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Radio wave propagation in water

  1. Dec 13, 2014 #1
    I'm trying to get my head around radio wave propagation in conductive media such as water, saturated rock, etc. What I know is that propagation is poor but better at lower frequencies and this has something to do with "skin depth". I have read what I can on skin depth but I struggle to relate it to a bulk medium such as a body of water.

    Is there a simple explanation of why conductive media are poor transmitters of radio energy? After all we use conductors to carry RF. Is it because a radio essentially detects the signal difference between two points (antenna and ground for example) and in a body of water the signal just bleeds away to ground?
     
  2. jcsd
  3. Dec 13, 2014 #2

    Drakkith

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    Staff: Mentor

    Hmm. It looks to me like the conductive medium acts like one big antenna, absorbing the energy of the EM wave as it travels and dissipating it as heat.

    Conductors are used to carry the signal in the equipment which generate and receive the signal, but these are specialized conductors constructed specifically to carry an RF frequency signal. Regular wires would lose too much of the signal due to several different effects. The higher the frequency, the higher the losses. See page 7 here: http://www.navymars.org/national/training/nmo_courses/NMO1/module10/14182_ch3.pdf
     
    Last edited: Dec 13, 2014
  4. Dec 14, 2014 #3
    Thanks, I think the absorption of energy is what I was missing
     
  5. Dec 14, 2014 #4

    sophiecentaur

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    Aamof, the conductor doesn't directly carry much of the energy (ideally , none). The energy that flows along a radio frequency transmission line is in the form of EM waves with the fields around and between the conductors. Current flows on the surfaces (ignoring skin depth) but the voltages on the surface are near zero (v low resistance) which keeps the EM wave travelling and localises it near the line. For example, the fields are in the spaces between the inner conductor and the screen of a co-ax cable or in between or right next to the two conductors in a twisted pair used in Ethernet connections or on the inside surfaces of a waveguide. As you have spotted, any resistance in the conductors constitutes a loss mechanism.
    The easiest materials to study are highly conductive metals, where the energy is reflected off the surface or good insulators which will be transparent to EM waves and mostly just affect the speed of the em wave passing through (refractive index and all that - remember?) along with a bit of loss on the way through.

    Water and other 'slightly conductive' substances are the worst cases to study because they have a Complex Refractive Index, slowing down waves and absorbing them at the same time. Submarines can communicate from below the surface only by using extremely low frequency signals (tens of Hz, at times), for which the skin depth allow some penetration to depths that are 'safe' for the submarine. To communicate as such low frequencies requires incredibly long wire transmitting and receiving antennas (hundreds of metres in length).
     
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