Cambium Networks PMP 450, Руководство по планированию

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PMP 450 Planning Guide
Anticipating Reflection of Radio Waves
In the signal path, any object that is larger than the wavelength of the signal can reflect the signal. Such an object
can even be the surface of the earth or of a river, bay, or lake. The wavelength of the signal is approximately
•
2 inches for 2.4, 3.5GHz, 3.6 GHz, 5.4-GHz and 5.8-GHz signals.
A reflected signal can arrive at the antenna of the receiver later than the non-reflected signal arrives. These two or
more signals cause the condition known as multipath. Multipath may increase or decrease the signal level and so
overall attenuation may be higher or lower than that caused by the link distance. This is problematic at the margin
of the link budget, where the standard operating margin (fade margin) may be compromised.
Noting Possible Obstructions in the Fresnel Zone
The Fresnel (pronounced fre
∙NEL) Zone is a three-dimensional volume around the line of sight of an antenna
transmission. Objects that penetrate this area can cause the received strength of the transmitted signal to fade. Out-
of-phase reflections and absorption of the signal result in signal cancellation.
The foliage of trees and plants in the Fresnel Zone can cause signal loss. Seasonal density, moisture content of the
foliage, and other factors such as wind may change the amount of loss. Plan to perform frequent and regular link
tests if you must transmit though foliage.
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pmp-0047 (March 2014)