a receiver located to the side of a directional antenna in such a way that it receives 50% of the antennaʼs output power with an unobstructed line of site. If an obstacle were in the first Fresnel zone surrounding that line of sight then the receiver would get less power. Again, the location of the Fresnel zone is unrelated to the directionality of the antenna. Itʼs only relative to the direct line between transmitter and receiver.

Calculating the Radius of the Fresnel Zones

The radius of the Fresnel zones may be calculated by the following formula.

Where:

Rn is the radius of the nth Fresnel zone M is a constant of proportionality equal to:

17.3 if Rn is in meters and D1, D2 are in kilometers and 72.1 if Rn is in feet and D1, D2 are in statute miles

FGHz is the frequency in GHz

N is the Fresnel zone number (Equal to 1 for the 1st Fresnel Zone)

D1 is the distance from the source to the obstruction

D2 is the distance from the destination to the obstruction

Obstructions in the First Fresnel Zone

It has been determined that the diffraction characteristics of the first Fresnel zone are the most critical to received signal strength. Calculations have been made concerning the impact of obstructions in the first Fresnel zone and itʼs been found that the first zone must be at least 60% clear of obstructions. The figure below shows a file cabinet in the Fresnel Zone between an 802.11 access point antenna and a PCMCIA card in a user’s notebook computer.

Figure 6.10 Interior Obstructions in the First Fresnel Zone

Copyright 2003 - Joseph Bardwell