Campbell Scientific RF401A-Series Spread Spectrum Radios - Manual (Page 44)

Appendix B. Distance vs. Antenna Gain, Terrain, and Other Factors
Ground Reflections
These are caused by the RF signal being reflected from the ground (or water),
and undergoing a phase shift so that it destructively interferes with the line-of-
sight signal. The conditions that cause this the most are propagation over
water, or over a low-lying fogbank. The reflected signal suffers little
attenuation, gets out of phase, and interferes with the main signal. If antennas
need to be sited near water, they should be positioned away from the water’s
edge so that the ground vegetation attenuates the reflected RF.
The result of the reflection and interference (worst case) is that the path loss
increases as the 4 th power of the distance, instead of the 2 nd power. This
changes the distance term in the path loss equation to: 40 x log ( d ) dB. Then,
with each doubling of distance, the path loss increases by 12 dB, instead of 6
dB.
Vegetation
Losses due to vegetation (trees, bushes, etc.) cause the path loss to increase by
the 3 rd to 4 th power of the distance, instead of the 2 nd power. This is just like in
the severe ground reflection case above.
Rain, Snow, and Fog
Below 10 GHz, these don’t have much effect on path loss (see Ground
Reflections).
B.3 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.
TABLE B-7 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)
TABLE B-7. 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
B-6