Electronic – High-gain omnidirectional antennas for Wi-Fi mesh

frequencyradiotransmitterwireless

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.

Best Answer

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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