On Thursday 03 August 2006 21:53, Reza - Asterisk Enthusiast wrote: > Ok... I am inspired with this > 8KM link. Perhaps a laser pointer would > help you to aim in less time? Did you just put the antenna of the router > on the LNB of the disk? Do you have any schematics/docs/journals that you > kept - with perhaps picture etc... of this pet project?
Yes you put a decent biquad or even patch antenna where the LNB goes. Nah we didn't really keep any of it... there are several key points though that make long links less easy than they seem. 1. Fresnel zones. If you're shooting over a distance there are a number of elliptical zones which you have to try and keep clear of signal-reflecting or absorbing material (buildings, trees, etc.). Just because you can see the far side doesn't mean the signal will get there with any kind of usable strength. http://www.zytrax.com/tech/wireless/fresnel.htm for more information. 2. Height. The earth is round, so if you're shooting for anything over about 10km or so you need to get up high to compensate for the curvature of the earth. This starts getting nontrivial when you have to worry about flight paths and zoning restrictions. Most tall buildings know they have a value in their height and lease out their roof space to wireless providers and cell companies. 3. EIRP. In order to go long distances reliably you need power. The FCC and CRTC are pretty close in terms of what they'll allow you to send through the air. Thankfully they let you use big-ass antennas if they're highly directional. Unfortunately highly directional means narrow beam widths which makes aiming less fun. 4. Antenna size. I got good results from regular old offset satellite dishes with various kinds of antennas at their focal points. Long distances require BIG dishes, and they're unsightly. They're also heavy, so when you're putting something big and heavy and perfectly-suited to catch the wind up on a perch, the perch must be very solid (and thus expensive), and safety mechanisms need to be in place to account for accidents such as mounts breaking. The record of 150-some-odd miles at one of the hacker cons was set with big-ass old satellite dishes. and over a ravine if I'm not mistaken... Fresnel zones relatively clear, nice climate, nobody cared about looks because it was temporary. 5. Weather. More air between the endpoints means more water vapor. Haze, rain, fog, snow... all of these things eat into your power budget. That's it off the top of my head. A laser pointer (well the cheap ones anyway) don't have very accurate or clear optics. You can see this by shining it across the house. The beam diameter is a couple of mm on your hand, but already an inch on the far wall. Considering it's a Class I laser, it hasn't got a lot of output power, and with the crappy optics you're not doing a good job of keeping that beam from spreading. Also.. Go to a hockey arena and try to keep the beam steady. Moving even a hair causes the (now probably a very dim foot-wide beam) to shake like crazy. You most certainly instead use GPS coordinates and a lot of careful tweaking. Kind of like aiming a satellite dish. It is fun stuff, and there is a LOT of science and math to be used if you want to get into the nuts and bolts of it all. I consider it a worthy passtime. :-) -A.
