Fresnel zone

From Academic Kids

In optics and radio communications, a Fresnel zone (pronounced as 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.

The concept of Fresnel zones may also be used to analyze interference by obstacles near the path of a radio beam.

This zone must be determined first, so as to keep it free from obstructions.

Maximum obstruction allowable is: 40%

Recommended obstruction is: 20%

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 cross section radius of the first Fresnel zone is the highest in the center of the RF LoS which can be calculated as:

<math>r = 43.3 \sqrt{d/{(4 f)}}<math>

where

  • r = radius in feet
  • d = distance in miles
  • f = frequency transmitted in gigahertz.

Missing image
Fresnel.JPG
Image:fresnel.JPG

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