New "SeaAerial" technology that uses a seawater plume as an antenna
The "SeaAerial" developed by Mitsubishi Electric shoots a jet of seawater into the air and uses the plume as an antenna. This is world's first antenna*1 using seawater that has successfully trialed signal reception for terrestrial digital broadcasts (full-segment).
The size of such antennas is determined by the frequency of the signals being transmitted and received. For low-frequency signals, the antenna needs to be large and requires a substantial support structure. But the "SeaAerial®" consists of just seawater and a pump and can create an antenna of the required size wherever and whenever it is needed. This makes it ideal for communications systems during disasters or as a temporary antenna for specific events. Its compactness also makes it easy to transport, so potentially it has a wide range of applications, including as a communications antenna for ships.
The excellent performance of the "SeaAerial®" rivals that of conventional metal antennas. The unique idea of a seawater antenna coupled with the advanced technology to make it practicable has already attracted considerable attention from various quarters.
The insulated nozzle that makes the unique idea of a seawater antenna feasible
Antennas come in various shapes and sizes, but an ordinary vertical antenna can efficiently send and receive signals when its height is one quarter of the signal wavelength. To successfully receive terrestrial digital broadcasts, the antenna must be 10−15 cm high. While generating a seawater plume of that height is simple, a number of other issues must be resolved to ensure that the plume works effectively as an antenna. Electricity flows through seawater, which contains 3−4% salt and will conduct electricity. But while this conductivity makes it suitable as an antenna, it also gives rise to some problems.
For the plume to act as an antenna, it must carry a high-frequency current. But when a current is applied to the plume, it dissipates in the surrounding ocean. That is the first problem. This was solved by using an insulated nozzle. At first glance, this looks like a simple metal tube, but the insulated nozzle actually embodies a number of original ideas that make use of the particular characteristics of high-frequency current. Because the nozzle length is one quarter that of the wavelength, it cuts off the flow of current into the surrounding ocean and allows the current to flow through the plume efficiently.
Plume thickness the key to overcoming the low conductivity of seawater
The second problem is the conductivity of seawater. While seawater conducts electricity, its conductivity is low compared with metal. So we need to compensate for that low conductivity. The answer lies in the thickness of the water plume. We ran exhaustive trial simulations and succeeded in identifying the optimal plume thickness for excellent high antenna performance.
Antenna efficiency in the compact antennas used in mobile terminals is between 30% and 80%. The figure for the "SeaAerial®" is 70%. This gives us antenna performance that is ample for practical applications. In the future, we expect to be able to cater to frequency bandwidths ranging from several megabits up to several gigabits simply by varying the height of the seawater plume. In a country like Japan that is surrounded by oceans, a seawater-based technology like "SeaAerial®" is likely to be effective and highly practicable.
- *1 As at January 2016 (based on Mitsubishi Electric research).
- * "SeaAerial" is a registered trademark of Mitsubishi Electric.