Saltwater might be 4 million times less conductive than copper. And yet, a saltwater antenna works, even at HF. Amazing!
We have all heard about the benefits of operating vertical antennas near the ocean shore, using water as a ground plane. But now for something completely different. Liquid antennas made of water?
Over the past decade, a few engineers have been experimenting with saltwater antennas. You may be surprised at how well these can work.
Check out these videos. First, here is Daniel Tam, KE6FRY from San Diego. Dan has been working with the navy to create multi-frequency antennas – 2 to 400 MHz – made of seawater. In this video, he takes you through the rationale for and operation of a monopole saltwater antenna.
Second, here is a demonstration of the SeaAerial saltwater loop antenna from Mitsubishi.
If you want a deep dive into monopole saltwater antenna, you can read Daniel’s patent for the electrolytic fluid antenna. Essentially, both of these antennas work as a radiating element (water) working against a ground plane.
Saltwater Antenna – How Can That Be?
These antennas claim a radiation efficiency of 60-80%, which is pretty amazing given how poor water is as a conductor. In fact, you might know that pure water is actually a good insulator. Conductivity in water arises when it contains impurities, such as sodium chloride or salt. Even so, the conductivity of saltwater is many, many orders of magnitude less than metals such as copper or aluminum.
But, apparently, saltwater is conductive enough to radiate electromagnetic signals. As we all know, an antenna is simply a device to transform guided waves on a transmission line into RF. However, the conductive process in metals versus liquids is entirely different. Metals conduct using free electrons. Liquids conduct using positive and negative ions.
It turns out that sodium and chloride ions are pretty good conductors. In these videos, you will notice that the water stream is insulated from the water ground plane. Your feed mechanism is a current probe, using magnetic induction to couple to the liquid antenna.
If you don’t want to pump water, you can simply use a non-conductive cylinder to hold the water, which is what is done often at VHF and higher frequencies.
Saltwater is 4 million times less conductive than copper. And yet, these things work!