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Is 2.4 GHz affected by rain more than 5.6 GHz?

Is 2.4 GHz affected by rain more than 5.6 GHz?
There is a common misconception amongst the wireless community that goes a little something like this…

  1. Humans use microwaves to cook food.
  2. Microwaves emit electro-magnetic radiation in the 2.4 GHz spectrum
  3. Microwaves work by exciting the water molecules.
  4. The excitation of the water molecules is a result of the transfer of energy from the microwave to the water molecules in the food.
  5. Hence the 2.4 GHz electro-magnetic (RF) radiation must be absorbed by the water in order to for the process to work.

Now, if we are to use this proposition as the basis for an analysis of the affect of water on IEEE 802.11b (or any other RF technology that works in the 2.4 Ghz spectrum) then what we'd expect to see would be a lot of useless wireless technology around during periods of rain – but we don't. The above proposition would also mean that if you put a wireless access point into a roof full of people then the signal would basically disappear as it was absorbed by the bags of water walking around the room (which is essentially what we are, large mobile bags of water). However in practice you find some signal degradation but no-where near the magnitude of what you would expect if the peak absorption frequency of water was around the 2.4 GHz mark.

Can you guess what I'm going to tell you next? All those 'wireless experts' that go on about the effect of water on 2.4 GHz technology are wrong. The peak absorption frequency is actually 22.2 GHz, no-where near the 2.4 GHz or 5 GHz spectrums that are currently in use by common WLAN equipment today. For more information I suggest reading the NASA Reference Publication 1108(02) entitled "Propagation effects on Satellite Systems at Frequencies Below 10 GHz."

In answer to your question, the effect of water (or more specifically, rain) is probably actually less on 2.4 GHz equipment than on 5 GHz equipment but in my experience I have found it to be negligible in both cases.

For those of you who are interested here's something very handy to know for the next time some wireless "expert" tells you that "like in a microwave, water absorbs RF" - A microwave works by exploiting the unusually strong dipolar property of water. Although water is electrically neutral, the dipolar state of water allows molecules to behave as if they were composed of small negative and positive charges on either end of a rod. The microwave produces alternative electro-magnetic field, effectively pushing, pulling, rotating and twisting the water molecules and this adding kinetic energy. This addition of the energy to the water molecules comes from the addition of kinetic energy into the molecule itself, not through an increase in the molecules (or any sub-components) excitation state as a result of RF absorbtion.

This was first published in July 2003

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