Fresnel zone

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In optics and radio communications (indeed, in any situation involving the radiation of waves, which includes electrodynamics, acoustics, and gravitational radiation), a Fresnel zone (pronounced /freɪˈnɛl/ fray-nell), named for physicist Augustin-Jean Fresnel, is one of a (theoretically infinite) number of concentric ellipsoids which define volumes in the radiation pattern of a (usually) circular aperture. Fresnel zones result from diffraction by the circular aperture.[1]

The cross section of the first (innermost) Fresnel zone is circular. Subsequent Fresnel zones are annular (doughnut-shaped) in cross section, and concentric with the first.

To maximize receiver strength, one needs to minimize the effect of the out-of-phase signals by removing obstacles from the radio frequency line of sight (RF LoS). The strongest signals are on the direct line between transmitter and receiver and always lie in the first Fresnel zone.

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Fresnel zones

If unobstructed, radio waves will travel in a straight line from the transmitter to the receiver. But if there are obstacles near the path, the radio waves reflecting off those objects may arrive out of phase with the signals that travel directly and reduce the power of the received signal. On the other hand, the reflection can enhance the power of the received signal if the reflection and the direct signals arrive in phase. Sometimes this results in the counterintuitive finding that reducing the height of an antenna increases the signal-to-noise ratio.

Fresnel provided a means to calculate where the zones are, where obstacles will cause mostly in phase and mostly out of phase reflections between the transmitter and the receiver. Obstacles in the first Fresnel zone will create signals that will be 0 to 90 degrees out of phase, in the second zone they will be 90 to 270 degrees out of phase, in third zone, they will be 270 to 450 degrees out of phase and so on. Odd numbered zones and even numbered zones are both constructive, but their phase varies for pi.[2]

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.[3]

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