On 6 Jul 2004 21:47:30 -0700,
(E-Mail Removed) (BA) wrote:
>I am thinking about setting up a wifi hotspot in my neighborhood. I
>have been doing some research but I can't find anything about range
>using a 12-15 decible omnidirectional outdoor antenna.
Too much gain for a hot spot. You'll have fabulous coverage of the
horizon and no coverage directly below the antenna. Let's do the
math:
http://www.netnimble.com/antennas.html
The -3dB beamwidth of a 12dBi antenna is about 6 degrees. If you
mount that antenna 100ft above the ground, the point where the -3dB
(half power) hits the ground is:
100ft / tan (3 degrees) = 1900 ft.
So, you'll have decent coverage from between 1900ft and the horizon,
and your coverage will get progressively worse between 1900ft and the
antenna base with lots of peaks and nulls.
http://www.netnimble.com/gp24hplot.gif
It's actually worse than that because high gain omni antenna tend to
have some uptilt to the antenna pattern if mounted over a reflective
ground (such as a metal horizontal tower strut or yardarm). Without a
clue as to topography, I can't recommend anything specific.
As for the ultimate range, that is fairly easy to guess(tm)
er...calculate. More math.
The basic sensitivity of the receivers are measured at 1*10^-5 BER
(bit error rate). This is a fairly good error rate and sets an
acceptable minimum. Using 802.11b, at 2Mbits/sec (which will give you
about half that in thruput), the typical receiver has about -85dBm
sensitivity. Assuming a WET11 or similar wireless client side bridge
radio, the internal patch antenna has about -1dBi gain (er...loss) and
is inflicted with about -2dB of internal connector and cable loss.
Adjust these numbers depending upon your confidence in the
manufacturers spec sheets.
Going to the other side of the puzzle, I'll assume that you're going
to plant your mythical access point on the roof, near the antenna, in
a Tupperware box, and are NOT going to use an overpowered and
overpriced amplifier. FCC part 15.247 limit you to +36dBm radiation.
You'll get about +15dBm out of the transmitter and loose about -3dB in
assorted connectors and pigtails.
Adding it all up, we have:
TX = TX power + feed loss + antenna gain =
+15dBm + -3dB + 12dBi = +24dBm
RX = Rx sens - feed loss + antenna gain =
-85dBm - -3dB - -1 dBi = -81dBm
Decent reliability requires an absolute minimum of 10dB of fade
margin. I've found 20dB to be a more tolerable minimum target value.
Therefore, the amount of free space loss (FSL), over the distance,
that results in those levels is:
FSL = +24dBm - (-81dBm) = 105dB loss.
For 2.4Ghz, FSL = 104.2dB + 20 log D
Where D = Distance in miles
For 105dB loss, we get a distance of 1.1 miles.
Note that this is at the very peak of the antenna pattern, which will
work when the laptop is at the same elevation at the antenna. Any
vertical difference will need to be adjusted for the antenna pattern
and will result in considerably less range.
There are plenty of online calculators to make the math easier. I
just wanted to explain how it works.
http://www.ydi.com/calculation/som.php
http://my.athenet.net/~multiplx/cgi-...eless.main.cgi
Note that fade margin or operating margin is a key part of the range
picture. 2.4GHz signals are not constant and are affected by
reflections, multipath, and propogation effects. You need some
overhead to insure a reliable path. This also assumes that there are
no obstructions in the RF path. There is a direct relationship
between your reliability and the fade margin.
Fade Margin Reliability
8dB 0.9%
18dB 0.99%
28dB 0.999%
38dB 0.9999%
48dB 0.99999%
Anyway, good luck on your hotspot. I suggest a much lower gain
antenna or if possible, panel antennas to illuminate the area of
interest.
--
Jeff Liebermann
(E-Mail Removed)
150 Felker St #D
http://www.LearnByDestroying.com
Santa Cruz CA 95060 AE6KS 831-336-2558
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