The terrain and humidity factor used in the microwave reliability calculation can be entered directly, or the value can be computed from humidity and terrain roughness information.
Lenkurt (1970) suggests the following values:
- 4 for very smooth terrain, including over water.
- 1 for average terrain, with some roughness.
- .25 for mountainous, very rough, or very dry areas.
The terrain and humidity factor can be computed using the formula (from Roelofs, 1986):
(2) a = K x (W/50)-1.3
- K is a constant based on local area humidity:
- 2 for coastal humid areas.
- 1 for average or temperate areas.
- .5 for dry areas.
- W is the roughness of the terrain. This is the standard deviation of the path elevations taken at one mile intervals, not including the end points:
- A is the average of the terrain at one mile intervals (excluding the end points).
- E is the elevation of each of the terrain points at one mile intervals (excluding the end points).
- ∑ (E – A)2 is the sum of the square of differences between each elevation point and the average elevation.
TAP™ keeps track of the source of the terrain/humidity value. If you calculate the value as described above, the Loss Mode will be marked “CALC”. However, if you enter any value directly, the mode will be marked “SPEC” (for specified).
- Engineering Considerations for Microwave Communications Systems, 1975, GTE Lenkurt Incorporated.
- Roelofs, Stan, “Fade Margin Requirements for Microwave Systems”, Microwave Reference Guide, 1986, Motorola.