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10724 Views 9 Replies Latest reply: Sep 19, 2009 6:49 AM by nsomarriba RSS

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RF signals and water

Sep 17, 2009 2:29 PM

nsomarriba 41 posts since
Feb 6, 2009

what happens with the RF signal when it hits water? does it reflect, retract, absorb...

  • Conwyn 7,907 posts since
    Sep 10, 2008
    Currently Being Moderated
    1. Sep 17, 2009 11:04 PM (in response to nsomarriba)
    Re: RF signals and water
  • Scott Morris - CCDE/4xCCIE/2xJNCIE 8,398 posts since
    Oct 7, 2008
    Currently Being Moderated
    3. Sep 18, 2009 10:48 AM (in response to nsomarriba)
    Re: RF signals and water

    Too many variables... And what-ifs.

     

    Bottom line as far as we are concerned with the wireless frequencies we use, it's unusable through water.

     

    The navy does some things with REALLY low frequency stuff (huge wavelength) through the oceans, but it's not perfect in any sense. 

     

    Within the link that Conwyn gave, there's another link that talks more in detail on this which was fairly interesting.

     

    On a perfectly smooth water surface, you can actually bounce signals.  but otherwise in choppy water it's akin to killing it.  Nothing I would plan a design around!

     

    Scott

  • CCBOOTCAMP - CCIE 228 posts since
    Sep 11, 2008
    Currently Being Moderated
    4. Sep 29, 2009 12:28 PM (in response to nsomarriba)
    Re: RF signals and water

    What are you trying to do? Microwave some goldfish at home? Pickup wifi at the bottom of your pool?

     

    thanks,
    Brad Ellis
    CCIE#5796 (R&S / Security) CCSI# 30482
    CCBOOTCAMP

    http://www.ccbootcamp.com

  • Jared 5,502 posts since
    Jul 27, 2008
    Currently Being Moderated
    6. Sep 18, 2009 5:54 PM (in response to nsomarriba)
    Re: RF signals and water

    I'd have to agree with Conwyn and Scott.  The density factors of water definately affect wifi.  After all, wifi was designed to use the air as your transport medium not water.

     

    Even a good rain storm affects wireless transmissions.  Take a pair of bridges and watch the perfomace during a good rainstorm.

  • John 2,289 posts since
    Jan 17, 2009
    Currently Being Moderated
    7. Sep 18, 2009 8:27 PM (in response to nsomarriba)
    Re: RF signals and water

    Hi

     

    That is a very good question and when designing close to water I.E. the sea, large ponds, lakes etc you have to take it into consideration for a good design.

     

    How it affects it all depends where the water is, and also depends on the frequency e.g.

     

     

    Rain

    A Microwave link of 38 Ghz frequency point to point is affected far more than a frequency of 2.4 Ghz, 7 Ghz point to point etc because the wavelength is

    very short on 38 Ghz. The higher the number the more it is affected by water/rain I.E. the data in the airwaves get hit/damaged which will eventually cause links to drop out.

    Note: I am trying to be basic here so it is easy to understand because i don't know your knowledge on Microwave.

     

    All countries have a design/number to work to depending on what part of the world they live in, England has 2 numbers. These numbers just represent what the rain rate is and you design the network to that I.E. a fade margin is designed in. That link however can only take so much rain and if the rain comes

    down heavier than the normal that link will drop out. You could design it better but that costs money.

     

     

    Large Ponds/Lakes, the Sea

    Think of these as a mirror which reflect, and that is just what a frequency will do when it hits large amounts of water like the above. If the signals reach the receiver 180 degrees out of sync then the whole thing is cancelled out and your link goes up and down.

     

    As the signal hits the water it reflects off (answering your question) in unpredictable directions. Apart from destroying your link this will also reflect in all directions and into other radio waves of the same frequency causing more destruction to your network.

     

    Car Parks

    What has car parks got to do with water ?

    This will have the same affect on the frequency as water will e.g

     

    There was a network in England which had a microwave dropping in and out between 10am and 6pm on a Saturday. After further investigation they

    discovered that a new super market had been built between the microwave link and between the hours of 10 and 6 the car park was full. When it was full of cars the frequency was bouncing off the cars and reflecting in all directions.

     

    Submarines

    As Scott mentioned submarines use a very low frequency and transmit huge distances. That is because they use water to their advantage by using a

    very low frequency which will cut through the water.

     

    The lower the frequency the greater the distance but power also makes a difference as well.

    You look at the maximum distance of 802.11 b and then 802.11 a.  802.11b goes further because the frequency is lower.

     

     

    Hope i never bored you too much with the details.

     

     

     

     

    Regards

     

    John

     

    Forgot to mention this.
    You mentioned Retract, i am assuming that was a typo and you meant refraction.
    Refraction
    A change in the speed of the wave, most common example is a wave entering water. So when a radio wave hits large amounts of water it will reflect and refract, the latter is what will happen to the submarine waves.
    Diffraction
    An example of this is a sector/AP designed to point down and cover a busy shopping square. However, the sector/AP is too far back on the roof so some of the signal
    hits the edge of the roof and gets sprayed away from its intended area.
    Absorption
    I have been in the desert a few times and trying to provide communications in the desert is very difficult because the sand absorbs the radio waves.
    Water, although an enemy most of the time to our communications; can be a very good friend in places like the desert.
    Reflection
    large amounts of water, rain for the high frequencies, large smooth surfaces, windows and especially those windows designed to keep the sun off. Hard surfaces as well but they absorb as well as reflect.
    John

     

    Message was edited by: John Forgot to add this part at the bottom.

  • Jared 5,502 posts since
    Jul 27, 2008
    Currently Being Moderated
    8. Sep 18, 2009 7:28 PM (in response to John)
    Re: RF signals and water

    John,

     

    That is some great stuff!

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