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The challenge of transmitting radio frequencies underwater.

  1. Apr 18, 2015 #1
    If both a transmitter and receiver antennae were submerged, and within a defined range and depth, what range might be available to a civilian operator?

    Are two divers able to communicate by radio?

    B
     
  2. jcsd
  3. Apr 18, 2015 #2
    There is a readable article on the subject at the following URL: http://users.tpg.com.au/users/ldbutler/Underwater_Communication.pdf
    The attenuation in sea water is very high, and increases with frequency. Therefore, a low frequency is desirable, but will necessitate a large antenna in order to obtain good efficiency. The typical excess attenuation caused by sea water at 1 MHz is about 40 dB/m. This will limit the range of a practicable radio system to perhaps 4m.
     
  4. Apr 18, 2015 #3

    davenn

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    Yup which is why they used VLF in the 10 - 30kHz range for the Omega system for submarines

    Dave
     
  5. Apr 18, 2015 #4

    Baluncore

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    There are alternatives to the high attenuation of MW radio.
    There is a new 475kHz amateur band. https://en.wikipedia.org/wiki/600-meter_amateur_radio_band
    An ultrasonic rather than an electromagnetic carrier could be used. Maybe with spread spectrum modulation.
    The divers could communicate through a magnetic field, inductively coupled at audio frequencies.
     
  6. Apr 19, 2015 #5

    jim hardy

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    I read as a kid lots of ww2 submariner memoirs. They were amazed when in the Great Lakes their radios worked down to 200 feet.

    Summer is coming. Waterproof a pair of CB walkie-talkies and try it out.
     
  7. Apr 19, 2015 #6

    Baluncore

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    The conductivity of seawater is definitely a major problem with RF underwater EM communication.

    There will be an optical band, probably a blue colour that gets through seawater best. That would be a good use for blue LEDs. Scattering by particles in the water would give good all round communication “vision”.

    I once needed a radio quiet location to calibrate a broadband IF system. The best place available was below the water line in the bow thruster compartment at the front of the ice-strengthened vessel. The only signals left in the IF were the LW aviation NDBs within 2000 km.
     
  8. Apr 19, 2015 #7
    I don't think a change from 1 MHz to 475 kHz is sufficient to alter the range very much, especially as the antenna for 475 kHz will be less efficient. Attenuation in dB/m is proportional to the square root of frequency. For the case of the Great Lakes, which is mentioned later, I think the water may be almost fresh, and so it will have lower conductivity and lower attenuation.
     
  9. Apr 19, 2015 #8

    dlgoff

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    Excellent idea. I did a search and here's a theses that covers this. repository.lib.ncsu.edu/ir/bitstream/1840.16/570/1/etd.pdf

    I took the liberty of uploading one of his graphs that shows the absorption spectrum of clear sea water. Blue it is.

    absorptionspectrum.jpg
     
  10. Apr 19, 2015 #9

    Baluncore

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    You are correct, it is obviously the conductivity of the sea water that is the real problem. Divers should be able to use LW, MW and even HF communications in rivers and fresh water lakes.

    The reference you gave earlier "Underwater_Communication.pdf" referred to the 1.8MHz amateur band. The change from 1.8MHz to 475kHz is significant, although maybe insufficient for seawater, it is a step in the right direction. I would not consider frequencies above 50kHz for communication through seawater or underground. It would be interesting to experiment with 475kHz in a lake. It should be possible to have a surface guided wave at shorter wavelengths.

    The dielectric constant of ice is between 3 and 4, liquid water is about 80. The conductivity of ice is very significantly less than seawater. To communicate with MW in seawater you need an iceberg or a lake of fresh surface water as the communication channel.

    The more I think about it, the more attractive blue light becomes for undersea wireless communications. I see no reason why we could not have several hundred metres of optical internet VoIP in seawater.
     
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