Fresnel zone is a theoretical "envelope" around the line of sight. While line of sight is important, it may not always be adequate. If obstructions penetrate the Fresnel zone, there will be signal attenuation
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 obstruction loss by removing obstacles in the first Fresnel zone
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.
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| In a parabolic reflector, all rays leaving the focal point are in phase |
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| In circular reflector, all rays leaving from focal point are NOT in phase. hence dish antennas are in parablic shape |
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| When the distance travelled by two waves are different, then the phase of each wave will be different. |
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