Campbell RF451, Инструкция по эксплуатации

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Appendix D. Distance vs. Antenna Gain, Terrain, and Other Factors
D-5
D.2.6 Receiver Sensitivity
Receiver sensitivity is usually specified in dBm for a specific bit error rate
(BER). The transceiver module used in the RF451 is specified at
–108 dBm at ~10 –4 raw BER.
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 fade
margin, and is a very good thing; a minimum of 10 dB of fade margin should
be sought.
D.2.7 Path 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 D-5. 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, also, 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 greater the antenna elevations relative to the terrain in the line-of-
sight path, the closer to free space conditions. Antenna height is everything!
D.3 Real World Distance Estimates
It is clear from the above discussion that free space path loss should seldom be
used as the basis for estimating real world distance.
Ground Reflections
As discussed in Section 5 of the Campbell Scientific application note The Link
Budget and Fade Margin , ground reflections will almost always be a factor in
terrestrial RF telemetry links. These are caused by the RF signal being
reflected from the ground (or water), and undergoing a phase shift so that it
destructively or constructively 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,