Fresnel Zone - Determining Fresnel Zone Clearance

Determining Fresnel Zone Clearance

The concept of Fresnel zone clearance may be used to analyze interference by obstacles near the path of a radio beam. The first zone must be kept largely free from obstructions to avoid interfering with the radio reception. However, some obstruction of the Fresnel zones can often be tolerated, as a rule of thumb the maximum obstruction allowable is 40%, but the recommended obstruction is 20% or less.

For establishing Fresnel zones, first determine the RF Line of Sight (RF LoS), which in simple terms is a straight line between the transmitting and receiving antennas. Now the zone surrounding the RF LoS is said to be the Fresnel zone.

The general equation for calculating the Fresnel zone radius at any point P in between the endpoints of the link is the following:

where,

F_n = The nth Fresnel Zone radius in metres

d₁ = The distance of P from one end in metres

d₂ = The distance of P from the other end in metres

= The wavelength of the transmitted signal in metres

The cross sectional radius of each Fresnel zone is the highest in the center of the RF LoS, shrinking to a point at the antenna on each end. For practical applications, it is often useful to know the maximum radius of the first Fresnel zone. From the above formula, the following formulas can be derived, using, and . Now we have an easy way to calculate the radius of the first Fresnel zone (F₁ in the above equation), knowing the distance between the two antennas and the frequency of the transmitted signal.

In SI:

where

• r = radius in metres
• D = total distance in kilometres
• f = frequency transmitted in gigahertz.

Or in imperial units:

where

• r = radius in feet
• D = total distance in miles
• f = frequency transmitted in Gigahertz.