When you think of line of sight, you probably, and understandably, assume that if you could draw a straight line from one wireless device to the other with no obstructions, you have line of sight between the two devices. After all, Merriam-Webster defines line of sight as:
- A line from an observer's eye to a distant point
- The line between two points; specifically : the straight path between a transmitting antenna (as for radio or television signals) and a receiving antenna when unobstructed by the horizon
So if the manufacturer of a wireless device says it has a line of sight range of 40 miles, and you deploy two of them with no obstructions between them, you will achieve the stated range. I strongly advise against betting a lot of money on this. It turns out that Merrian-Webster is a little off base on definition #2 above.
In the fuzzy world of wireless communications, they use quite a different definition for line of sight. It is of interest to note that they usually refer to it as “Line of Sight”. Maybe that gives them the license to have their definition differ from Merrian-Webster’s.
When RF device manufacturers quote a Line of Sight range for their products, they take into consideration not only the ability to have a visual line of sight which is unobstructed, they require that the RF Fresnel zone is also unobstructed. The Fresnel Zone is an elliptical area immediately surrounding the visual path and varies in thickness depending on the length of the signal path and the frequency of the signal. The Fresnel Zone is much like the Twilight Zone, but a lot scarier (unless you are old like me, you may not get that joke). The Fresnel Zone can be visualized as follows:
For 900MHz, the radius of the Fresnel Zone for a range of 40 mi. is approximately 240 ft. (73m).
So if you bought two 900MHz serial data communication devices with a stated Line of Sight range of 40 mi. (64km) and took them out to the local salt flats to test, how would you expect them to perform? Unless you have really, really long arms, they will not work at all because the lower half of the Fresnel Zone would totally obstructed by the ground. To achieve the stated 40 mi (64km) Line of Sight range, you would need to build two towers, each at least 300 ft. (91m) tall!