Rain Fade Margin

Value to Enter:

Enter the dB loss per mile, or per kilometer, associated with Rain Fade for the entire end-to-end link. This value can be derived from standard tables or from field measurements. If you're unsure, and if you live in an area where rainfall can be heavy and prolonged, use 0.8 dB/mile (0.5 dB/km) as a 'worst-case' value for Wi-Fi or WiMAX systems.

Significance of This Value:

For 802.11b/g networks (2.4GHz) you may find attenuation up to 0.08 dB/mile (0.05dB/km) in the heaviest of rain conditions (4 inches/hr). A thick fog may result in as much as 0.03 dB/mile (0.02 dB/km) attenuation. For 802.11a and WiMAX (5.8 GHz) the attenuation my be up to 0.8 dB/mile (0.5 dB/km) in heavy rain and 0.11 dB/mile (0.07 dB/km) in fog. Even a light rainfall or heavy dew that collects as droplets on tree leaves can introduce as much attenuation as a heavy continuous rainfall.

In the United States, use the Rain Fade Region Map with an accompanying Average Rainfall Table to get an idea of the rainfall rates then refer to the Rain Fade Margin Graph for specific attenuation values at various frequencies.

Background and Technical Perspective:

Connect802's projects in Malaysia required careful assessment of Rain Fade for microwave links through the dense rainforests.

A spectrum user requires varying degrees of detail when estimating the coverage or reliability of a transmission system. For frequency management and general planning purposes it may be sufficient to make simple (and optimistic) assumptions based on Free Space Path Loss (which accounts for spatial spreading losses.) Additional detail can be incorporated into a design model to improve the accuracy of the predictions. The International Telephony Union (ITU) has a number of recommendations described in the ITU Radio Regulations (ITU-R P.620-5, ITU-R P.837-4, ITU-R P.1546-1, and revisions in ITU-R P.620-5). Weather monitoring stations around the world report rainfall rates which are used to determine both average rain amounts as well as local gradients in rainfall (where, for example, a low-lying area receives more or less rainfall than a higher-elevation location a few miles away.)

As the transmitted signal energy passes through water droplets it is both absorbed and scattered to varying degrees. Absorption is the aspect where part (or all) of the energy is converted to heat as it penetrates the water droplet. Scattering is the disruption of the direction of travel of the RF signal as it passes through the non-uniform shape, and differing density of the water droplet. Scattering may cause refraction of the wave (bending due to the difference in density between the air and the water) or diffraction (bending of the signal around the edges of an object that is smaller than 1 wavelength.)