Fresnel zone
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In optics and radio communications, a Fresnel zone (pronounced FRA-nel Zone), named for physicist Augustin-Jean Fresnel, is one of a (theoretically infinite) number of a concentric ellipsoids of revolution which define volumes in the radiation pattern of a (usually) circular aperture. Fresnel zones result from diffraction by the circular aperture.
The cross section of the first Fresnel zone is circular. Subsequent Fresnel zones are annular in cross section, and concentric with the first.
To maximize receiver strength you need to minimize the effect of the out of phase signals. To do that you must make sure the strongest signals don't bump into anything - they have the maximum chance of getting to the receiver location. The strongest signals are the ones closest to the direct line between transmitter and receiver and always lie in the 1st Fresnel Zone.
The concept of Fresnel zones may also 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, we must 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 Fresnel zones at any point P in the middle of the link is the following:
where,
Fn = 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
λ = The wavelength of the transmitted signal in metres
The cross section radius of the first Fresnel zone is the highest in the center of the RF LoS which can be calculated as:
where
Or even:
where
- r = radius in metres
- D = total distance in kilometres
- f = frequency transmitted in gigahertz.
