MDS 05-3628A01, Rev. B
MDS 4790M (MPT 1411) I/O Guide
75
Earth Curvature
As the distance of a communication link increases, the clearance problem is
compounded by the earth’s curvature. Radio waves traveling through typical
atmospheric conditions bend slightly, which is represented by treating the earth
as though it were slightly flatter than it actually is. Experience has shown that
if we consider the earth’s radius to be 4/3rds of its actual size, we get good
agreement between theory and measured propagation results.
Figure 37 shows a representation of the 4/3 earth “radio horizon.” This figure
shows that under normal radio propagation conditions, a station with its
antenna 15 meters above flat terrain will have a radio horizon approximately
15 kilometers away, well beyond the visual horizon.
Invisible place holder
Figure 37. Antenna Height vs. Theoretical Radio Horizon
Fade Margins
Variations in the temperature and humidity of the atmosphere with elevation
cause the signals to bend more or less, resulting in fading at the receiver. The
longer the path is, the more likely that deep fades will occur, hence the greater
the fade margin required.
Different parts of the world have differing propagation conditions, which can
be categorised as favorable, average, or adverse. In general, mountainous areas
have favorable propagation conditions, while tropical areas and those near
large bodies of water have adverse conditions.
Calculating Path Loss
Assuming that we have satisfied the line-of-sight and first Fresnel zone
clearance requirements, we can calculate the path loss. At 450 MHz, the loss
between two isotropic radiators (0 dBi antennas) that are 1 km apart is 86 dB.
For every doubling of distance, the loss increases by an additional 6 dB.
Knowing this, the output power (+37 dBm), and the receiver sensitivity (–115
dBm), we can calculate antenna size and tower height requirements to cover
any desired distance.
(0)
0
(2)
(4)
(6)
(8)
(10) (12) (14) (16) (18) (20) (22) (24) (26) (28) (30) (32) (34)
(0)
0
(50)
15
(100)
30
(150)
46
(200)
61
(250)
76
(36)
Radio Horizon Over Level Terrain in Kilometers (Miles)
Normal Conditions
Antenna
Height in
Meters
(Feet)
58
55
52
48
45
42
39
35
32
29
26
23
19
16
10
13
6
3
