Darpa, the research institute of the US Department of Defense, is working on small transmitters for ultra-low-frequency communication. This should make communication possible in places where this has been difficult until now, such as under water, in tunnels and in caves.
Darpa writes that so far not much progress has been made in the field of this form of mobile communication. With its Ameba program, the institute aims to use low frequencies between ‘several hundred’ Hertz and 3 kHz for low bitrate communication. For example, a diver must be able to send text messages to a submarine or a drone. The technique works with small transmitters, which, however, do not use oscillating current signals like normal radios to generate radio signals via an antenna.
Instead, the transmitters use materials that contain strong magnetic or electric fields. By mechanically moving these materials, it is possible to generate ulf and vlf frequencies, Darpa said. This can be achieved by, for example, moving an electret or a bar magnet at high speeds. According to the institute, several developments in chemistry and materials are needed to develop these transmitters for practical use, for example, in order to accommodate powerful magnetic fields in a smaller amount of material.
In addition, developments in design and engineering are needed to provide packaging and ways to generate radio signals through movement. Darpa also wants to use frequencies between 3kHz and 30kHz to enable communication between the Earth’s surface and the ionosphere. In this way it should be possible to bridge an entire hemisphere. Until now, communication via very low frequencies has been cumbersome, because the long wavelengths require large transmission installations. For example, an American installation for communication with submarines during the Cold War consists of 26 towers with a height of up to three hundred meters and the energy consumption is a few megawatts.
The Ameba transmitters must consume less than 20W and must not weigh more than ten kilograms, according to an accompanying document.