Microwave Rain Attenuation Calculations
Q: When I calculate rain attenuation in the microwave link budget program, the attenuation lookup returns different values for a given rain distance and rate. Which one should I use?
A: Since rain attenuation is not an exact science, different models give different results. Of the four models, three (Medhurst, Ryde, CCIR) usually give results within a dB or so, but the Crane model is often more conservative, giving a higher loss value than the others.
You can choose which of the models you want to use. Or when you return to the Microwave Link Budget form, you can enter a value that is your choice of some intermediate value. For example, you might enter a value that is between the lower value of the three models and the higher value of the Crane model.
There really isn't a "right" answer - if your application requires a particular model you should use that one. Or if you want to be conservative you can choose the "worst case" (highest loss) value, or if you want to be optimistic you can choose the lowest loss. You can get a good idea of the range of performance by running the link budget with the rain attenuation at both extremes.
Also note that the models used have specific frequency ranges:
Model Minimum Frequency Maximum Frequency CCIR 3.0 GHz 100 GHz Crane 1.0 GHz 100 GHz Medhurst 7.5 GHz 30 GHz Ryde 4.0 GHz 13 GHz
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