Line of sight is the direct free-space path that exists between two points. Using binoculars on a clear day, it is easy to determine if visual line of sight exists between two points that are miles apart. To have a clear line of sight there must be no obstructions between the two locations. Often this means that the observation points must be high enough to allow the viewer to see over any ground-based obstructions. Frequently if the Access Point is not visible from the ground at a location, it is visible from the roof. Topographical analysis may also indicate that a roof mounted antenna pole may lift the antenna high enough to have clear radio line of sight - even if visual line of sight does not exist.

The following obstructions might obscure a visual link:

• Topographic features, such as mountains
• The curvature of the Earth
• Buildings and other man-made objects
• Trees

If any of these obstructions rise high enough to block the view from end to end, there is no visual line of sight.

Obstructions that can interfere with visual line of sight can also interfere with radio line of sight. But one must also consider the Fresnel effect. If a hard object, such as a mountain ridge or building, is too close to the signal path, it can damage the radio signal or reduce its strength. This happens even though the obstacle does not obscure the direct, visual line of sight. The Fresnel zone for a radio beam is an elliptical area immediately surrounding the visual path. It varies in thickness depending on the length of the signal path and the frequency of the signal.

As shown in the picture above, when a hard object protrudes into the signal path within the Fresnel zone, knife-edge diffraction can deflect part of the signal and cause it to reach the receiving antenna slightly later than the direct signal. Since these deflected signals are out of phase with the direct signal, they can reduce its power or cancel it out altogether. If trees or other 'soft' objects protrude into the Fresnel zone, they can attenuate (reduced the strength of) a passing signal. In short, the fact that you can see a location does not mean that you can establish a quality radio link to that location.

There are several options to establish or improve the line of sight:

• Raise the antenna mounting point on the existing structure
• Build a new structure, i.e. radio tower, which is tall enough to mount the antenna
• Increase the height of an existing tower
• Locate a different mounting point, i.e. building or tower, for the antenna
• Cut down problem trees