How to Plan a Microwave Radio Link.

Whether your link is point-to-point or point-to-multipoint, the first thing to do is to verify that it will have not only clear line of sight, but at least 60 percent of the first Fresnel zone clear of obstructions as well.

The longer thedistance, the more important this is. If the Fresnel zone is blocked, then you will get a lower signal level on the distant end than expected — even if you can literally “see” the other antenna in the distance.But even if your Fresnel zone is partially blocked, it is still possible to get a link, provided that your system was designed to have a strong signal at the other end of the link.

In planning long-range microwave links where you are not sure that you have unobstructed line-of-site and clear Fresnel zone, an RF path analysis should be done. There are many software packages available that have terrain data and can create a path profile from a set of latitude/longitude coordinates. But these programs can only indicate for certain if a link will not work due to terrain obstruction.

A clear path on paper is not a guarantee that your link will work, since it does not show trees or buildings. So even a “clear” link might have 80-foot trees in the way that could block the signal.You could be wasting your time and money if you ignore Fresnel zone issues — or worse yet, no line-of-site and attempt to set up a link anyway. You will likely not have a reliable link, if one at all.

But assuming that you do have clear line-of-site and 60 percent of the first Fresnel zone clear (or nearly clear),how can you know if you will have a good link or not? How much gain do your antennas need to have? How much coax cable loss is too much? If your link is at 2.4 GHz, should external amplifiers be used? Or given your fixed base station antenna with a pre-set gain, how far can you reach with the different types of client antennas? And which clients will need amplification?