Liquid radio

Sticking out like a hundred sore thumbs

A BIG American warship bristles with more than 100 large copper antennae that send and receive signals for its weapons, its radar and its voice and data communications. A lot of aerials, then, but still not enough. The navy wants its ships to carry even more of them. Fulfilling that desire has, however, stymied experts for decades. If placed too close together, antennae interfere with each other's signals. They also get in the way of aircraft and weapons. And, crucially, naval antennae—many of them more than 20 metres tall—make warships more easily visible to enemy radar.

At the American navy's Space and Naval Warfare Systems Command (known as SPAWAR for short), in San Diego, a team of more than 30 engineers is trying to solve such problems. In 2007 the team's leader, Daniel Tam, thought of a possible answer, appropriately enough, while taking his morning shower. The sodium and chloride ions in salt water conduct electricity. Could a spout of seawater, he mused, replace a metal antenna?

After a trip to a hardware store, Mr Tam discovered that indeed it could. With an $80 water pump, a $15 rubber hose and a $20 electrical device called a current probe that was easily plugged into a hand-held radio, he produced a spout roughly four metres tall from the waters of San Diego Bay. With this he could send and receive a clear signal. Over the intervening years his invention, dubbed the “pee antenna” by incredulous colleagues, has been tweaked and improved to the point where it can transmit over a distance of more than 50km (30 miles).

To make a seawater antenna, the current probe (an electrical coil roughly the size and shape of a large doughnut) is attached to a radio's antenna jack. When salt water is squirted through the hole in the middle of the probe, signals are transferred to the water stream by electromagnetic induction. The aerial can be adjusted to the frequency of those signals by lengthening or shortening the spout. To fashion antennae for short-wave radio, for example, spouts between 18 and 24 metres high are about right. To increase bandwidth, and thus transmit more data, such as a video, all you need do is thicken the spout. And the system is economical. The probe consumes less electricity than three incandescent desk lamps.

A warship's metal antennae, which often weigh more than 3½ tonnes apiece, can be damaged in storms or combat. Seawater antennae, whose components weigh next to nothing and are easily stowable, could provide handy backups—and, eventually, more than backups. Not all of a ship's antennae are used at once, so the spouts could be adjusted continuously to obtain the types needed at a given moment. According to SPAWAR, ten such antennae could replace 80 copper ones.

Fewer antennae mean fewer things for enemy radar to reflect from. Seawater is in any case less reflective of radar waves than metal. And if a ship needed to be particularly stealthy (which would mean keeping its transmissions to a minimum), her captain could simply switch the water spouts off altogether.

One disadvantage of water spouts is that they can be torn apart by the wind. SPAWAR's researchers have, however, found that their antennae work just as well if encased in a plastic tube. The tube can be sealed at the top so that the water goes up the middle, bounces off the top and then trickles down the inside of the tube's wall to the bottom, where it may be recycled.

That innovation also means that SPAWAR's invention need not be restricted to the navy. The closed-tube design allows saline aerials to be deployed on land, too. Indeed, one has already been tested successfully by a group of marines. It worked, as expected, with brine made from fresh water and a few pinches of salt. But if salt is not to hand, never fear. It also worked fine when the spout was fed with Gatorade.