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

    RF signals and water


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

        • 1. Re: RF signals and water
          • 2. Re: RF signals and water



            thanks. I appreciate it.



            • 3. Re: RF signals and water
              Scott Morris - CCDE/4xCCIE/2xJNCIE

              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!



              • 4. Re: RF signals and water
                CCBOOTCAMP - CCIE

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


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


                • 5. Re: RF signals and water

                  No, just trying to have a better understanding of the limitation of RF due to environmental factors.

                  • 6. 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.

                    • 7. Re: RF signals and water



                      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.




                      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.



                      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.









                      Forgot to mention this.
                      You mentioned Retract, i am assuming that was a typo and you meant 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.
                      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.
                      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.
                      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.


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

                      • 8. Re: RF signals and water



                        That is some great stuff!

                        • 9. Re: RF signals and water



                          Thanks for the explanation and no it is not a bore on the other hand i've enjoyed reading it.