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."

Comments

physics

[Anonymous Coward]

so how did they overcome the earth horizon limitations?

Re:physics

[LordHunter317]

Earth horizon limitation would be more like 300 miles. I know people who've hit that problem talking at 24GHz.

Beat that!

Re:physics

[Anonymous Coward]

Earth horizon limitation would be more like 300 miles.

I thought at sea level it was more like 20 miles.

Re:physics

[Allnighterking]

OK in a sense they don't have to. First, 72 miles isn't that great a curve to deal with and second once you get into this frequecy range there are a number of Technics available for "bouncing" signal off of the Troposphere for really long shots. (100's even 1000's of miles) However for this one this technic isn't required.

Try this.. Draw a circle on a piece of paper. Then Draw two lines out from the center of the circle at right angles extend those two lines through the circle and beyond. You will notice that sooner or later it will be possible to draw a line from the top of those two original lines that will no longer intersect the circle itself. This is the same reason you see two things One very tall MicroWave (2.4 gig is in the MicroWave band) with dishes that look like the are pointed down. (The picture you drew will show you the angles. ) Of course there are limits, like how high you can get the antenna (Mountains help) and note that the longer the shot the larger the antenna should be (concentration of more signal) but you should be able to recieve cleanly down to -90dbm0 no problem (or even lower).

Factors that will affect the signal are. Atmospheric conditions (two antennna's swaying in the wind is the simplest example) Sun Spots buildings or trees in the line of site, and frequency. 2.4ghz can shoot further than say 7ghz can just as low frequency radio goes through the earth rather than around it. In general the rule of thumb is any time the thickness of a material exceeds the length of a single cycle the radio wave is blocked. (yes this is true of all wave transmissions and yes some materials can be made that are transparent, but I did say "in general").

Doing a 72 mile shot over water isn't really that remarkable. At 7ghz I've seen 50+ shots over water. (Despite the statement at the end of the article shots over water are IMPROVED not inhibited by the water in the microwave range. )
If anything the neat part or even unique part is that they did it at such a low cost.

The real question is...

[drachenfyre]

how would you warchalk this link? I mean, besides the technical difficulties of writing on water. Do you put a really REALLY large ear on the ground near the antenna? In all seriousness, this is quite an impressive feat, especially using a highly crowded spectrum. I am curious though how it handles atmospheric interference. Over 72 miles the beam width on this thing must be miniscule and I would think any atmospheric disturbance would send the link crashing. Then again, I've never passed 802.11b traffic more than 2 or 3 miles.

Isn't this in violation of FCC Part 15.247?

[GMontag]

(b) The maximum peak output power of the transmitter shall not exceed 1 Watt. If transmitting antennas of directional gain greater than 6 dBi are used, the power shall be reduced by the amount in dB that the directional gain of the antenna exceeds 6 dBi.

I am just guessing at what they mean in the article by "high-gain". They say they are using a 1 watt bi-directional amp. My personal definition of high gain is a lot higher than 6dBi.

Am I misinterpreting this?

Re:Aluminum Vs. Silicon

[RollingThunder]

$2000 for dishes and the towers they're mounted on.

That's dead cheap for a large tower, especially if you have to pay a company to build it for you.

Check out Mt Washington NH @ 27 Miles

[Anonymous Coward]

We have had a system running for more then 3 years from the summit of Mt Washington, to the North Conway valley in NH. No worries about freznel zone, as the 6000+ foot mountain works as a fine tower.

Somewhat unique in this case though is that we have winds that exceed 75 MPH more then 30% of the time. Try figuring out how to build an antenna and mounting system that remains accurate and doesn't blow away into Maine under those conditions!

There are several pics in the photo gallery of the summit dish. Here are several:

• picture 1
• picture 2
• picture 3
• virtual tour

You may also find the weather stats of this location interesting.

Re:Before you ask, the horizon is still a problem.

[GigsVT]

Clarification:

6dbi is the limitation before you must start reducing power. Above that you must reduce power on a scale proportional to gain.