[SLL] Looking for around the block wireless (medium range?)

[Robert Woodcock]

On Fri, Nov 02, 2007 at 11:43:03AM -0700, Xeno Campanoli wrote:
> I've got at least two neighbors now who are out of range from my little
> netgear transceiver, and whom I could get on Linux if I could just get
> them a network connection.  M is about 8 blocks away on the other side
> of the Catholic church, and B is about 6 blocks down the hill.  Does
> anyone know of a wireless transceiver that will work with that distance
> and does Linux?

Since the FCC limits the power on 802.11 transmitters, this is all about the
antennas.

Assuming your wireless access points and/or cards have external antenna
connectors, you can find stuff to plug in to them at places like netgate.com
or hyperlinktech.com, and there's also some selection locally at Vetco
Surplus and Fry's. There's also a lot of stuff you can make on your own -
check out the AntennaHowto at www.seattlewireless.net.

Directional antennas work by taking all of the radio energy that would
normally be transmitted in all directions and focusing it in one direction
(compare a flashlight bulb by itself vs. a flashlight bulb + reflector).
There's a couple measures of how focused an antenna is - the most quoted one
is gain, which is a logarithmic scale. Every 3 decibels of gain is a
doubling of power. 10 decibels of gain is a tenfold increase in power. This
power boost occurs both in transmit and receive (if you shine a powerful
light directly into a flashlight, it'll be focused where the bulb is). A 2dB
gain antenna is fairly omnidirectional. A 10dB gain antenna is good for
covering a room from a single corner. A 20dB gain antenna is like a
flashlight - you'll need to take a little care in aligning it to the target.

Now, even if you substitute 24dB dishes for your 2dB dipoles, increasing the
radio power received by a factor of 26000 (1.26^44), it may still not be
enough to go through obstacles (radio waves at wireless ethernet frequencies
don't go around curves much at all - it's very directional, very
line-of-sight). Anything that warms up when you put it in the microwave will
absorb some energy, and this is referred to as attenuation. I estimate a
single cinder block wall will attenuate 12-15dB of signal (meaning only 3 to
6% of the signal gets through). If you're going through trees, well, don't
get the link going in winter and necessarily expect it to work when all the
leaves come back. If you're going through the crest of a hill, then forget
it.

If you can't get close enough to line-of-sight, then you'll need to go to a
lower frequency. I know there's some proprietary stuff for 900MHz.
-- 
Robert Woodcock - rcw at blarg.net
"I am the object of criticism around the world. But I think that since I am
being discussed, then I am on the right track."
	-- Kim Jong Il, "Orient Express" by Konstantin Pulikovsky