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Rf into seawater penetration?

Discussion in 'Electronic Basics' started by John Fields, Feb 9, 2004.

  1. John Fields

    John Fields Guest

     
  2. <snippety>

    Let me put it this way. Ther are good reasons why the Navy uses
    VLF transmitters to communicate with their subs. ;-)

    Honestly, I'd be surprised if a toy-car transmitter was usable to
    more than a foot or so of depth.

    Remember what you're dealing with. Seawater is a pretty good
    electrical conductor, and the ocean is the world's biggest ground plane.


    --
    Dr. Anton Squeegee, Director, Dutch Surrealist Plumbing Institute
    (Known to some as Bruce Lane, KC7GR)
    kyrrin a/t bluefeathertech d-o=t c&o&m
    Motorola Radio Programming & Service Available -
    http://www.bluefeathertech.com/rf.html
    "Quando Omni Flunkus Moritati" (Red Green)
     
  3. George

    George Guest


    RF or IR won't work through several meters of water. Water is used to
    absorb them in labs. I don't know much about antennas but you should
    probably consider having an antenna on a float with the signal sent
    down a wire to the device below. The only EM radiation I can think
    that might penetrate coherently that deep is like blue-green laser.
     
  4. George

    George Guest

    Oh yeah, and the fact that it's seawater increases the absorption
    rate. The absorption rate is proportional to conductivity which is
    proportional to the amount of ions in the water.
     
  5. Rubicon

    Rubicon Guest

    Hello,

    Does anyone have an accurate estimate of how deeply, if at all a
    common childs remote controlled toy cars transmission can penetrate
    seawater to control the car? A difficult question I admit!

    Also:
    Are there any RF frequencies that are better at penetrating seawater
    than others and if so what are they?

    The application is for a 15m max, close range transmitter of some sort
    (RF, IR?) to remotely instruct an under water (up to 7meters deep -
    tidal) device to activate a solenoid or motor. Most probably it's be a
    motor or servo of some sort as solenoids are so expensive and using a
    cheap toys components would be an advantage for costs sake.

    Bearing in mind the current global situation you can rest assured it's
    for nothing sinister but for a device my employer has in mind for
    rescuing lost and preventing stolen boating equipment.

    Funny that after over three years of constantly hearing every single
    step and problem involved in its double hulled home built construction
    that's about all I can remenber of his boat apart from the fact it's a
    Hartely design. I think? Oh it was suggestred by another that I use an
    auger and bit to drill through from the inside below the waterline to
    the fiberglass outer layer so that when it got into heavy seas it'd
    burst and sink. Ahhh... I can almost see it now...

    Cheers,

    Andrew.
    Remove the ZZ from E-Mail address to contact me.
     
  6. Very low frequency (approx 10 kHz) waves are routinely used for contact
    to submarines, but the antennas are rather big.

    I do however remember reading a paper on a communication device used for
    cave rescue, called the mole-phone. Apparently it relies on the magnetic
    rather than the electrical component of the field and hence works
    several hundred meters through rocks. Conceivably this should also work
    under water.
     
  7. Eric R Snow

    Eric R Snow Guest

    I was just reading, a month or so ago, in Nuts & Volts about a system
    that uses inductance as the transmitting medium. The article described
    a setup where the transmitting antenna was a wire loop surrounding a
    room. Then a coil was used to pick up, through inductance, the signal.
    The pickup used in the article was one from Radio Shack that is
    normally used as a phone pickup. The kind that uses suction to stick
    to the phone, The article mentioned this type of system for
    communication with cavers. Maybe that's what a "mole phone" is. But I
    know hardly anything about electronics and have no idea if this would
    work very well through salt or fresh water. I do remember reading that
    the sound quality was good enough for voice but lacking for music. As
    a scuba diver I can attest to how well sound travels through water. It
    seems that an ultrasonic transducer mounted to the inside wall of a
    stainless steel enclosure would work well. I'm sure false triggering
    could be avoided the same way IR remotes work. The signal is modulated
    at a certain frequency so the reciever pays attention to that. Maybe
    the transducer could just be pulsed on and off at a certain frequency
    to achieve the same thing.
    ERS
     
  8. Rubicon

    Rubicon Guest

    Hello.

    Yes... I thought as much after so many sub movies over the years
    however as they go much deeper than the approx 7m depth intended I
    wasn't completely sure about the penetration of "regular" RF into
    seawater.
    A search before posting here resulted in Navy related documents and
    hi-tech articles Thanks for eliminating that avenue.

    The small bouy with an antenna would be easier for a "rank" amateur
    like me to rig up but it's not what he wants.

    Ultrasound is what I originallly thought of and it looks like the way
    to go. The schematics I've seen so far on the net are ultrasound
    switches and prone to naturally produced activation. Does anyone know
    where to find a modulated ultrasound switch schematic?

    In addition I'm wondering if the transmitter and receiver work well in
    water resistant enclosures? Are there any special housing requirements
    here?

    Thanks for all your help.

    Andrew.




    I had also for some reason thought of a pulsed strobe triggering a
    sensor but that seems a little





    Remove the ZZ from E-Mail address to contact me.
     
  9. Rubicon

    Rubicon Guest

    Sorry, missed this part.
    Remove the ZZ from E-Mail address to contact me.
     
  10. Rubicon

    Rubicon Guest

    Thankyou all for your help.

    Andrew.



    Remove the ZZ from E-Mail address to contact me.
     
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