Wifi Tower

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Hey guys, I don' t have a design ready to post yet, but my question
is.. How high should my tower be for my vertical sector wifi antenna?
I'm currently still undecided on which antenna to get, but my radio is
gonna be a 400mw mini-pci nic and the antenna is going to be a 17-24db
vertical sector antenna. I'm looking to cover distances of 4-15miles.


Re: Wifi Tower



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Yawn.  Be prepared to hear 4-15 miles of interference.  The higher the
antenna, the more junk it hears.  To the best of my knowledge, nobody
makes a 17-14dBi vertical sector antenna (unless your sector is about
5 degrees wide).

Instead of just handing you a number with such a vague description,
I'll just describe what *YOU* get to calculate.

1.  Figure out your maximum service range.  Over 10 miles is a problem
with most 802.11b links due to timing problems.  You may need hardware
that has the timing tweaked to handle long ranges.

2.  Figure out how much power you are going to run at both ends of the
link.  Notice that I said BOTH ends.  It makes no sense to have an
high power central access point, that everyone can hear, but a lower
powered client radio that the central access point cannot hear.  In
other words, the tx power and rx sensitivity at both ends should be
roughly the symmetrical.

3.  Figure on a 19dBi or 24dBi dish antenna at the client.  You can't
do it properly with less antenna gain.  Run the link numbers with the
example at:
http://en.wikibooks.org/wiki/FAQ_for_alt.internet.wireless/Wi-Fi#Link_Calculations
Aim for a fade margin of no less than 20dB or your support people will
be getting phone calls in the middle of the night from irate
customers.

4.  If your link calculations are acceptable, then work on the
topography issues.  You'll need line of sight *PLUS* the Fresnel Zone
clearance.  At 17 miles, the Fresnel Zone at midpoint is about 65
radius.  
  http://www.terabeam.com/support/calculations/fresnel-zone.php
  http://www.connect802.com/antenna_c_main.php
If there are any buildings, trees, or mountains within the Fresnel
Zone, you will have signal loss possibly knife edge diffraction
problems.  That also means that if the client end of the link is at
ground level, to have 65ft of clearance at the other end means that
the tower has to be at least 130ft off the ground.

However, that assumes that the earth is flat.  At 17 miles, that's not
the case.  At 17 miles, your towers will need to be at least 30ft high
to avoid the earths curvature or 60ft high if you assume that the
client radio is at ground level.
  http://www.connect802.com/height.htm
The best way to do this is to use a Topographic mapping program to
check the link.  I use Topo! by National Geographic when I'm in a
hurry, and Radio-Mobile with SRTM maps when I want accurate numbers.
  http://www.cplus.org/rmw/english1.html

5.  If your clearance calculations look good, you can start worrying
about coverage area and downtilt angle.  If you just aim your science
fiction 17-24dBi antenna at the horizon to maximize the range, the
narrow antenna vertical beamwidth will ruin coverage close to the
central antenna tower.
  http://www.terabeam.com/support/calculations/downtilt-cover.php
This coverage calculation will largely determine the type and number
of antennas required.

Good luck.

--
Jeff Liebermann     jeffl@comix.santa-cruz.ca.us
150 Felker St #D    http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann     AE6KS    831-336-2558

Re: Wifi Tower



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Oops, I lied (and goofed).

There are sector antennas up to 20dBi gain:
  http://www.hyperlinktech.com/web/antennas_2400_out_sector.php
Vertical beamwidth of 5 degrees at 20dBi gain.  Yech.

My favorite vendor, Superpass, only has panels up to about 15dBi gain
unless you want to go to 45 or 60 degree sectors:
  http://www.superpass.com/2400-2483M.html

I couldn't find one with 24dBi gain.  As gain increases 3dB with
doubleing the size of the antenna, a 24dBi panel will probably be
truely huge.
--
Jeff Liebermann     jeffl@comix.santa-cruz.ca.us
150 Felker St #D    http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann     AE6KS    831-336-2558

Re: Wifi Tower


Wow, Alot of info there :| Gonna take me a while to digest this. Thanks
soo much for the time and effort in explaining. I will be back with
more questions as soon as this info digest :D


Re: Wifi Tower


Jeff,

I know who to hire when I need a wireless guy :)

Good stuff.


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Hi Jeff,


Til-tek also makes 14-17dbi vertical sectors with 120 and 160 degree
patterns

I've used them in the valley,  works fairly well,, but i've seen the
same siginal and SNR at the center as well as at 180 degrees to the
antenna on a 120.

sooo,,,



Bob
robert smith consulting

Re: Wifi Tower


Here's my first draft. -> http://tekmanx.serveftp.com/~tekmanx/pole.gif


Re: Wifi Tower



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Calculate your wind load.  It looks way to thin to be self supporting
in even a light wind.

--
Jeff Liebermann     jeffl@comix.santa-cruz.ca.us
150 Felker St #D    http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann     AE6KS    831-336-2558

Re: Wifi Tower



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One more detail.  Your prospective 17-20dBi panel antenna has a
vertical beamwidth of 5 degrees.  That's rather narrow.  If your pole
leans a few degrees forward or backwards, your antenna could easily
end up pointing into the sky, or into the dirt.  High gain antennas
required a VERY stable mounting.

Tower manufacturers and distributors:
  http://www.valmont.com
  http://www.radiancorp.com/ROHNNET/rohnnet2004/html2004/index.html
  http://www.glenmartin.com
  http://www.texastowers.com/online.htm
  http://www.ustower.com
  http://www.alumatower.com
  http://force12inc.com
  http://www.alliedtower.com/product.htm

--
Jeff Liebermann     jeffl@comix.santa-cruz.ca.us
150 Felker St #D    http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann     AE6KS    831-336-2558

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