Pumping Up An Antenna From A Stream Of Sea Water

Our Hackaday readership represent a huge breadth of engineering experience and knowledge, and we get a significant number of our story tips from you. For instance, today we are indebted to [sonofthunderboanerges] for delivering us a tip in the comment stream of one of our posts, detailing an antenna created by coupling RF into a jet of sea water created with a pump. It’s a few years old so we’re presenting it as an object of interest rather than as a news story, but it remains a no less fascinating project for that.

The antenna relies on the conductivity of sea water to view a jet of water as simply another conductor to which RF can be coupled. The jet is simply adjusted by altering the flow rate until it is a quarter wavelength long at the desired frequency, at which point it is a good analogue of a metal whip antenna. The RF is coupled at the base by a ferrite cored transformer that clips around the nozzle ejecting the water, and a bandwidth from 2MHz to 400MHz is claimed. If you work with RF you will probably wince at the sight of salt water coming near the RF connector, as we did.

The advantage of the system is that it allows antennas of multiple frequencies to be created at very short notice and using very little space or weight when not in use. The creator of the antenna at the US Navy’s SPAWAR technology organization points to its obvious application on Navy warships. Whether or not the sailors are using these antennas now isn’t clear, but one thing’s for certain, the idea hasn’t gone away. Early last year Popular Mechanics reported on a similar project under way courtesy of Mitsubishi, in Japan.

The piece that spawned the tip in a comment was our coverage of the novel Poynting vector antenna, though as this drone-hoisted antenna shows, we’ve brought you a few unusual designs in our time.

30 thoughts on “Pumping Up An Antenna From A Stream Of Sea Water

        1. The clamp is too big to stay on when I am peeing. And when you are almost done peeing you would need to switch to a UHF burst transmission. Synchronization is still being worked out,

          1. I was not saying that you should use pee stream as antenna. Wet any length of string using pee, then use that string as antenna.

            But, Your Idea is not bad…

            (I hope that moderators will not block me)

    1. This isn’t for emergencies, and it allows you to have any length, and I suppose can be aimed in different directions. I’ve a feeling the Navy are aware of the existence of bits of wire, so I don’t suppose they’d waste their time on this.

      Not that the US military doesn’t waste untold billions on nonsense, but this one seems a bit more down to earth than killing goats with your mind.

  1. SPAWAR, jets of water happy sailors. And then there is that often referenced antenna standard the wet noodle. I would assume the length is somewhat fluctuating of the water jet.

  2. I wonder how well this functions in real life… when there is wind… heavy wind.
    Back in the day of cars with large antennas there were antennas that could retract automatically, using a telescopic antenna and a piece of cable that was winded/unwinded by a motor at the base of the antenna.
    Seriously, using water as an antenna sounds like fun on paper but must have more negative then positive sides.

    1. you have to understand the point behind this design was to be stealthy over anything else.

      if I remember currently the original articles on this talked about how its possible to track normal antenna even when the radio if off. all antenna will passively resonate when a strong enough signal matches the Antennas and this lets you find the location of the antenna. the point of the pump antenna is there antenna before you turn it on or after you turn it of so nothing to track.

      1. I hate to be Mr Obvious, but wouldn’t a telescopic antenna do the same job? Telescope it down when you’re done, so it’s length has changed. You could keep it in an earthed container, ships are still made of metal right?

  3. An interesting theoretical study, but I don’t see any practical application to this. I can’t imagine that it would stay stable enough to work as a good antenna on a ship, given the motion of the ship itself in multiple directions simultaneously. Then there’s the wind (a constant when a ship is under way, and nearly constant anywhere on shore near an ocean), which would likely break the stream into droplets, rendering it ineffective and non-resonant at the intended frequency. I could see radio amateurs experimenting with this (indeed I think I have but was unable to find the article in 30 seconds of Googling), but I can’t see it ever becoming a practical way of creating an antenna. There are too many other easier solutions out there.

    1. those easier solution are also easier to track, the whole point of this style was to make a stealth antenna. there are way to make antenna passively resonate even when the radio is off thous giving away the location of the ship. using this type of antenna make that impossible as there is only an antenna as long as you pump and its gone the second you stop pumping.

    2. You’re right that an antenna like this introduces numerous issues, but keep in mind that it is intended to deal with other issues. Military vessels have trouble finding enough space for all the antennas they need. That’s particularly true of carriers where antennas have to be kept away from aircraft flight paths. The stress on stealth creates still more issues, since that antenna becomes a radar reflector. A salt-water-fountain antenna offers a solution, since it could be created in a couple of seconds and disappear just as quickly.

      My hunch that its only practical military use may be with submarines or SEAL teams. It’d offer a way for a nuclear sub or SEALs to extend the base of the antenna just above the water, create the long water antenna in a flash, send a compressed VLF-HF message, and then disappear.

      For radio hams, a brine solution in a vertical plastic tube might permit a tunable antenna by adjusting the level of brine in the tubing. That’d work, but it’s likely to be more complex and heavier than inexpensive antenna tuners like those sold by QRP Guys:

      http://www.qrpguys.com

      This idea seems to be solution in search of a problem. But keep in mind that the military often does that kind of research. A failed idea may suggest a practical solution.

      –Mike Perry, WA4MP

  4. This feels like something I would expect to see on an episode of MacGyver. Also would be an extremely impractical way of making an antenna from a tall building – Just dump water in a laminar stream off the edge…

  5. Lot’s of naysayers here huh.

    I don’t know how seawater compares to actual metal in terms of performance but if it really does work then I can see the advantage. It’s very easy to re-tune. See what they said there.. 2 to 400 MHz. That’s quite a range and I would think that it’s linear, chose any frequency within that range not just selected bands.

    Ok.. so tuners have been mentioned. Yes, those are a thing. A random wire with a tuner might be brought to a low enough SWR that it doesn’t fry your final amplifier but that doesn’t mean it works as well as an actual resonant whip which seems to be what you get with a liquid antenna.

  6. Interesting. They said 2-400 MHz. At VHF/UHF a few feet of wire is going to be a better solution. It is small, light, less expensive and a lot more reliable. In emergency situations power is often an issue. It would make more sense to use the power needed for the pump for higher transmit power to a short wire or to extend the life of your batteries or other power source.

    Medium and HF frequencies might be interesting because of the large wavelengths. To get a quarter wave on 2 MHz you would need a jet of water 116’ high. That is not going to be a small pump, probably something on the scale used for firefighting. That would not be too big an issue on a ship, but would be a consideration for something portable.
    Of course you could use a shorter stream and tune it. The lower resulting bandwidth might be an issue. Wind or other factors could change the resonant frequency quickly.

    One advantage of the sea water antenna is you would not need a big upport for your low frequency antenna.

  7. So now people that live in housing developments that don’t allow antennas should be able to put in a sea water fountain and get on the air. Of course, if you can convert your flag pole to an antenna, that would probably make more sense.

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