Campbell Scientific RF401A-Series Spread Spectrum Radios - Manual (Page 43)

Appendix B. Distance vs. Antenna Gain, Terrain, and Other Factors
If the received signal strength is greater than the receiver sensitivity, a link can
be established. Any excess signal strength above the receiver sensitivity is
“link margin”, and is a very good thing; a minimum of 6 dB of link margin
should be sought.
B.2.7 Path Loss
We have combined in this section the normal “free space” path loss (only seen
in mountaintop to mountaintop scenarios) with loss due to ground reflections,
diffraction, leaf/forest absorption, etc. It is all loss!
A starting point is the “free space” path loss. Here are two equations for this:
Lp (dB) = 32.4 + 20 x log( f ) + 20 x log ( d ) dB (f in MHz, d in km)
Lp (dB) = 36.6 + 20 x log( f ) + 20 x log ( d ) dB (f in MHz, d in miles)
Here is a table showing the free space path loss (in dB). Note the effect of
frequency.
TABLE B-6. Free Space Path Loss
Frequency Distance
1 mi. 2 mi. 4 mi. 8 mi. 10 mi. 16 mi. 22 mi. 26 mi. 30 mi.
400 MHz 89 95 101 107 109 113 115 117 118
915 MHz 96 102 108 114 116 120 123 124 125
2.4 GHz 104 110 116 122 124 128 131 133 134
Notice the relationship between path loss and distance: each time you double
the distance, you lose 6 dB of signal under free space conditions. Or, put
another way, if you add 6 dB of gain (for example with 6 dB of additional
antenna gain, or 6 dB less cable loss), you can double the distance for free
space conditions.
As mentioned before, free space conditions are the ideal, but seldom actually
seen. The higher the antenna height relative to the terrain in the line-of-sight
path, the closer to free space conditions. Antenna height is everything!
Here are some additional propagation effects that increase the path losses:
Diffraction
This is caused by objects close to the line-of-sight path. Real world examples
of this would be hills, buildings, or trees. The object may not be in the direct
line of sight, but if it is close enough, it will cause the RF to diffract around the
object, giving additional path loss. “Close enough” is a function of frequency,
path length, and position of the obstacle along the path.
An example at 900 MHz: a 10 mile path length with an obstacle halfway along
the path will see diffraction “losses” from an obstacle within ~70 ft. of line-of
sight. The amount of loss would be from 6 dB to 20 dB, depending on the
obstacle surface. A sharp edge (like a rock cliff) would give the minimum loss
(6 dB), while a rounded hill would give the maximum loss (20 dB).
B-5