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Hopefully this is the right SE site a question of this nature.

Basically what I want to do is set up a wireless transmitter using a high-gain omni/directional antenna as a test-bed mesh network between my brother and I. The range is well within the 30+ miles I see advertised on antennas for sale. What I don't understand is the frequencies and channels they advertise. 2.4Ghz 802.11g seems to fit within UHF, but I haven't found any clear indication that this is true, and I can't escape feeling like I'm oversimplifying this, or missing something.

PS: I'm happy to read any material supplied if an explanation is too verbose to propose here.

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    \$\begingroup\$ 30 miles for a consumer grade wifi....that doesn't sound right.... Are you talking about WiMax stff? \$\endgroup\$ – rfusca Nov 9 '11 at 22:18
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    \$\begingroup\$ High gain and omni-directional are mutually exclusive by basic physics. Furthermore, 30 miles at legal WiFi power levels simply ain't gonna happen. Then at that wavelength you need line of sight. \$\endgroup\$ – Olin Lathrop Nov 9 '11 at 22:34
  • \$\begingroup\$ @rfusca No, but again this stems from an attempt to inform myself, so I could be way off. See: mcmelectronics.com/product/MS-1000 for an example. \$\endgroup\$ – David Metcalfe Nov 9 '11 at 23:04
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    \$\begingroup\$ @David - That's a TV antenna... Its range is talking about the range you might get from a tv station...who is broadcasting far more powerfully than you'd be able to. \$\endgroup\$ – rfusca Nov 9 '11 at 23:18
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    \$\begingroup\$ UHF covers a very wide range. It's not very useful to classify equipment as 'UHF' when you're concerned about power levels, which are restricted to much smaller bands. Shame on them for advertising that way. The TV antennas operate at several hundred MHz, and are unidirectional broadcasts from a source of several hundred kilowatts of power. You're trying to set up a mesh with transmitters restricted to 1 watt at 2.4 GHz, which won't reflect off the atmosphere and will be attenuated much more rapidly by matter (air, trees, hills) between antennas. Does that help clear things up? \$\endgroup\$ – Kevin Vermeer Nov 10 '11 at 0:31
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There's a tradeoff between antenna size (on both ends), transmit power, and data rate.

Many TV broadcast stations transmit relatively high data rates at medium distances to relatively small receive antennas by spending a lot of money at the transmit end: well over a 1,000,000 watt of power on a huge TV transmitter tower. Perhaps the antenna you mentioned can receive a TV broadcast from such a tower 30 miles away. But it is highly unlikely that the same antenna you mentioned can receive a WiFi broadcast at the maximum allowed WiFi power (1 watt) at the same distance, even if you happen to have a huge TV transmitter tower available.

There seem to be 2 directions you can go with this:

  • You have a fixed distance you must directly communicate over, how to accomplish this?
  • You want to experiment with Wi-Fi mesh; what are good ways to get a little more range, and what range is reasonable?

fixed distance

There seem to be several technologies that Real Soon Now can communicate 30 miles or more using unlicensed equipment (at slower data rates than WiFi):

WiMAX IEEE 802.16

IEEE 802.22TM ( IEEE 802.22 news )

"Wi-Fi-like technology"

My understanding is that none of them are really available commercially yet -- it is currently not possible to transmit 30 miles with off-the-shelf, unlicensed transmitter.

So if you absolutely must communicate that distance as soon as possible, you need to get some sort of license -- perhaps a ham radio license -- and use the kind of higher-powered transmitters that require a license in order to transmit legally.

Wi-Fi mesh

You could set up a wireless mesh network with a bunch of nodes between your place and your brother's place, so packets hop from one node to the next over the whole distance.

I see some theoretical discussion over at the WSN wiki and some more practical tips at the SeattleWireless wiki.

You might be able to use fewer in-between nodes by taking the advice of websites that discuss tweaking off-the-shelf WiFi devices to squeeze more range out of them, such as "Poor Man's WiFi". Generally these people replace the omnidirectional antenna of standard WiFi devices with highly directional antennas.

Even though the FCC requires people to set a lower transmit power on high-gain antennas (b), higher gain does give you significantly more range. Many of these places report success over line-of-sight distances well over 2 miles (a). 30 miles in one hop seems to be possible, but I suspect breaking it up into several shorter hops will be much easier. In principle, the more directional the antenna, the longer the range. However, highly-directional antennas are difficult to get aligned properly -- and if they aren't aligned properly, you get less signal than you would with a simpler, less directional antenna.

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First you need to understand what is meant by "high-gain" and "omnidirectional".

A true omnidirectional antenna is one which radiates equally well in all directions. Think of the sun, it sends out light equally well in all directions, very little of which actually reaches the earth.

Such a true omnidirectional that radiates equally in all directions is known as an isotropic radiator. Wikipedia has a nice writeup at http://en.wikipedia.org/wiki/Isotropic_radiator

Now here on earth most of the time we are more concerned in sending the signal out horizontally rather that straight up into space or straight down into the earth. Any "gain" antenna achieves it's gain from reducing the power sent in some directions and re-directing it into other directions. It is important to remember that an antenna does not create power, only focus it in certain directions. Think of it like a flashlight which uses a parabolic reflector (just like a satellite dish) to favor some directions over another.

So what you are looking for is not an isotropic antenna but one which radiates well in all horizontal directions. This is achieved by reducing the energy sent out in the vertical dimension. The pattern would look like a pancake parallel to the earth. The flatter the pancake (or radiation pattern), the more gain in the horizontal dimension.

All professional grade antennas made by reputable manufacturers will have technical data sheets showing plots of the radiation patterns.

Of course you may be able to easily construct an antenna on your own if you are handy. There are lots of plans on the internet.

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