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[wxsig] Wind application

Daron J. Wilson daron at wilson.org
Thu Mar 30 19:35:52 UTC 2006


> I work in this industry.  We see these claims all the time.
> 
> Just use 802.11b as a benchmark....how far does that go?  I've seen
> industry claims of up to ...yes,
>  they are smoking something...20 miles.
> 
> You might be best using something at 900 MHz that is frequency hopping
> instead of direct sequence
> modulation.

More important than the claimed range is your consideration of basic RF
system design including antennas, path and propagation.  We have a 2.4Ghz
802.11b link running 18.5 miles, point to point, 24dbi reflectors on each
end, it's been running for a couple years now:

http://209.237.77.10

With a signal strength on the bridges of 70-80% I'm quite sure I could
squeak another mile or so out of it.  Keep in mind, there are complex timing
issues in the protocol that will cause reduced throughput from retries and
collisions, but it can work.

First, determine if you have line of sight between the two locations.  If
you do, 2.4Ghz should do well.  If you don't have an actual line of sight,
how close to line of sight is it?  If you are talking about trees or brush
slightly in the way, 900 Mhz should punch through there, if it's more, you
may end up dropping down to 450 to get around it.

Second, design the system with maximum antenna gain so you can put keep the
transmitter power at the remote site as low as possible (less RF out, less
drain on the battery/solar panel).

Third, the frequency that you need to measurements from the instrumentation
is important, if you intend to have a record or two per second, then the
spread spectrum stuff that transmits all the time is fine.  If you are
interested in a record every 5 or 10 seconds, something more conventional
that comes up and transmits a burst of data might be more efficient (low
power FM radio, Kantronics KPC3+ for example).

Good Luck!

Daron N7HQR

www.ocrg.org






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