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Appendix H. Distance vs. Antenna Gain, Terrain, and Other Factors
H-6
Rain, Snow, and Fog
Below 10 GHz, these don’t have much effect on path loss (see Ground
Reflections).
Real World Distance Estimates
From the above discussion of departures from the ideal “free space” path loss,
it is clear that we should usually use something other than the 2 nd power
distance table.
Here is a table which gives calculated path loss (Lp) values at 900 MHz for the
2 nd , 3 rd , and 4 th powers of distance; the equations (for 915 MHz) are:
Lp (2
nd
power) = 95.8 + 20
×
log ( d ) dB (d in miles)
Lp (3 rd power) = 95.8 + 30 × log ( d ) dB (d in miles)
Lp (4 th power) = 95.8 + 40
×
log ( d ) dB (d in miles)
Example calculated Lp values (in dB)
TABLE H-1. 900 MHz Distance vs. Path Loss (Lp in dB) per Three Path Types
Path Type 2 mi. 4 mi. 6 mi. 8 mi. 10 mi. 14 mi. 18 mi. 22 mi. 26 mi. 30 mi.
2 nd power 102 108 111 114 116 119 121 123 124 125
3 rd power 105 114 119 123 126 130 133 136 138 140
4 th power 108 120 127 132 136 142 146 149 152 155
The following table helps select a Path Type in the above “Distance vs. Path
Loss” table to best fit your situation.
TABLE H-2. Path Type vs. Path
Characteristics Selector
Path Type Path Characteristics
2 nd power Mountaintop to mountaintop
or Tall antenna towers
Line-of-sight
3
rd
power Dominantly line-of-sight
Low antenna heights
Some trees
4 th power At water’s edge (very reflective)
Across field of grain (reflective)
Lots of Trees (absorptive)
Examples
Some examples will help illustrate the tradeoffs in a link analysis. These
examples will all use the RF401 900 MHz radio, and will use –107 dBm as the
required power level at the radio receiver. This is 3 dB higher than the quoted
sensitivity of –110 dBm, which will give us a 3 dB margin.