Rain fade is the deterioration of the microwave RF signal levels caused by rain precipitation on either end of the satellite VSAT link. Typically, Ku and Ka band links are affected (frequencies above 11GHz) and can occur at the VSAT remote or the Teleport end of the link. RF energy is absorbed and scattered by the rain droplets and affects the higher frequencies more because of the wavelength of the signal and the size of the rain and shape of the droplet. Cross polarization isolation will also be reduced.
Line of Sight
Rain storms many km away from the antenna could still have a detrimental effect, especially if the elevation angle is quite low and the storm is in the line of sight of the antenna and can last for prolonged periods.
Water, snow and Ice on the surface of the antenna and RF feed assembly will also cause a decrease in signal levels by approx. 5-15%. A Radome will protect the VSAT electronics from the weather and corrosion but when it rains the water on the surface of the Radome will attenuate the signal.
Clouds are made from water vapour, right? cause significant loss on high frequency satellite links, the typical attenuation at Zenith (0° Elevation) it is only a few dB below 100 GHz.
Possible ways to overcome the effects of rain fade are;
- Site diversity
- Uplink Power Control (UPC)
- Adaptive Coding and Modulation (ACM)
- Hydrophobic coatings on the antenna or Radome
- C-Band VSAT communications
- Link budgets
- Antenna heated blankets