Slashdot Mirror
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."
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.
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....
(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.
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)