Codan Radio Radio Repeater Systems Training Guide - Manual (Page 51)

RADIO REPEATER SYSTEM | TRAINING GUIDE
Appendix C: Calculating Path Loss Page 43
APPENDIX C: CALCULATING PATH LOSS
We see from the profi le of the path from BRIDGE ARS to BRIDGE Base Station (Offi ce) that it is
not line-of-sight. The connecting line passes through a “lump” or obstruction. In most cases, if any
paths that you draw (particularly long paths) are not line-of-sight, calculation of path loss becomes
extremely complicated, and you are advised to seek professional help. Since this is a very short path
we can make allowances. If the free space path loss for the distance is 105 dB or less, and there is
an obstruction within 20% of the center of the path that does not exceed 150 m, the path will likely be
quite safe for year round use. Under these conditions, when doing your path calculations, add 20 dB
for shadow loss. Another example, directly related to our example is: If the free space path loss is 95
dB or less and the obstruction within 20% of the center of the path does not exceed 500 m, add 35 dB.
It is always best under these circumstances to measure the path loss. The maximum path loss should
not exceed 130 dB.
Although an algebraic computation is more accurate, using the nomogram on the next page will provide
line-of-sight path, or path losses near enough for our purposes. The answer will be within 6 dB for 90%
of the time. The graph shows that our free space path loss from BRIDGE ARS to BRIDGE Base Station
(7.8 km or 4.8 miles at 150 MHz) is 89 dB. Following the instructions on profi ling above, we would add
35 dB to account for the obstruction. Note that our free space path loss calculation is based on using
VHF (150 MHz). If we were limited to UHF use, we would not use this site, as the path loss at 450 MHz
(-99 dB plus the obstruction loss) would exceed our suggested maximum path loss. In addition, our
calculations are based on good “take-offs” at both ends of the path. This means that the antennas are
clear of any near fi eld obstructions such as trees and buildings (within 150 m and on the path line).