Microwave signals using frequencies greater than 10GHz suffer from a phenomenon called Rain Fade. This is caused by the frequencies both uplink (14GHz) and down link (10 to 13GHz) having signal wavelengths so short that rain, snow or even heavy cloud cover can attenuate the down link signal dramatically.

At frequencies greater than 10 GHz (Ku12 GHz) or (Ka 20 GHz) the length of the falling rain droplets are very close to a resonant sub multiple of the signals wave length. Therefore the rain drops are able to absorb and depolarize the microwave signals passing through the earth’s atmosphere. The amount of attenuation depends on the rain intensity and also rain noise .when the microwaves pass through the rain drops they excite the water molecules which increases there temperature .This effectively sucks energy from the signal which increases the systems noise temperature which in turn affects the systems final signal to noise ratio.

The uplink operator faces a similar problem as the uplink frequencies for Ku 12 GHz downlink frequencies operate at 14GHz. In cases of heavy rain the uplink is normally equipped with uplink power control. These uplink power control systems adjust the strength of uplink signals at IF frequencies to compensate for varying weather conditions.

The receive site can only install a Dish / LNBF combination which will provide service availability of 99.5% through out the year. However how much rain fade protection is available depends on the receive site location. Rain Climatic Zone maps provide eight carefully selected regions world wide. These maps provide information carefully calculated which predict the amount of time that the signal will be disrupted due to excessive rain or snow.

The availability of a specific Ku down service can be improved by increasing performance margins by increasing the antenna size or selecting a lower noise LNBF.

It is recommended by most satellite operators that 4Db extra rain fade margin be calculated into operational service link budgets for Ku receive sites through out New Zealand.

Satellite system designers always address the issues of rain attenuation and build in a rain fade margin in both uplink and downlink calculations. Typically rain fade margins are around 4 Db above the minimum satellite receiver threshold point (Free View / Sky approximately 6Db) at FEC ¾ and 6 to 8 Db for commercial installations.

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