Fresnel Effect

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The early 19th century French physicist, Augustin Fresnel (pronounced “Freh-Nel”), made an important observation about the behavior of light. Fresnel noted that a ray of light passing near a solid object is subject to diffraction, or bending. This diffraction caused the intensity of the original light beam to increase or decrease depending on how near the object was to the beam. This characteristic of electromagnetic radiation is known as the Fresnel Effect.

Light and radio waves are subject to the same laws of physics, including the Fresnel effect. If an object like a mountain ridge or building is close to the radio signal path, it can affect the quality and strength of the signal. Radio waves diffracted by such objects can affect the strength of the received signal. This happens even though the obstacle does not directly obscure the direct visual path. This area, known as the “Fresnel Zone,” must be kept clear of all obstructions.

60% of the Fresnel Zone is the generally accepted antenna reception area which must be kept clear to prevent signal degradation. The network architect has several options to resolve propagation degradation due to the Fresnel Zone problem. Here are some options including:

• Raise the antenna mounting point on Building A and/or Building B

• Build a new structure, e.g. a radio tower tall enough to provide adequate clearance
• Increase the height of the existing antenna mounting point by installing a taller mast with stabilizing guy wires
• Locate a different mounting point, e.g. building or tower, for the antenna
• Remove the obstacle (gee, let’s move the mountain)

Trees and weather can also impair radio frequency signal transmission in the Fresnel zone. This includes broadleaf trees, which can collect large amounts of water increasing radio frequency signal attenuation until evaporated.

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[tags]antenna,radio frequency,fresnel effect,augustin fresnel,fresnel zone[/tags]