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Remote Feed: 72-Mile 802.11b Link
An anonymous reader writes "A 72-mile link was installed last month from San Diego to San Clemente Island, using standard 802.11b WLAN gear and high-gain, 2-foot parabolic antennas. More in this Computerworld article."
Seriously, however, broadcast medium networks like 802.11b are best used for distribution, not long distance point-to-point links (fiber is ultimately cheaper on a bit-for-bit basis), but this demonstrates that you can build a really cheap 802.11b distribtuion network to solve the Last Mile Problem. Another nail in the coffin of Ma Bell...
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I have always been under the impression that for part 15 devices that the power is given in mv/m.
According to http://www.radioinnovation.com/Howto/how_pass.htm the maxiumum power for a part 15 device in the 2.4ghz range is allowed an average power density of 50 mV/m at a range of 3 meters, and is a transmitter power of -3.4 dBm when used with a perfect 1/2 wave dipole. -3.4dBm is, http://www.qsl.net/vk6zse/wattsdbm.htm, between 500-800 microwatts.
Now I realize that they are using parabolic antennas, but are they still meeting that average power density, I suspect that ERP is likely greater than 1 watt when using directional antennas.
They put it on a high tower, for 72 miles the earth bulge is ~90 feet so with fresnel zone allowance you would need an ~210 foot high tower. As to the other complaint about insecurity, at these distances the antenna's required will make an extremely straight beam, in fact the beam width is probably only in the mid single digits which is part of what makes shots this long hard, they are extremely hard to line up. Basically you would need to be on a tower inbetween the two sites and somehow intercept the information without knocking out the signal, not a trivial task. Besides they are sending seismograph data, not Top Secret documents.
There are 4 boxes to use in the defense of liberty: soap, ballot, jury, ammo. Use in that order. Starting now.
Amateur radio operators have been doing stuff like this for years on frequencies above and below 2 Ghz. Here's a listing of distance records .
Tired of being "punished" by the Slashdot $rtbl since 2002. I'm now over at http://soylentnews.org/ .
Which is probably why they shot over water. No trees, shorter towers. It's great to see this stuff getting tested, especially by educators who tend to publish their results. Hams have been enjoying this sort of fun for a long time now, and the basic problems are still in front of you. You have to have line of site (plus some extra height for the Fresnel effect), and you still have to buy and point dishes and since 1 watt WAPs aren't sitting on the shelf, you still have to get a pair of expensive little amplifiers. These things can at least be purchased now, and if you want to set up such a link, attend your local Amateur Radio shindig and you'll find piles of retired microwave enthusiasts, eager for the chance to lend a hand....
5 miles from the nearest excahnge
you shouldn't need 2' parabolic dishes for this - it ought to be possible with a well-aimed pair of yagi style antenna. (cringely article)
1 watt + 2ft dish at 2.4ghz would put a useable signal into a satellite in geosync orbit. 72 miles seems modest as far as range goes... the gain with a good 2ft dish at 2.4ghz is pretty extreme. If it is line-of-sight at 72 miles the signal should be way way above the noise.
Only if you have a boat...
-- If god wanted me to have a sig, he'd have given me a sense of humor.
here's a link to more information via HPWREN's web page: hpwren San Clemente
Frank Dwyer (dwyer@sdsc.edu)
Assistant Director, IT Services
San Diego Supercomputer Center
(i) Systems operating in the 2400-2483.5 MHz band that are used exclusively for fixed, point-to-point operations may employ transmitting antennas with directional gain greater than 6 dBi provided the maximum peak output power of the intentional radiator is reduced by 1 dB for every 3 dB that the directional gain of the antenna exceeds 6 dBi.
Fab-corp sells a 24 dBi parabolic. If my math is right, that allows you 18 dBi of gain.
'SBEMAIL!' is better than a goat!!
Yes at 802.11b frequencies the radio waves do not bounce, the will pretty much head out into space. At these frequencies it is pretty much line of sight, you may get a little bending in the wave but not much. You will need height at the transmit/receive site to overcome the horizion. Southern California does have one seasonal effect on radio waves however. In the summer when a high pressure area sits in the pacific ocean radio links between the west coast and hawaii are possible. This is called Troposhperic Ducting and people in So-Cal may hear radio stations both AM and FM coming in from Maui. The stronger the duct, the higher the frequency that can be passed between two points. I had a 2 meter repeater (Amatuer Radio) that was meant to be low-level, local area only. When ducting was happen my range went from about 20miles to well over 400miles. It would cover the entire Sacramento/ San Joaqiun valley stoping only when you went out of the duct. People 400 miles away sounded as if the were next door.
A perfectly fed 2ft. diamater dish at 2.4 GHz has a gain of 24db, but even a Pringles can feed will give you a dish with 21db of gain. More importanly, the spatial extent of the signal will be minimized, allowing for cellular reuse.
Personally, at 2.4 GHz I'd go with a loop Yagi-Uda array instead, as they're still cheap, and much more wind resistant, something you care about when pointing matters. Make enough of them and they'll be as cheap or cheaper than the Yagis used for UHF TV reception.
Bottom line: There's nothing inherently expensive about gain antennas, and they're the cheapest way to improve the link equation.
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I am a student working on the HPWREN project responsible for this link, and you can find out much more information about this link and the wireless network in general at our website:
http://hpwren.ucsd.edu
Also note the November 1st news item that deals specifically this with link, and includes photographs of the setup here:
http://hpwren.ucsd.edu/news/021101.html
--Avoid metagame thinking, browse with scores hidden (This sig is in violation of itself)
the 802.11b link was interesting but I want to know more about there 10mile+ 45mb 802.11a links.
See my blog http://ilovecookes.blogspot.com/ for light hearted technical information.
Dude, many cellphones transmit at more than 5 watts. And they're right next to your brain when they're on.