Aligning Antennas on a Long Distance Link
The key to successfully aligning antennas on a very long distance link is communication. If you change too many variables at once (say, one team starts wiggling an antenna while the other tries to take a signal strength reading), then the process will take all day and will probably end with misaligned antennas.
You will have two teams of people. Ideally, each team should have at least two people: one to take signal readings and communicate with the remote end, the other to manipulate the antenna. Keep these points in mind while working on long distance links.
- Test all equipment ahead of time. You don't want to fiddle with settings once you're in the field. Before separating the equipment, power everything on, connect every antenna and pigtail, and make sure you can establish a connection between the devices. You should be able to return to this known good state by simply powering on the device, without having to log in or change any settings. Now is a good time to agree on antenna polarity (see chapter two if you don't understand what polarity means).
- Bring backup communications gear. While mobile phones are usually good enough for working in cities, mobile reception can be bad or nonexistent in rural areas. Bring a high powered FRS or GMRS radio, or if your teams have amateur radio licenses, use a ham rig. Working at a distance can be very frustrating if you are constantly asking the other team “can you hear me now?” Pick your communication channels and test your radios (including the batteries) before separating.
- Bring a camera. Take some time to document the location of each site, including surrounding landmarks and obstructions. This can be very useful later to determine the feasibility of another link to the location without having to travel there in person. If this is your first trip to the site, log the GPS coordinates and elevation as well.
- Start by estimating the proper bearing and elevation. To begin, both teams should use triangulation (using GPS coordinates or a map) to get a rough idea of the direction to point. Use a compass to roughly align the antenna to the desired bearing. Large landmarks are also useful for pointing. If you can use binoculars to see the other end, all the better. Once you have made your guess, take a signal strength reading. If you are close enough and have made a good guess, you may already have signal.
- If all else fails, build your own landmark. Some kinds of terrain make it difficult to judge the location of the other end of a link. If you are building a link in an area with few landmarks, a self-made landmark such as a kite, balloon, flood light, flare, or even smoke signal might help. You don't necessarily need a GPS to get an idea of where to point your antenna.
- Test signal in both directions, but only one at a time. Once both ends have made their best guess, the end with the lowest gain antenna should make fix their antenna into position. Using a good monitoring tool (such as Kismet, Netstumbler, or a good built-in wireless client), the team with the highest gain antenna should slowly sweep it horizontally while watching the signal meter. Once the best position is found, try altering the elevation of the antenna. After the best possible position is found, lock the antenna firmly into place and signal the other team to begin slowly sweeping around. Repeat this process a couple of times until the best possible position for both antennas is found.
- Don't touch the antenna when taking a reading. Your body will affect the radiation pattern of the antenna. Do not touch the antenna, and don't stand in the path of the shot, when taking signal strength readings. The same goes for the team on the other side of the link, too.
- Don't be afraid to push past the best received signal. As we saw in chapter four, radiation patterns incorporate many smaller sidelobes of sensitivity, in addition to a much larger main lobe. If your received signal is mysteriously small, you may have found a sidelobe. Continue sweeping slowly beyond that lobe to see if you can find the main lobe.
- The antenna angle may look completely wrong. The main lobe of an antenna often radiates slightly to one side or the other of the visual dead center of the antenna. Don't worry about how the antenna looks; you are concerned with finding the best possible position to achieve the greatest possible received signal.
- Double-check polarization. It can be frustrating to attempt aligning a dish only to discover that the other team is using the opposite polarization. Again, this should be agreed upon before leaving home base, but if a link stays stubbornly weak, a double check doesn't hurt.
- If nothing works, check all components one at a time. Are the devices on both ends of the link powered on? Are all pigtails and connectors properly connected, with no damaged or suspect parts? As outlined in chapter eight, proper troubleshooting technique will save you time and frustration. Work slowly and communicate your status well with the other team.
By working methodically and communicating well, you can complete the job of aligning high gain antennas in just a short while. If done properly, it should be fun!