Why is rain attenuation in satellite communication?

Any satellite communications system network operator using a Ku-Band system (12/14 GHz or higher frequencies) will face the effects of rain fade at some time. But to understand why this weakening occurs with Ku-Band transmissions, you must first understand the causes of satellite rain fade. Two of the most common causes are listed below.

• Absorption - Part or all of the energy generated when a radio wave strikes a rain droplet. The droplet is converted to heat energy and absorbed by the droplet.
• Scattering - A non-uniform transmission medium (the raindrops in the atmosphere) causes energy to be dispersed from its initial travel direction.

Scattering can be caused by either refraction or diffraction:

• Refraction - The refractive index of the water droplets encountered by the radio wave.
• Diffraction - the travel direction of the radio wave also changes as it propagates around the obstacle in its path (a water droplet).

It refers primarily to the absorption of a microwave Radio Frequency (RF) signal by atmospheric rain, snow or ice, and losses are especially prevalent at frequencies above 11 GHz. It also refers to the degradation of a signal caused by the electromagnetic interference of the leading edge of a storm front. Rain attenuation or fade can be caused by precipitation at the uplink or downlink location. However, it does not need to be raining at a location for it to be affected by rain fade, as the signal may pass through precipitation many miles away, especially if the satellite dish has a low look angle.

From 5 to 20 percent of rain fade or satellite signal attenuation may also be caused by rain, snow or ice on the uplink or downlink antenna reflector, radome or feed horn.