A few rules of thumb...
The lower the frequency, the more likely the signal will "curve"
or diffract around obstacles.
The bigger and more conductive the obstruction, the more it will absorb.
Microwaves at 2.4 GHz are used to heat food because water absorbs
at that frequency, thus 802.11b,g and CC2420's don't work so well
when it's raining below sea level...
MS
David Gay wrote:
On 2/20/07, Philip Levis <[EMAIL PROTECTED]> wrote:
On Feb 20, 2007, at 10:42 AM, Jacob Sorber wrote:
> You are asking for a simple answer where none exists. Wireless
> channels are tricky. While line-of-sight is straightforward, "not
> in line of sight" could mean a lot of things. What is obstructing
> the signal? A building? A person? A hill of dirt and rock? Is
> it raining? What is the humidity? I have yet to find a radio that
> is immune to obstructions. If you find one, I would love to see
> it. My experience has been that the 2.5Ghz radios are usually more
> robust to obstructions than the CC1000 radios on the Mica2/Mica2Dot
> motes, but not always. When one mote is a meter under water the
> CC1000 seems to do much better than the CC2420, though neither work
> very well in that situation. Also a rain storm can reduce the
> range some. The best thing to do is to program two motes and take
> them outside and see. It will only take a few minutes. If you
> need a more technical explanation for why node A can't hear node B,
> then you need to do some reading in the wireless comm literature.
> This might be a good place to start ( www.eecs.berkeley.edu/~dtse/
> cu_day1.ppt).
>
My understanding is that the waves which are really resistant to
obstructions are extremely low frequency (ELF) ones: think < 100Hz.
That's what submarines use to communicate when submerged, for example
[1].
Lower frequencies propagate better through water (which is why a
CC1000 at 400 or 900MHz is better than a CC2420 at 2.4GHz). And
propagation through fresh water is significantly better than through
salt water (if I remember the numbers correctly, salt water ==
essentially no range at the frequencies under discussion, while fresh
water might get a few meters).
Lots more detail on all this is available if you look up electrical
engineering source materials rather than sensor network ones... (i.e.,
tinyos-help is probably not the best place to find out about this, try
finding a friendly professor with RF knowledge).
David Gay
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