Building Wireless Community Networks, Second Edition

Point-to-Point Links

From a radio perspective, point-to-point links are very straightforward to set up. You should always follow more or less the same steps when evaluating the possibility of a link:

• Establish that you have line of sight from end to end.

• Measure the distance between the points and calculate the path loss.

• Add in the capabilities of your equipment to determine your link budget.

• Go out and hook up your gear.

If you intend to make a long-distance point-to-point link, first find out the latitude, longitude, and altitude of each end point. You can find this by physically going to each site and marking the coordinates with a GPS, or you can estimate using topographical maps or software (see Chapter 6 for some examples of how to do this). With the coordinates and altitude of both sites, you can calculate a bearing and tilt angle, so you know roughly where to point the antennas on each end. A decent GPS can help here by giving you a bearing to and from each point. You should also check out the online wireless design CGIs at http://www.qsl.net/n9zia/wirelesspage09.html for help with many of the calculations you’ll need to perform.

Obviously, if you can see the other point through binoculars or a telescope, this is a good first step. Ideally, there should be very little on the ground between the two points. The closer the path is to an actual valley, the better. Take a look at Chapter 6 for details about how to calculate the path loss and link budget for your link. I’ve mentioned ...