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Seawater Fountain Transmits and Receives Signals Like an Antenna

Feb. 12, 2016
Researchers have developed an antenna system that turns the earth's most abundant resource—seawater—into an antenna for long-range communications.

Researchers from Mitsubishi Electric have developed an antenna system that turns the earth's most abundant resource—seawater—into an antenna for long-range communications.

The system, which can be almost completely submerged underwater, sends a fountain of saltwater into the air through an insulated nozzle. The nozzle itself is connected to a water pump, external power source, and a radio frequency cable. The water pump shoots the seawater through the specialized nozzle, allowing the stream to transmit and receive RF signals. Mitsubishi says that the height of the saltwater stream can be adjusted to operate over different frequencies.

While an unusual concept, conductive liquids have been investigated as a way to make antennas that can change their frequency and bandwidth. The research has drawn more attention in recent years as electronic devices begin to transmit data over a huge number of spectrum bands, all of which need different antennas. As a result, manufacturers are pumping devices full of filters and specialized antennas to tune into GPS, Wi-Fi, and other signals.

But researchers have not always been satisfied with this brute force approach. Several companies are developing filters that are capable of tuning into multiple spectrum bands. At the same time, researchers are tapping into synthetic materials, also known as metamaterials, which can be used to control the frequency and polarization of antennas. Liquid antennas, on the other hand, can adjust their frequency by changing the liquid’s shape.

Most liquid antennas have been made with liquid metal alloys. Last year, chemical and RF engineers from North Carolina State University built an antenna that could adapt to different frequencies by changing its length inside a tube. The alloy of gallium and indium could change its shape when a small voltage was sent into the material. While it was more suited to larger antennas that operate at lower frequencies, the system could even be used in microfluidic chips to transmit millimeter waves.

Although seawater is around 1,000 times as conductive as drinking water, it is still significantly less conductive than metal alloys. For that reason, the researchers struggled to get the antenna’s efficiency high enough to actually transmit data. But using computer simulations, the researchers were able to find the ideal diameter of the seawater plume, resulting in about 70% antenna efficiency.

Mitsubishi says that it tested this antenna, also known as SeaAerial, with digital television broadcasts. The company said that broadcasts sent through the antenna could be watched normally.

Although seawater is an exotic material for an antenna, Mitsubishi is not the first to experiment with it. In 2011, the U.S. Navy’s Space and Naval Warfare Systems Center (SPAWAR) patented an antenna that could change its bandwidth and frequency depending on the width and height of a saltwater stream. Shooting seawater through a magnetic coil created an antenna that, like Mitsubishi’s, could transmit and receive radio signals.

Daniel Tam, an engineer for the SPAWAR program, said in a video about the technology that a waterspout 2 feet high could function as a millimeter-wave antenna. The antenna could operate at very high frequencies (VHFs) and high frequencies (HFs) when the height of the stream was adjusted to 6 feet and 80 feet, respectively.

In a short summary of the technology from 2011, the Navy said that the antenna was designed to consolidate some of the antennas used on warships and submarines to communicate over many different frequency bands. 

Mitsubishi’s antenna could also be used on either land or sea. The researcher say that it could replace large low-frequency antenna towers on land with antenna fountains. Ships could also take it into seas where communications are limited. “It needs just a pump and a nozzle,” the company said in a statement.

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