Codan Radio Radio Repeater Systems Training Guide - Manual (Page 57)

RADIO REPEATER SYSTEM | TRAINING GUIDE
Appendix D: Calculating Signal Levels Page 49
Note from the profi le in Appendix B that there is a mountain near the center of the BRIDGE ARS to IRON
MTN. ARS path. It does not reach high enough to block the line-of-sight dotted line drawn between the
proposed sites but it does come pretty close. In fact, at 22 km from one end, the obstruction is shown
as being only 110 meters below the line. This leads to another path loss characteristic known as the
0.6 Fresnel Zone.
Figure D-1: 0.6 Fresnel Zone
Figure D-1 shows an ellipse with the antennas as the locus at each end of the path. This ellipse is called
the 0.6 Fresnel Zone. For your information a table showing the points spaced on the ellipse spaced 5
kilometers apart for the length of our path at the frequencies in which we are concerned is shown below.
Investigation of the table shows that at 850 MHz the 0.6 Fresnel Zone is 67 meters from the center of
the path. If the obstruction were to enter this Fresnel Zone we would have to add a diffraction loss to
our free space path loss. Although we have simplifi ed the problem for our discussion, the 0.6 Fresnel
Zone clearance will be adequate and the path will probably be OK.
DISTANCE 150 MHz 450 MHz 850 MHz
1 km 42 m 25 m 18 m
6 km 99 m 59 m 43 m
11 km 127 m 75 m 55 m
16 km 144 m 85 m 62 m
21 km 153 m 91 m 66 m
26 km 156 m 92 m 67 m
31 km 153 m 91 m 66 m
36 km 144 m 85 m 62 m
41 km 128 m 76 m 55 m
46 km 101 m 60 m 43 m
51 km 46 m 27 m 20 m
The cross section of the Fresnel zones at any point is a series of concentric circles surrounding the
center of the path. Note that the obstructions within these zones may be to the sides as well as below
the path.
67 m
110 m
0.6 Fresnel Zone
Line-Of-Sight Path
Bridge A.R.S.
1327m
Iron Mtn A.R.S.
1692m