The Fresnel Zone

When setting up a wireless link, the first rule is to achieve line of sight between the transmitter and receiver. Line of sight means that you can physically see the other device from either location fully and clearly without any obstructions. While some wireless links will tolerate some amount of obstruction to line of sight, the best performance will come from true line of sight.


Physical obstructions like buildings must be avoided. The wireless link must be located so that it is above the building or positioned so the signal travels around the building. Physical obstructions are obvious and can be avoided with proper planning.


Trees, while also a physical obstruction, are more problematic. As trees grow taller, a tree that may not currently be an obstruction could become one at a later time. Unless the tree is maintained below the space required for the link, it will eventually partial or fully obstruct the link.


Deciduous trees present special problems. A deciduous tree may provide minimal or no obstruction during fall and winter when the leaves are gone but will quickly become an obstruction when the leave return in spring and summer.


Tree blocking signal


The second consideration is the Fresnel Zone. The Fresnel Zone is a three-dimensional oblong area between a wireless transmitter and receiver. The Fresnel Zone is an important consideration for wireless links because the area must remain clear of obstructions and interference. Additionally, the Fresnel Zone gets larger as the distance between the transmitter and receiver increases.


Fresnel Zone


Most of the signals that travel from a radio transmitter to receiver travel in a direct path between the two points. However, a significant amount of the signals travels off-axis. The signals that travel off-axis can deflect off objects, which can cause them to arrive out of phase and cause destructive interference.


The primary factor in the size of a Fresnel Zone for a wireless link is the distance between transmitter and receiver. The larger the distance between the two devices, the larger the Fresnel Zone. As the transmitter and receiver are spaced farther apart, more consideration must be given to the size of the Fresnel Zone and any potential obstructions in the Zone.


Additionally, as the distance between transmitter and receiver increase, the curvature of the Earth also becomes a factor. Even if the area between the devices is clear, the ground itself could become an obstruction due to the size of the Fresnel Zone.


Fresnel Zone requirements are also affected by the operating frequency of the link. Generally, higher frequencies have narrower Fresnel Zones, but a link could operate over a wide frequency range. Manufacturer specifications and guidance should always be sought for this information.


The most common solution to overcoming Fresnel Zone obstructions is to raise the link higher of the ground. The height above the ground (assuming no other potential obstructions) will depend upon the distance and operating frequency of the link. If ground-based obstructions (buildings, trees) existing in the link of sight or within the Fresnel Zone, the link may have to be raised even higher. Since some links will tolerate some obstruction or Fresnel Zone interference, the manufacturer of the link should always be consulted for specific guidance.


You can learn more about the Fresnel Zone in the NTC Yellow Book: Video Security Systems Handbook.

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