Black Box LW0050A Manual page 125

In conclusion, the received signal power is above the sensitivity threshold, so the
link should work. The problem is that there is only a 2-dB difference between
received signal power and sensitivity. Normally, a higher margin is desirable
because of fluctuation in received power as a result of signal fading.
S F
IGNAL ADING
Fading of the RF signal is caused by several factors:
• Multipath
The transmitted signal arrives at the receiver from different directions, with
different path lengths, attenuation, and delays. The summed signal at the
receiver may result in an attenuated signal.
• Bad Line of Sight
An optical line of sight exists if you can see one antenna from the other—there
are no obstructions between them.
Radio-wave clear line of sight exists if a certain area around the optical line of
sight (Fresnel zone—see the next page) is clear of obstacles. A bad line of sight
exists if the first Fresnel zone is obscured.
• Link Budget Calculations
• Weather conditions (Rain, wind, etc.)
At high rain intensity (150 mm/hr), the fading of an RF signal at 2.4 GHz may
reach a maximum of 0.02 dB/km.
Wind may cause fading due to antenna motion.
Transmitter
Figure D-6. Multipath Reception.
APPENDIX D: Radio Signal Propagation
Multipath
reception
Receiver
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