That ref you give looks like a pretty good analysis. Although
I can't attest to the accuracy of any of it, it's got lots of
cool equations. I assume you are referring to the graph on pg 17,
where the no-problem-with-rain statement is made...That particular
page is a really fabulous example of what I call the PowerPoint
Information Removal Feature, which is installed by default.
Aside from not labeling the axes, the traces are annotated
in urgo-finnish or something. Perhaps there were speaker's notes
attached (which, as part of another PIRF default, you just can't
print without reading the help file) but as it stands I don't
see any proof of anything, other than, of course, the PIRF default
usage, on that slide...
At least your observation agrees with my RoT. And being a RoT I
don't have to provide concentration/absorption curves...although
I suppose I could do it with PowerPlonk...heh
MS
Aleksandr N. Sadkov wrote:
In theory Rain doesn't affect on Path Loss at 2.4GHz, for example
(http://www.radionet.com/_FileRoot/318040.pdf)
But I have measured great drop in RSSI during the rain, I guess it's due to
antenna effects.
Regards, Sadkov Aleksandr.
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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