A Radio Antenna You Can Turn Off With a Faucet
At SPAWAR Systems Center Pacific, a US Navy research lab in San Diego, California, engineers pointed a nozzle at the sky and fired a plain jet of seawater into the air, then used that jet to send and receive radio signals. No metal rod, no mast, no rigid structure at all, just water rising and falling back down.
It functions as a real antenna, made from something most of us associate with dousing fires, not transmitting them. When the pump shuts off, the antenna doesn’t power down so much as vanish, the column falls, and there’s nothing left standing where it was.
Why Would Water Carry a Signal at All?
A normal antenna works because electrons slide back and forth along a conductive shape, tuned so those oscillations line up with a radio wave. Metal is good at this because its atoms hold their outer electrons loosely, letting current flow with little resistance.
Seawater is nothing like that. It’s mostly water with dissolved salt, sodium and chloride ions drifting through it rather than a lattice of metal atoms passing electrons hand to hand. It does conduct electricity, but far less efficiently than copper or aluminum. So how do you take a stream of salt water, a fairly weak conductor, and get it to behave electromagnetically like a solid antenna element?
The Trick Is in the Nozzle, Not the Water
The answer is a device called a current probe. As the seawater shoots upward, it passes through this probe, which induces a magnetic field around the rising column. That field, not the water’s own conductivity, is what actually carries the signal to and from the connected radio gear. Engineer Daniel Tam worked out that the water itself just needs to move through the probe correctly, it doesn’t need to be a great conductor at all.
In any antenna, resonant frequency is tied to physical length, and the water column’s height plays that exact role. A jet about 2 feet tall (0.6 m) resonates in the UHF band; raised to roughly 6 feet (1.8 m), it drops into VHF; higher still, it reaches HF. SPAWAR’s own figures describe an 80-foot column (24.4 m) covering frequencies from 2 to 400 MHz. Change the water pressure, change the band.
Several Antennas, One Nozzle Array
Since each band corresponds to a specific water height, multiple jets set at different heights can transmit at once without stepping on each other, the same principle every radio system already relies on to keep bands from interfering. A patent covering the design describes three separate sub-streams, tuned to HF, VHF, and UHF, operating side by side from the same basic setup.
On a conventional ship, each frequency band needs its own physical rod, built to a fixed length for that job alone. Here, the same pump, nozzle, and current probe can be reconfigured in seconds, no rewiring, no new hardware, just a different jet height.
Why the Navy, Specifically, Cares About This
Warships reportedly carry around 80 separate antennas bristling from the deck, a figure from press coverage rather than an official Navy count. Each is a fixed structure competing for limited space, adding to the ship’s radar signature, and vulnerable to damage. SPAWAR has suggested that as few as 10 seawater antennas of varying heights could, in theory, cover the same frequency range as all 80 fixed rods.
A water jet can be aimed anywhere there’s open deck, switched on only when needed, then shut off entirely, leaving no silhouette for an enemy sensor to lock onto and no rod that can be sheared off in a hit. Treat this as the idea driving the research, not a feature already standard across the fleet.
What We Still Don’t Know
Public material doesn’t explain in detail how the current probe generates its magnetic field, or how much water flow and pump pressure are needed to keep a jet stable enough for practical use. One press account puts its usable range at 30 miles (48 km), but that figure comes from a single source and hasn’t been independently confirmed.
It’s also unclear whether this has been tested aboard an operational ship or only under controlled research conditions; the available record treats it as a proof of concept rather than a finished, fielded system. Whether a jet of seawater ever takes its place among the antennas on a warship’s deck remains an open question.