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Appendix H. Distance vs. Antenna Gain, Terrain, and Other Factors

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

power

distance table.

Here is a table which gives calculated path loss (Lp) values at 900 MHz for the

, 3

, and 4

powers of distance; the equations (for 915 MHz) are:

Lp (2

power) = 95.8 + 20 × log ( d ) dB (d in miles)

Lp (3

power) = 95.8 + 30 × log ( d ) dB (d in miles)

Lp (4

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.

power 102 108 111 114 116 119 121 123 124 125

power 105 114 119 123 126 130 133 136 138 140

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

power Mountaintop to mountaintop

or Tall antenna towers

Line-of-sight

power Dominantly line-of-sight

Low antenna heights

Some trees

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.

H-6

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