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.
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. It is headed by Troy Olsson, DARPA’s program manager in the Microsystems Technology Office. 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.
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.
“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. 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. 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.
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.
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.”
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