The rain attenuation due to rainfall can be entered directly or computed by three different methods (see Ryde & Ryde, Medhurst, and CCIR).
When you direct TAP™ to calculate rain attenuation, you will enter a rainfall rate (in either inches or millimeters per hour) and the portion of the path affected by rain attenuation entered in the current units (miles or kilometers), or as a percentage of the total path.
After entering all of the required values, the attenuation values computed for the three models are displayed. When you select the desired value and return to the microwave link budget program, the selected attenuation is incorporated in the fade margin and reliability calculations performed in this program.
TAP™ keeps track of the source of the rain attenuation value. If you accept the Medhurst value as described above, the Loss Mode will be marked “MED” in the link budget program to remind you of the attenuation model used to generate attenuation. However, if you enter any value directly, the mode will be marked “SPEC” (for specified).
The loss from atmospheric absorption can be entered directly or computed by pressing the “Calculate Loss” button for the Absorption Loss field.
Absorption loss is computed as a function of the frequency using the complete length of the path. The loss value is determined from curves based on van Vleck (1947), providing separate values for and water vapor losses. The absorption loss is the sum of these two losses.