Definition & Calculation of Fresnel Clearance Zone
What is Fresnel Zone?
Unlike the popular view of a Line of Sight which a clear unobstructed clear straight line. Radio frequency line of sight is defined by Fresnel Zones which are ellipse shaped areas between any two radios. The primary Fresnel zone is required to be at least 60% clear of any obstruction to ensure the highest performance of wireless link.
The radius of the Fresnel Zone at its widest point can be calculated by
the formula listed below, where d is the link distance in Kilometers, f is the frequency
in GHz, and r is the radius off of the center line of the link in feet. For further information
on this formula, please review the figure below.
Note: The final value represents a first order approximation and should only be used as a guide. No guarantees or warranties are implied accordingly.
Fresnel Zone Example
Notes:
- The Fresnel Zone is the area around the visual line-of-sight that radio waves spread out into after they leave the antenna. You want a clear line of sight to maintain signal strength, especially for 2.4 GHz wireless systems. This is because 2.4 GHz waves are absorbed by water, like the water found in trees.
- Typically, 20% Fresnel Zone blockage introduces little signal loss to the link. Beyond 40% blockage, signal loss will become significant.
- This calculation is based on a flat earth. It does not take the curvature of the earth into consideration. The effect of this is to budge the earth in the middle of the link. It is recommended for long links to have a microwave path analysis done that takes this and the topography of the terrain into account.
- The formula for determining the radius of the widest point of the fresnel zone (in meters) is:
17.32 * square root of (d/4f)
where d is the distance (in kilometers) between the two antennas and f is the frequency (in GHz) at which you are transmitting. - The formula for determining the radius of the widest point of the fresnel zone (in feet) is:
72.05 * square root of (d/4f)
where d is the distance (in miles) between the two antennas and f is
the frequency (in GHz) at which you are transmitting.