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.
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.
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.
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.
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:
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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.
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.
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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.
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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.
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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.
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coverage calculation will largely determine the type and number of antennas required.
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
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.
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.
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