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."

Aluminum Vs. Silicon

[n9fzx]

More proof that a cheap $2 aluminum dish will beat a $200 silicon power amplifier any day -- and it uses the spectrum more efficiently!

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...

1 Watt Max?

[Student_Tech]

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.

Re:physics

[afidel]

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.

DX

[Goody]

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 [arrl.org].

Before you ask, the horizon is still a problem.

[Art Popp]

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....

Re:Very Useful

[mocktor]

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 [adsp.net] style antenna. (cringely article [pbs.org])

Re:Aluminum Vs. Silicon

[afidel]

Dishes with this kind of signal amplification are not cheap, notice the $3,000 price quote. Assume $1,000 for the 2 proxim AP's this leaves over $2,000 for the dishes. Even with free AP's I decided the 25 mile link to my ISP's hq would not be worth it because of the cost of dishes. Fibre may be cheap to rent in some instances but the cost of running a 72 mile aquatic link would have been astronomical compared to the cost of this link. Also this has almost nothing to do with last mile problems specifically because this is a point to point link with a $3,000 price tag. No home user is going to shell out a $3,000 setup fee. Point to multipoint links are feasible with different kinds of antennas and they do partially solve the last mile problem, but Ma Bell has been darn good at beating back competition over the years.

72 miles is not so far for this..

[Gwena]

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.

Re:bahh

[Tenebrious1]

Only if you have a boat...

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

[GigsVT]

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....

Heh, to an unlicensed operator, who is probably violating all kinds of ERP FCC limits? Not a chance.

If anything, the only thing hooking up with hams will do is convince you to get licensed, because they likely won't talk to you much until you do, especially if they think you are going to violate FCC rules and possibly cause QRM.

link to HPWREN web site

[NspktrGdgt]

here's a link to more information via HPWREN's web page: hpwren San Clemente [ucsd.edu]

Re:Isn't this in violation of FCC Part 15.247?

[DustMagnet]

You are allowed more than 6dB gain. You just need to read a little more of the rules you quoted.

(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.

Re:physics

[rspress]

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.

Re:Aluminum Vs. Silicon -- Cheap Antennas

[n9fzx]

Time to haul out my trusty Collins Space Systems Slide Rule Calculator (circa 1974):

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.

Re:physics

[iofire]

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 [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 [ucsd.edu]

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

[Art Popp]

Heh, to an unlicensed operator, who is probably violating all kinds of ERP FCC limits?

It's really two questions.

To an unlicensed operator?

Yes, of course they will help unlicensed people. They were all unlicensed until they became hams, and most of them know it. I've had help on piles of radio projects from those nice folk, and returned the favors when they wanted to interface 'puters to their "rigs."

...who is probably violating all kinds of ERP FCC limits?

Who said anything about violating ERP (Effective Radiated Power) limits for FCC rules. The fellow in the article specifically mentioned abiding by those rules.

As for QRM (abbr. for interference), how much QRM is generated from a 1 watt tight-beam microwave hop.

This issue is quite different from that of CB radio enthusiasts that transmit at 300 times (yes, times) the FCC limits for that band, stomp all over the adjacent ham band with horrible amounts of interference, and then ask the hams for help when they've blown the finals on their tube amps. Yes, that category of CBer is often treated poorly at ham gatherings. And appropriately so.

Amateur radio isn't called called Amateur because they're beginners. It's Amateur because it's "not for profit." These fellows invest inordinate amounts of time and money participating in a community of radio enthusiasts, and if you are trying to stay within the rules and and achieve long distance radio communication there will be no end to the help/advice/parts available from them.

What about the 802.11a stuff they are doing?

[LWATCDR]

the 802.11b link was interesting but I want to know more about there 10mile+ 45mb 802.11a links.

Re:2 foot antenna?

[alienw]

Dude, many cellphones transmit at more than 5 watts. And they're right next to your brain when they're on.