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Omni Directional

May 18th, 2009

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How do you find the range of a wi-fi antanna?

I am trying to set up a few hot spots and am trying to figure how much gain I need. What is the difference between 8dBi, 12dBi and 15 dBi and which is better?

I am trying to cover two or three city blocks with low trees coverage. The antenna will be on a roof and will have a clear line of sight. I want to use omni directionals and want to examine the type of coverage areas I can get.

Also I will be using a wireless-g router.

Thanks

That is good if your omni is centrally located and has a clear line of sight, that's half the battle. Do a site survey anyway for a few reasons. First, you'll know what other WLANs might be running in the area and you'll then be able to choose the best channel, 1, 6 or 11 for 802.11b/g. Second, a clear visual line of sight is not always necessarily a clear RF line of sight... because of multipath interference, dead zones due to reflection, diffraction, 2.4GHz cordless phones, etc.

As for the antenna gain, every 3 dBi increase is twice the power... so obviously the 15dBi antenna is the strongest, but be aware of something... the higher the gain, the more the RF energy is flattened out, so the signal will be shaped less like a donut and more like a pancake. If your high gain antenna is mounted up high like a rooftop, the best part of the signal might miss your clients down on the ground because the strongest part of the signal might be too far above the ground. For this reason, some antennas have a built-in electronic downtilt. Check the specs and buy antennas that fit your installation. Also for this reason, you just might get better results with an 8dBi versus 15dBi.

I have explained in some of my other answers that this is the same reason why some home users put high gain antennas on their Linksys or Netgear in the basement and wonder why their signal actually gets WORSE up in the 2nd floor bedrooms.

If you're outdoors, then every 6dBi increase will double your distance, as a rule of thumb. Best bet is to get a laptop and download freeware NetStumbler for example, and mount your antenna and then walk all around the property to do your own basic site survey.

I don't know where you are in the world, but also be aware that your signal might look great now (in the fall/winter) but then your signal might stink in the spring/summer, because of all the leaves on the trees. So keep in mind seasonal differences like this.

Definitely use an access point that has antenna diversity (2 antennas) for the best results. Have your pair of high gain omnis mounted at least a few inches apart (at least 1 wavelength, about 5 inches) and a little farther apart is even better, like a couple feet, in many circumstances.

Finally, I'm going to reiterate that for what you described, I'll recommend 8dBi and not 12 or 15. The 8 dBi should have an RF radiation pattern with about 50 degrees elevation. Once you get into 12 and 15dBi, it's more like 10 degrees or less, so placing it on a roof means you'll shoot the signal OVER the whole area, say, 30 feet above the ground, missing all your clients who are at ground level.

One last thing, look into using lightning arrestors since you're talking about an outdoor/rooftop installation. The lightning arrestor installs between the antenna and the access point, along the antenna cable, that is, inline with the antenna. You need to have a good electrical earth ground wire attached to the arrestor. It's normally an open circuit. When a nearby lightning strike creates a spike on the antenna/wire, it instantly changes to a short circuit and directs the spike down the ground wire. Note I said "nearby" because pretty much NOTHING will protect your equipment from a direct lightning hit. Bottom line here, even if the building has a lightning rod, you should still use lightning arrestors.

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