Campbell RF451, Instruction Manual

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Appendix D. Distance vs. Antenna Gain, Terrain, and Other Factors
D-6
and interferes with the main signal. This phenomenon gives rise to the 2-Ray
Multipath Propagation Model for estimating real world distances.
The path geometry defined by the relative elevations of the antennas and the
distance between the antennas is a significant factor in determining the degree
of interference from the reflected wave. The terrain along the path of
propagation is always assumed to be relatively flat and smooth.
The equation for estimating path loss using the 2-Ray propagation model is:
Lpath (2-Ray) = 120 – 20log (h
tx
• h
rx
) + 40Log (d) (h in meters, d in km)
TABLE D-6 compares path loss (Lpath) calculations for the free space and
2-Ray propagation models. The antenna elevations used for the 2-Ray path loss
are 9 m (30 ft) and 3 m (10 ft) respectively.
TABLE D-6. 915 MHz Distance vs. Path Loss (Lpath in dB) per Two Propagation Models*
Path Type 2 mi. 4 mi. 6 mi. 8 mi. 10 mi. 14 mi. 18 mi. 22 mi. 26 mi. 30 mi.
Free Space 102 108 111 114 116 119 121 123 124 125
2-Ray 112 124 131 136 140 145 150 153 156 159
*(30 ft and 10 ft antenna elevations)
Often the maximum path distance may be constrained more by the antenna
elevations than the path loss. This is because the maximum unobstructed line
of sight distance in kilometres over a flat earth is defined by the following
equation:
LOS
MAX
= (4.124 √ h
1
) + (4.12 √ h
2
) (h is in meters)
For the preceding antenna elevations, the maximum unobstructed line-of-sight
distance is 19.5 km (12 miles).
TABLE D-7 helps select a Path Type in TABLE D-6 to best fit your situation.
TABLE D-7. Path Type vs. Path
Characteristics Selector
Path Type Path Characteristics
Free Space
Mountaintop to mountaintop
or Tall antenna towers
Line of sight
2-Ray
At water’s edge (very reflective)
Across field of grain (reflective)
Lots of Trees (absorptive)