If the distance between your sites is 0-5 miles, then determining line of sight is mostly common sense. Beyond that it gets tricky, owing to a factor called "multipath", as we'll explain. Meanwhile, here's what you should know - and solid ground for any technical meeting:
What exactly is "line of sight"? Let's start with a definition. A line of sight connection is one where local and remote antennas can see each other and that the visual path is clear of obstruction - phone and power lines excepted. That being said, the distance of the transmission and the width of the radio beam determines the amount of path clearance that you need for a solid wireless connection.
And now we introduce "multipath". A point-to-point microwave transmission is cigar-shaped, initiating at a focal point from each antenna and fanning out across space, intersecting mid-path with the beam coming from the remote antenna. The area of beam intersection is the widest transmission point and that's called the "Fresnel zone". The longer the path distance the wider the Fresnel zone and hence, the more potential for signal reflections or "multipath" issues. As stated earlier, multipath isn't much of a factor until you get beyond five miles. And that brings us to the other prime influence effecting multipath and that's the angle of the beam as it leaves the antenna.
A chief benefit of licensed microwave is that those radios are coupled with high performance parabolic antennas (commonly called "dish", but actually described as "drum-shaped"). These antennas focus the beam to between 0.9 and 2.0 degrees, depending on diameter of the dish. Larger dishes are for longer paths and the larger the dish, the tighter the focal point of the beam as it leaves the antenna. The narrow focal point of the radio beam explains why licensed microwave works in many cases where line of sight is insufficient for unlicensed wireless. Unlicensed radios (e.g., 2.4 and 5.8 GHz) use comparably sloppy grid or panel type antennas that cast beams upwards of thirty degrees. Now consider the greater magnitude of path reflections between a two mile beam that starts at thirty degrees versus one that starts at a degree and a half.
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