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Fade margin:
A = | P
S
| − | P
R
| = 79 − 67.1 = 11.9 dB
Fade margin is now only 12 dB which corresponds to link availability > 99.99% of the time in a year.
Technical literature often gives the minimum fade margin of 20 dB. For very long links (more than
10 km) fade margin will, indeed, be approximately 20 dB. For shorter links, however, such large
margin is not necessary. It is helpful to first conduct the calculation above to receive an idea of the
attenuation affecting the link.
The result
To achieve the required transmission capacity and link availability for link distance of 4 km, transmit
power -22 dBm and 99 cm antennas were selected for both sides of the link.
Sources for Chapter Chapter 2, Implementation Notes :
[1] Lehpamer, H.: Microwave transmission network, Second edition, ISBN: 0071701222, McGraw-Hill
Professional, 2010.
ITU-R recommendation used:
• ITU-R P.453-10 – The radio refractive index: its formula and refractivity data
• ITU-R P.530-14 – Propagation data and prediction methods required for the design of terrestrial
line-of-sight systems
• ITU-R P.837-1 and 6 – Characteristics of precipitation for propagation modelling
• ITU-R P.838-3 – Specific attenuation model for rain for use in prediction methods
• ITU-R P.310, ITU-R P.526, ITU-R P.676, ITU-R P.834, ITU-R P.835
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Implementation Notes