Question: How do rain, snow, ice and fog impact wireless performance?
Answer: In the process of distributing wireless to a geographic area, it is often necessary to place transceivers outdoors. In doing so, a prudent designer understands that one cannot afford to overlook the effects of the environment on the life of the equipment and the performance of the wireless system. The major environmental effects include the following:
- Weather-related precipitation in the form of rain, snow, ice and fog
- The seasonal effects on propagation of signal and equipment reliability
- Electromagnetic interference caused by machinery, power distribution or other nearby transponders
- Local topography and site structures
Weather-related precipitation in the form of rain, snow, ice and fog is often overlooked due to the common belief and correct understanding that precipitation has little effect at the most used frequencies. In fact, falling rain and snow does not become a real problem until around 10 GHz and above. Fog starts to be an issue at around 20 GHz. The more common 2.4 GHz signals may be attenuated only up to 0.05 dB/km (0.08 dB/mile) by torrential rain (4 in./h). Thick fog produces up to 0.02 dB/km (0.03 dB/mile) attenuation. At 5.8 GHz, torrential rain may produce up to 0.5 dB/km (0.8 dB/mile) attenuation, and thick fog up to 0.07 dB/km (0.11 dB/mile).
As you can see, these rain attenuation numbers are very small for the commonly available frequencies. I am as willing as anyone else to call these low numbers unimportant, but falling rain is not the whole story. Standing liquid / condensation is the collection of rain, snow or dew. Even though rain itself may not cause major propagation problems, rain, dew, or snow will collect on the leaves of trees and other surfaces and will produce noticeable attenuation — up to 3dB at 2.4 GHz or half power — until it evaporates. Sheeting liquid describes rain falling off of eaves, roofs, gutters and other structures. Sheeting is of particular concern if it is near the transponder, for even a light rain or snow melt from a medium-sized roof can cause a major amount of attenuation of the signal — in excess of 20 dB at 2.4 GHz is not uncommon. Sheeting is best avoided by careful antenna positioning with regards to any structure.
Even in small amounts, ice or snow buildup on an antenna can be a major problem for both attenuating signal and changing the characteristics of the antenna. Careful selection of equipment is very important because some surfaces and materials are better at shedding liquids than others. An antenna that performs well in Arizona may have disappointing results in Michigan.
Often overlooked are the effects of rain, snow and ice on the life and severability of equipment. Water may find a way into connectors and equipment. If allowed to do so, water may short-out or cause corrosive damage to connectors and equipment. Once it freezes, water will cause ice expansion, which can crack housings, allowing for more water to enter. In many cases, standard “weather proofing” or equipment which is “weather proofed” may not be enough. Equipment that is rated against a standard, rather than “weather hardened” or “ruggedized,” can be selected. Selection against a recognized standard along with other environment-specific techniques gives the network designer a proven solution to the problem of moisture. Be aware in both radio and network equipment that degradation usually precedes breakdown. Network monitoring and scheduled maintenance are the best protections against the effects of the environment in any given location.
Conclusion. At most commonly used frequencies the effects of precipitation are more damaging than the precipitation itself. A prudent designer will identify these and other like issues in his plan and employ a regiment of technologies and methods to overcome problems associated with the environmental conditions of a specific geographical area. Equipment selection against a recognized standard, equipment placement with regard to structure and various weatherproofing techniques are necessary. Vigilance to the network condition, if possible in realtime, as well as an effective maintenance schedule are invaluable.
John Welch is a senior wireless systems engineer at Apprion (Mountain View, Calif.; www.apprion.com). Welch has over thirty years experience in technology and engineering with radio frequency, digital and analog devices and various wireless communication and video systems. He brings over 12 years of specialized wireless implementation as well as design experience with industrial, metropolitan and military wireless infrastructures and systems.
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