA program would generate the signals by mechanically moving materials harboring strong electric or magnetic fields.
In principle, this is as simple as taking a bar magnet or an electret—an insulating substance, such as a cylinder of quartz (silica) glass, in which positive and negative electric charges are permanently segregated to create an electric dipole— and moving it at rates that will generate ULF and VLF frequencies. To open up practical new capabilities in national security contexts, however, the challenges include packing more powerful magnetic and electric fields into smaller volumes with smaller power requirements than has ever been achieved before for a ULF or VLF transmitter. That will require innovations in chemistry and materials (new magnets and electrets), design (shapes and packing geometries of these materials), and mechanical engineering (means of mechanically moving the magnets and electrets to generate the RF signals).
The AMEBA transmitter will exploit the magnetic component of the electromagnetic (EM) field because it is the magnetic field which is capable of penetrating conductive media. Furthermore, in the frequency ranges of interest, the background clutter for the magnetic field is lower than that of the electric field. Thus, the goal of AMEBA is to maximize the magnetic component of the EM field, regardless of the source mechanism.
The goal of phase 1 is to demonstrate a 1 fT magnetic field strength at a 1 km free-space distance from the transmitter. This field strength is intended to demonstrate the capability of messaging underground with ~100 meters of through-earth propagation.
The goal of phase 2 is to demonstrate a 10 fT magnetic field strength at a 1 km freespace distance from the transmitter. This field strength is intended to demonstrate the capability for messaging underwater with ~30 meters through seawater propagation.
Technical Area 2: Propagating RF (3 kHz – 30 kHz)
This frequency range, referred to as VLF, is well-known for the ability of EM waves to couple to the naturally occurring Earth-ionosphere waveguide. This coupling enables propagation of signals with very little attenuation around the globe. The Earth’s waveguide is formed between the ground and the different layers of the ionosphere at 75-85 kilometers above the Earth’s surface.
At 10 kHz, the EM wavelength measures 30 km and the far-field starts at ~5 km from the source. Once the VLF EM field is coupled to the waveguide, it can propagate over very long distances, which allows over-the-horizon messaging. This is in contrast to high-frequencies that require line-of-sight, relaying or bouncing off the ionosphere. The AMEBA approach will enable the deployment of transmitters with size and power consumption compatible with man-portable applications and capable of closing communication links at distances greater than 100 km
“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 Olsson. “With AMEBA, we expect to change that. And if we do catalyze 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.”
HRL awarded Project AMEBA to develop man-portable low-frequency radio antennas
HRL Laboratories, LLC, has received an award to participate in project AMEBA, the Defense Advanced Research Project Agency (DARPA) 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.
“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,” McKnight said.
“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 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.
“We’re just vibrating a stack of magnetic material and the magnetization 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 communication 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.
Under AMEBA program, SLAClab created a pocket-sized antenna that could enable mobile communications in challenging environments, such as undersea & underground.
Very low frequency communication systems (3 kHz–30 kHz) enable applications not feasible at higher frequencies. However, the highest radiation efficiency antennas require size at the scale of the wavelength (here, >1 km), making portable transmitters extremely challenging.
Conventional transmitter techniques are inadequate due to large size and high loss. Researchers have shown that a strain-based, piezoelectric transmitter can overcome many of the fundamental limitations of conventional electrically small antennas (ESA). These transmitters can resonate in a very small footprint while exhibiting low losses.
Facilitating transmitters at the 10 cm scale, Researchers led by Mark A. Kemp demonstrated an ultra-low loss lithium niobate piezoelectric electric dipole driven at acoustic resonance that radiates with greater than 300x higher efficiency compared to the previous state of the art at a comparable electrical size. A piezoelectric radiating element eliminates the need for large impedance matching networks as it self-resonates at the acoustic wavelength
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