If this were an outdoor RF implementation and the two antennae were parabolic dishes mounted on buildings across the street from each other, the obstruction could be a tree or another building interposed in the Fresnel Zone.

The first Fresnel Zone must be at least 60% unobstructed. If more than 40% of the zone is blocked it can be assumed that severe degradation of the signal strength will result. This is an important realization. Even though there might be a direct line-of-sight between the wireless adapter in a client computer and the access point on the ceiling, the radius of the first Fresnel zone surrounding the straight line-of-sight path must be taken into consideration. An interior object (like a file cabinet or cubicle wall) may be obstructing the first Fresnel zone and causing unexpected degradation of the signal. When considering outdoor RF implementation this becomes more critical due to the fact that there are fewer opportunities for reflected signals to bounce around in a room and make their way to the receiving antenna. Moreover, outdoor RF typically utilizes directional antennae, further reducing the possibility of multipath reflections.

To assess the clearance in the first Fresnel zone you must first apply the formula to determine the radius of the zone at the distance being assessed. You can consider the maximum radius of the zone that always occurs exactly halfway between the two antennae and use that value as your guideline. From the straight line defined by the direct line-of-sight between the two antennae, remember that the first Fresnel zone must be at least 60% unobstructed. A completely unobstructed zone would, of course, be better!

Practical Examples of the Fresnel Zone Calculation

Letʼs look at some practical examples using F=2.437 GHz (the center frequency of 802.11b channel 6).

An end-user, sitting in their cubicle, is 50-feet from the access point. What is the radius of the first Fresnel zone at the point where their metal file cabinet is between their notebook computer and the access point? The file cabinet is 4-feet away from the user.

D1 = 46ʼfrom access point

D2 = 4ʼfrom client wireless adapter

First Fresnel Zone = 1.22 feet (1-foot, 2.64-inches) 60% radius = .737 feet (8.44-inches)

For the same end-user, what is the radius of the first Fresnel zone at the point where an upright shelving unit is located halfway between the user and the access point?

D1 = 25ʼfrom access point

D2 = 25ʼfrom client wireless adapter

First Fresnel Zone = 2.26-feet (2-feet, 3.1-inches 60% radius = 1.358-feet (1-foot, 4.3-inches)

What is the maximum radius of the first Fresnel zone when the end-user is 150-feet away from an access point? Remember that the maximum radius occurs halfway between the two antennae.

D1 = 75ʼfrom access point

D2 = 75ʼfrom client wireless adapter

First Fresnel Zone = 3.92-feet (3-feet, 11-inches) 60% radius = 2.35-feet (2-feet, 4.2-inches)

Copyright 2003 - Joseph Bardwell